1 00:00:13,440 --> 00:00:18,680 In 1862, an American called Thomas Wentworth Higginson wrote, 2 00:00:20,360 --> 00:00:23,800 "I think that if required on pain of death 3 00:00:23,800 --> 00:00:27,680 "to name instantly the most perfect thing in the universe, 4 00:00:27,680 --> 00:00:31,840 "I should risk my fate on a bird's egg." 5 00:00:37,080 --> 00:00:40,640 I think, if pressed, I might do the same. 6 00:00:45,440 --> 00:00:49,560 It's a remarkable structure that protects a new life 7 00:00:49,560 --> 00:00:51,000 from the outside world 8 00:00:51,000 --> 00:00:54,200 and at the same time allows it to breathe. 9 00:00:54,200 --> 00:00:56,440 HEARTBEAT 10 00:00:57,920 --> 00:01:03,120 It's strong enough to withstand the full weight of an incubating parent 11 00:01:03,800 --> 00:01:07,440 and fragile enough to allow the chick to crack it. 12 00:01:14,480 --> 00:01:16,800 But how is an egg constructed? 13 00:01:16,800 --> 00:01:21,880 And, perhaps more importantly, why is it that way? 14 00:01:22,080 --> 00:01:25,560 In this programme, we're going to follow an egg 15 00:01:25,560 --> 00:01:30,440 from its creation to the moment when life breaks out of it. 16 00:01:35,080 --> 00:01:37,960 Piece by piece, we will reveal 17 00:01:37,960 --> 00:01:41,720 what lies behind nature's most perfect thing. 18 00:01:56,320 --> 00:02:01,280 This is the egg many of us see each morning at breakfast. 19 00:02:01,440 --> 00:02:03,920 ROOSTER CROWS 20 00:02:03,920 --> 00:02:06,560 And it's the egg about which there is 21 00:02:06,560 --> 00:02:11,240 the biggest body of scientific research, 22 00:02:11,240 --> 00:02:14,600 because the poultry industry has invested millions 23 00:02:14,600 --> 00:02:17,880 in finding out what makes the perfect egg. 24 00:02:27,920 --> 00:02:32,360 The very familiarity we have with this very common article 25 00:02:32,360 --> 00:02:34,840 that we find in our kitchens 26 00:02:34,840 --> 00:02:40,080 may well blind us to the wonder and beauty of the eggs 27 00:02:40,400 --> 00:02:44,360 in terms of structure and colour and shape 28 00:02:44,360 --> 00:02:49,040 that are produced by the 10,000 different species of birds 29 00:02:49,040 --> 00:02:51,520 that are alive in the world today. 30 00:02:54,120 --> 00:02:59,360 It's time to reintroduce some wonder into this miracle of nature. 31 00:03:06,120 --> 00:03:09,760 Each egg serves one purpose - 32 00:03:09,760 --> 00:03:13,400 to nurture new life. 33 00:03:13,400 --> 00:03:17,400 From the world's smallest, the bee hummingbird egg 34 00:03:17,400 --> 00:03:19,720 that weighs less than half a gram, 35 00:03:19,720 --> 00:03:24,760 to the biggest, the ostrich egg that's 40,000 times heavier. 36 00:03:26,040 --> 00:03:30,160 And yet there is an astonishing diversity of design 37 00:03:30,160 --> 00:03:35,360 not only in terms of size and shape but also colour and pattern. 38 00:03:38,600 --> 00:03:43,760 It was the sheer beauty of the egg that prompted, in many people, 39 00:03:44,120 --> 00:03:48,120 including me, questions about their biological perfection. 40 00:03:49,560 --> 00:03:52,800 And, perhaps the most important question of all, 41 00:03:52,800 --> 00:03:55,840 why lay an egg in the first place? 42 00:04:02,560 --> 00:04:05,000 WIND WHISTLES 43 00:04:07,240 --> 00:04:12,480 Antarctica - one of the most extreme environments on earth. 44 00:04:14,440 --> 00:04:18,000 Emperor penguins are beginning the daunting task 45 00:04:18,000 --> 00:04:21,960 of incubating their eggs in sub-zero temperatures. 46 00:04:23,800 --> 00:04:27,960 So why haven't they evolved to develop young inside the body, 47 00:04:27,960 --> 00:04:29,560 where it could stay warm? 48 00:04:32,640 --> 00:04:36,880 Well, penguins are related to the birds that once flew. 49 00:04:36,880 --> 00:04:40,320 And keeping weight to a minimum for anything that flies 50 00:04:40,320 --> 00:04:42,360 is of the utmost importance. 51 00:04:43,560 --> 00:04:45,560 But there's also another reason. 52 00:04:47,680 --> 00:04:50,120 Birds are hotter than mammals. 53 00:04:50,120 --> 00:04:55,360 The internal temperature of all adults is 40 degrees. 54 00:04:55,600 --> 00:04:58,800 No embryo can develop at such high heat. 55 00:05:00,560 --> 00:05:02,880 But since they lay eggs, 56 00:05:02,880 --> 00:05:06,000 they can incubate their embryos at lower temperatures. 57 00:05:09,840 --> 00:05:13,640 Laying eggs, however, does create a lot of problems. 58 00:05:15,960 --> 00:05:19,520 The embryo must now be protected, warmed and nourished 59 00:05:19,520 --> 00:05:21,560 outside the body. 60 00:05:23,600 --> 00:05:28,880 And those aren't just challenges for birds that breed in polar extremes. 61 00:05:32,680 --> 00:05:35,600 BIRDSONG 62 00:05:39,000 --> 00:05:41,920 It's the beginning of spring in Oxfordshire. 63 00:05:45,560 --> 00:05:49,760 The increasing warmth and light of the new season will transform 64 00:05:49,760 --> 00:05:53,000 the woods into a riot of life. 65 00:05:54,560 --> 00:05:57,480 Now is the time to produce young. 66 00:06:05,720 --> 00:06:09,000 This is a female great tit. 67 00:06:14,960 --> 00:06:17,560 And this is a blue tit. 68 00:06:17,560 --> 00:06:22,200 They're both extraordinarily skilful in caring for their eggs. 69 00:06:23,560 --> 00:06:26,600 Over the next few weeks, I will watch them both 70 00:06:26,600 --> 00:06:30,200 as they lay, incubate and hatch their eggs. 71 00:06:31,760 --> 00:06:36,280 Right now, it's the very beginning of the season. 72 00:06:36,280 --> 00:06:38,760 In just over 24 hours, 73 00:06:38,760 --> 00:06:43,160 this female will lay the first egg of her clutch. 74 00:06:43,160 --> 00:06:45,520 She's busy and stressed. 75 00:06:47,680 --> 00:06:50,360 Creating an egg is hard work. 76 00:06:50,360 --> 00:06:55,600 It requires additional nutrients, including calcium for the shell. 77 00:06:55,800 --> 00:06:59,400 And because tits lay large clutches, 78 00:06:59,400 --> 00:07:02,920 they need large amounts of additional calcium. 79 00:07:04,160 --> 00:07:09,200 Some birds can extract the calcium they need from their bones, 80 00:07:09,200 --> 00:07:11,200 but not tits. 81 00:07:11,200 --> 00:07:13,040 To create her clutch, 82 00:07:13,040 --> 00:07:17,920 she will need to find more calcium than she has in her entire skeleton. 83 00:07:20,200 --> 00:07:24,640 And this is what she's looking for - fragments of snail shell. 84 00:07:26,000 --> 00:07:28,160 During the laying period, 85 00:07:28,160 --> 00:07:33,400 a female will spend half her time picking up fragments like this, 86 00:07:34,040 --> 00:07:36,360 so that when she goes to roost at night, 87 00:07:36,360 --> 00:07:38,920 her gizzard is packed full of this material 88 00:07:38,920 --> 00:07:41,720 with which she will make the shell for her egg. 89 00:07:44,680 --> 00:07:47,680 We don't know exactly how many snail shells 90 00:07:47,680 --> 00:07:50,840 she has to eat to produce an egg. 91 00:07:50,840 --> 00:07:53,440 But we do know that without them, 92 00:07:53,440 --> 00:07:57,360 she would lay eggs with very thin, breakable shells, 93 00:07:57,360 --> 00:08:00,000 or even eggs with no shell at all. 94 00:08:01,400 --> 00:08:04,480 And she's running out of time - 95 00:08:04,480 --> 00:08:08,280 the egg is already forming inside her. 96 00:08:08,280 --> 00:08:13,040 Calcium or no calcium, there's no stopping the egg's arrival now. 97 00:08:19,080 --> 00:08:23,560 But exactly what happens when an egg is created? 98 00:08:25,480 --> 00:08:28,280 To understand that, we must go back 99 00:08:28,280 --> 00:08:30,920 to the very beginning of an egg's existence. 100 00:08:42,080 --> 00:08:46,200 Forget everything you know about human conception. 101 00:08:48,960 --> 00:08:51,200 Birds do it differently. 102 00:08:53,200 --> 00:08:55,800 As day breaks over the River Thames, 103 00:08:55,800 --> 00:09:00,480 great crested grebes and mute swans court in the morning light. 104 00:09:08,040 --> 00:09:12,880 But each dawn mating is not sparking an egg to life. 105 00:09:15,800 --> 00:09:20,640 In humans, fertilisation occurs within just hours of insemination. 106 00:09:22,480 --> 00:09:25,720 In birds, there's a long delay. 107 00:09:27,760 --> 00:09:30,840 The females store sperm. 108 00:09:30,840 --> 00:09:34,720 Sometimes for a few days or even a week. 109 00:09:36,720 --> 00:09:42,000 And on the other side of the world, one bird stores it for longer still. 110 00:09:48,400 --> 00:09:52,680 A female albatross flying across the vast southern seas. 111 00:09:55,000 --> 00:09:58,600 The albatross and its close relatives store sperm 112 00:09:58,600 --> 00:10:01,640 for far longer than any other bird. 113 00:10:04,840 --> 00:10:10,000 It can be two months between mating and laying a fertilised egg. 114 00:10:13,680 --> 00:10:18,480 In the past, sea bird biologists used to have a rather romantic idea 115 00:10:18,480 --> 00:10:21,800 about this period spent away from the colony. 116 00:10:21,800 --> 00:10:24,800 For a start, they called it the honeymoon period. 117 00:10:25,840 --> 00:10:28,000 But that rather missed the point, 118 00:10:28,000 --> 00:10:31,360 because the female goes away on her own. 119 00:10:34,200 --> 00:10:37,640 So why does she fly hundreds of miles away from home 120 00:10:37,640 --> 00:10:41,760 with her mate's sperm, still unused, inside her? 121 00:10:43,880 --> 00:10:48,400 Because she's busy building up this... 122 00:10:48,400 --> 00:10:49,960 Yolk. 123 00:10:53,800 --> 00:10:57,720 This is the bird's reproductive tract. 124 00:11:00,080 --> 00:11:04,680 Here in the ovary, one of the ova is filling with yolk. 125 00:11:06,640 --> 00:11:10,200 On the yolk's surface sits a tiny disc 126 00:11:10,200 --> 00:11:15,480 that contains all the female genetic material needed to create an embryo. 127 00:11:18,080 --> 00:11:20,880 The albatross has now to collect enough food 128 00:11:20,880 --> 00:11:24,800 to enable her to amass a yolk so big 129 00:11:24,800 --> 00:11:27,400 that it can be transformed into a chick. 130 00:11:29,040 --> 00:11:33,000 And only when she's done that will the egg be fertilised. 131 00:11:36,160 --> 00:11:40,840 Remarkably, it takes more than one sperm to start new life. 132 00:11:40,840 --> 00:11:44,680 The extra sperm probably releases substances 133 00:11:44,680 --> 00:11:47,200 that start the embryo's development. 134 00:11:50,120 --> 00:11:52,320 Minutes after fertilisation, 135 00:11:52,320 --> 00:11:56,280 the egg starts its 24-hour journey down the oviduct. 136 00:11:59,040 --> 00:12:03,000 First, it's swathed with albumen, the egg white, 137 00:12:03,000 --> 00:12:06,600 that contains the water needed by the growing chick. 138 00:12:08,040 --> 00:12:11,000 That done and enclosed within a membrane, 139 00:12:11,000 --> 00:12:13,480 it travels on to the uterus, 140 00:12:13,480 --> 00:12:17,560 where it will be given its protective armour, a shell. 141 00:12:21,840 --> 00:12:26,360 The shell is actually quite separate from what it contains. 142 00:12:27,560 --> 00:12:31,800 To help understand this, we can do a simple experiment. 143 00:12:37,320 --> 00:12:41,120 These are unfertilised quail eggs. 144 00:12:41,120 --> 00:12:43,360 And this is vinegar. 145 00:12:44,880 --> 00:12:46,680 What the vinegar will do is 146 00:12:46,680 --> 00:12:49,080 to reverse the process of shell formation 147 00:12:49,080 --> 00:12:51,920 by eating away the shell from the outside. 148 00:12:58,000 --> 00:13:03,040 These thousands of tiny bubbles are carbon dioxide. 149 00:13:03,040 --> 00:13:03,080 They're the result of the acetic acid in the vinegar 150 00:13:03,080 --> 00:13:06,080 They're the result of the acetic acid in the vinegar 151 00:13:06,080 --> 00:13:09,160 reacting with the calcium carbonate of the shell. 152 00:13:13,280 --> 00:13:17,160 In 24 hours, the shell has dissolved. 153 00:13:21,600 --> 00:13:25,600 And this is the egg as it would have been 154 00:13:25,600 --> 00:13:28,920 when it first arrived at the uterus - 155 00:13:28,920 --> 00:13:33,560 a yolk surrounded by a thin layer of albumen 156 00:13:33,560 --> 00:13:38,800 all contained and supported by a loose, soft bag. 157 00:13:40,800 --> 00:13:46,040 And unexpectedly, it's this bag, the membrane, not the shell, 158 00:13:46,200 --> 00:13:48,360 that gives the egg its shape. 159 00:13:50,840 --> 00:13:56,000 So now, back inside the uterus, the egg is almost complete. 160 00:13:56,640 --> 00:14:00,960 Calcium carbonate, carried by blood vessels, is deposited 161 00:14:00,960 --> 00:14:03,200 on the soft egg membrane, where 162 00:14:03,200 --> 00:14:07,720 it will harden and set, forming the shell. 163 00:14:07,720 --> 00:14:11,120 Then other cells begin to discharge pigment, 164 00:14:11,120 --> 00:14:15,040 like paint being squirted from hundreds of tiny paint guns. 165 00:14:16,480 --> 00:14:18,480 As the egg slowly revolves, 166 00:14:18,480 --> 00:14:22,120 yet more cells spray out spots and streaks. 167 00:14:24,160 --> 00:14:28,200 It's taken just under 24 hours for the egg to be fertilised 168 00:14:28,200 --> 00:14:31,200 and enclosed in a hard shell. 169 00:14:32,760 --> 00:14:38,000 And now, within the dark uterus, it waits like an actor in the wings, 170 00:14:38,480 --> 00:14:42,560 ready to make its appearance on life's stage. 171 00:14:44,240 --> 00:14:49,480 But which end will emerge first from the bird - big end or little end? 172 00:14:53,600 --> 00:14:58,840 Well, let's start with the chicken and a study conducted in 1896. 173 00:15:01,120 --> 00:15:04,560 A German scientist called Heinrich Wickmann 174 00:15:04,560 --> 00:15:07,760 poked a pencil up a chicken's bottom. 175 00:15:07,760 --> 00:15:10,200 ROOSTER CROWS 176 00:15:12,480 --> 00:15:16,200 Using eight very tame chickens, 177 00:15:16,200 --> 00:15:20,200 Wickmann used a pencil to mark 178 00:15:20,200 --> 00:15:23,880 a cross on the end of the egg 179 00:15:23,880 --> 00:15:28,240 that he could see just inside the hen's oviduct. 180 00:15:28,240 --> 00:15:30,960 This enabled him to establish 181 00:15:30,960 --> 00:15:35,000 that an hour or so before the egg is laid, 182 00:15:35,000 --> 00:15:37,880 the pointed end is pointing outwards 183 00:15:37,880 --> 00:15:41,760 and then immediately before it's ejected, 184 00:15:41,760 --> 00:15:44,760 it turns round, like that, 185 00:15:44,760 --> 00:15:48,120 and comes out blunt end first. 186 00:15:52,080 --> 00:15:55,680 But this isn't the way all birds lay their eggs. 187 00:15:55,680 --> 00:15:58,800 Some lay pointed end first. 188 00:16:00,000 --> 00:16:04,440 But do they also turn their eggs inside them as a chicken does? 189 00:16:05,720 --> 00:16:09,080 That is still something of a mystery. 190 00:16:09,080 --> 00:16:11,000 And perhaps it will remain so 191 00:16:11,000 --> 00:16:14,640 because no-one since Wickmann has been bold enough 192 00:16:14,640 --> 00:16:16,640 to use a pencil to find out. 193 00:16:27,240 --> 00:16:30,280 Working out what happens inside wild birds 194 00:16:30,280 --> 00:16:34,520 before they lay is, understandably, a rather more tricky business. 195 00:16:36,800 --> 00:16:39,600 But we can observe what happens afterwards. 196 00:16:41,280 --> 00:16:44,520 We've seen that eggs come in different sizes, 197 00:16:44,520 --> 00:16:47,840 but is their size always directly proportional 198 00:16:47,840 --> 00:16:50,760 to the size of the bird that laid them? 199 00:16:57,480 --> 00:17:00,760 And what about the number of eggs that birds lay? 200 00:17:03,400 --> 00:17:07,760 Some, like the nocturnal kiwi, lay just one huge egg 201 00:17:07,760 --> 00:17:10,640 that weighs a fifth as much as the bird that produced it. 202 00:17:14,600 --> 00:17:18,920 Others, like the male ostrich, incubate enormous clutches 203 00:17:18,920 --> 00:17:21,240 to which several females have contributed. 204 00:17:23,080 --> 00:17:28,000 These are the largest eggs laid by any living bird... 205 00:17:28,000 --> 00:17:31,600 ..but they're tiny when compared to the bird which produced them. 206 00:17:32,840 --> 00:17:36,280 Each egg weighs just 2% of the adult. 207 00:17:38,200 --> 00:17:41,840 Eggs are perfect in so many different ways. 208 00:17:41,840 --> 00:17:43,400 And they have to be, 209 00:17:43,400 --> 00:17:47,200 because they have to be laid in so many different conditions, 210 00:17:47,200 --> 00:17:49,440 from the poles to the tropics, 211 00:17:49,440 --> 00:17:53,320 wet and dry, in nests and without. 212 00:17:55,560 --> 00:17:58,200 But they all need heat. 213 00:18:03,120 --> 00:18:07,280 A Welsh churchyard viewed by a thermal camera. 214 00:18:10,200 --> 00:18:13,800 It might look like night, but actually, it's day. 215 00:18:16,640 --> 00:18:19,560 What the camera is recording is heat. 216 00:18:23,640 --> 00:18:27,920 There's a bird here that has a surprising incubation strategy 217 00:18:27,920 --> 00:18:30,800 that's never been filmed before. 218 00:18:30,800 --> 00:18:32,960 We need the thermal camera 219 00:18:32,960 --> 00:18:35,480 to give us an understanding of what she does. 220 00:18:38,440 --> 00:18:43,520 The bird is a goldcrest. It's the smallest bird in Europe. 221 00:18:45,160 --> 00:18:48,200 She weighs no more than a teaspoon of sugar. 222 00:18:51,040 --> 00:18:53,680 Her eight eggs may look tiny too, 223 00:18:53,680 --> 00:18:56,960 but in relation to her they are huge. 224 00:18:56,960 --> 00:19:01,240 Each one weighs 16% of her body weight. 225 00:19:01,240 --> 00:19:04,160 In human terms, that would be like giving birth 226 00:19:04,160 --> 00:19:07,480 to eight whopping 18lb babies. 227 00:19:13,200 --> 00:19:17,440 But her tiny body can only cover two or three of her large eggs 228 00:19:17,440 --> 00:19:20,240 at any one time. 229 00:19:20,240 --> 00:19:24,160 Bird embryos develop at around 37 degrees, 230 00:19:24,160 --> 00:19:27,000 so how will she keep them all warm? 231 00:19:30,480 --> 00:19:34,240 Well, the secret to her success is revealed 232 00:19:34,240 --> 00:19:38,080 when we turn our standard camera off 233 00:19:38,080 --> 00:19:40,920 and our thermal camera on... 234 00:19:45,360 --> 00:19:47,880 She has hot legs. 235 00:19:49,760 --> 00:19:53,840 She's pumping extra blood through them to radiate heat. 236 00:19:53,840 --> 00:19:57,920 And now her actions have been caught on film for the first time. 237 00:20:00,840 --> 00:20:04,560 No other bird on earth is known to do this. 238 00:20:12,120 --> 00:20:16,760 As scientists continue to closely observe nesting behaviour, 239 00:20:16,760 --> 00:20:19,600 they're discovering there's much more to incubation 240 00:20:19,600 --> 00:20:21,640 than was previously thought. 241 00:20:23,240 --> 00:20:27,320 In the woods of Oxfordshire, the tits are about to lay. 242 00:20:29,080 --> 00:20:32,840 How exactly do they care for their eggs? 243 00:20:34,680 --> 00:20:37,400 Like most birds about to start incubating, 244 00:20:37,400 --> 00:20:39,720 this female great tit has shed the feathers 245 00:20:39,720 --> 00:20:42,680 from a patch of skin on her abdomen. 246 00:20:45,560 --> 00:20:49,080 It's known as a brood patch. 247 00:20:49,080 --> 00:20:53,280 By controlling the flow of the blood in its naked skin 248 00:20:53,280 --> 00:20:57,200 she can regulate the amount of heat she gives to her eggs. 249 00:20:59,800 --> 00:21:02,480 The next stage of her incubation strategy is, 250 00:21:02,480 --> 00:21:06,720 perhaps surprisingly, to stop doing so. 251 00:21:09,600 --> 00:21:14,560 One egg a day - that's how most birds lay. 252 00:21:14,560 --> 00:21:17,920 So this clutch has taken eight days to produce 253 00:21:17,920 --> 00:21:19,960 from the first egg to the last. 254 00:21:21,160 --> 00:21:24,600 The cool temperatures of the woods is not high enough 255 00:21:24,600 --> 00:21:26,400 to start their development. 256 00:21:27,880 --> 00:21:32,000 So while she's still producing eggs, she doesn't incubate them. 257 00:21:35,960 --> 00:21:40,880 If she starts them off together, then they will hatch together. 258 00:21:41,080 --> 00:21:44,720 And that will enable her to care for her chicks as a group 259 00:21:44,720 --> 00:21:47,520 when they're all at their most vulnerable. 260 00:21:51,440 --> 00:21:55,040 It's now that her behaviour becomes more complex 261 00:21:55,040 --> 00:21:56,920 than researchers first thought. 262 00:22:00,520 --> 00:22:05,040 Scientists working in these woods have recently made a discovery - 263 00:22:05,040 --> 00:22:10,080 great tits are controlling the speed at which their eggs develop 264 00:22:10,240 --> 00:22:12,600 in response to the weather. 265 00:22:14,440 --> 00:22:19,000 As we all know, there's nothing less reliable than a British spring. 266 00:22:23,680 --> 00:22:26,560 Some days are cold and grey. 267 00:22:28,600 --> 00:22:33,240 That is when we might expect that her eggs need to be kept warm. 268 00:22:36,920 --> 00:22:39,760 Other days, it might be sunny. 269 00:22:39,760 --> 00:22:44,200 Perhaps a chance for the birds to spend less time on the nest. 270 00:22:48,000 --> 00:22:50,560 But new research is revealing 271 00:22:50,560 --> 00:22:53,640 that exactly the opposite is what happens. 272 00:22:57,800 --> 00:23:02,960 On warm days, perhaps surprisingly, parents incubate for longer 273 00:23:03,120 --> 00:23:05,240 than they do on cold days. 274 00:23:05,240 --> 00:23:08,920 So when it's warm, development of the egg speeds up, 275 00:23:08,920 --> 00:23:11,360 when it's cold, it slows down. 276 00:23:13,600 --> 00:23:17,880 The research into why the birds are, apparently, being so contrary, 277 00:23:17,880 --> 00:23:22,760 is being led by Dr Ella Cole from the University of Oxford. 278 00:23:29,440 --> 00:23:33,400 We've recently discovered that by varying the amount of time 279 00:23:33,400 --> 00:23:36,440 they actually spend incubating eggs each day, 280 00:23:36,440 --> 00:23:40,080 they can manipulate their hatching date, 281 00:23:40,080 --> 00:23:44,920 and they do this so they can find enough food for their chicks. 282 00:23:48,520 --> 00:23:53,040 This is the caterpillar of a winter moth, 283 00:23:53,040 --> 00:23:57,760 and each chick will need to eat about a thousand of these 284 00:23:57,760 --> 00:24:00,240 in the first two weeks of its life. 285 00:24:00,240 --> 00:24:05,280 That means, in turn, that the parents must time 286 00:24:05,280 --> 00:24:09,960 the incubation of the eggs so that when the eggs hatch 287 00:24:09,960 --> 00:24:13,320 there will be a glut of these caterpillars around. 288 00:24:15,640 --> 00:24:20,760 That caterpillar peak lasts just two short weeks. 289 00:24:21,360 --> 00:24:24,360 And warmer weather starts it earlier. 290 00:24:24,360 --> 00:24:27,240 So by taking their cues from the weather, 291 00:24:27,240 --> 00:24:31,480 the birds ensure that their eggs will hatch at exactly the same time 292 00:24:31,480 --> 00:24:33,280 as their food appears. 293 00:24:34,840 --> 00:24:37,640 It's quite remarkable that the tits are actually able 294 00:24:37,640 --> 00:24:40,240 to do this fine-scale adjusting 295 00:24:40,240 --> 00:24:43,920 even in the late stages of incubation. 296 00:24:45,400 --> 00:24:50,400 Laying an egg enables the birds to do something a mammal can't do. 297 00:24:50,400 --> 00:24:53,160 The parents, in fact, have some control 298 00:24:53,160 --> 00:24:55,160 over when their eggs will hatch. 299 00:25:04,000 --> 00:25:09,080 Now let's see what happens inside the egg when incubation begins. 300 00:25:10,680 --> 00:25:14,400 The hard shell certainly provides excellent protection. 301 00:25:15,720 --> 00:25:19,600 But the embryo within must be connected to the outside world 302 00:25:19,600 --> 00:25:21,120 so that it can breathe. 303 00:25:22,880 --> 00:25:27,880 Minute pores lead from the surface to the embryo's blood supply. 304 00:25:29,840 --> 00:25:33,880 This chicken egg has 10,000 of them. HEARTBEAT 305 00:25:33,880 --> 00:25:38,200 And they enable the developing embryo to take in oxygen 306 00:25:38,200 --> 00:25:40,480 and expel carbon dioxide. 307 00:25:45,600 --> 00:25:49,640 But a porous egg is inevitably a vulnerable one. 308 00:25:51,120 --> 00:25:53,800 What else might be able to get in? 309 00:26:04,720 --> 00:26:09,520 Well, some birds lay their eggs in rather strange places. 310 00:26:17,520 --> 00:26:19,160 Here in Dorset, 311 00:26:19,160 --> 00:26:22,640 hundreds of mute swans gather each year to breed. 312 00:26:27,360 --> 00:26:31,520 For the last few weeks, pairs have built nests in reed beds 313 00:26:31,520 --> 00:26:34,080 on the edge of a tidal lagoon. 314 00:26:36,840 --> 00:26:40,040 Now, they're just beginning to incubate. 315 00:26:43,920 --> 00:26:47,800 One threat to the eggs is hard to avoid. 316 00:26:54,480 --> 00:26:59,440 Most nests are likely to be flooded at least once during the season, 317 00:26:59,440 --> 00:26:59,600 so it's important that the eggs should be waterproof. 318 00:26:59,600 --> 00:27:02,800 so it's important that the eggs should be waterproof. 319 00:27:06,880 --> 00:27:10,760 And indeed a swan's egg has an outer layer 320 00:27:10,760 --> 00:27:14,760 that waterproofs it without suffocating the chick. 321 00:27:18,440 --> 00:27:21,280 But water isn't the biggest danger. 322 00:27:27,000 --> 00:27:29,840 It's what's carried in the water. 323 00:27:34,720 --> 00:27:39,960 Eggs can be infected by bacteria, and bacteria can travel in water 324 00:27:40,560 --> 00:27:42,800 and so get in through the pores. 325 00:27:44,080 --> 00:27:48,160 Adult birds have an immune system that can fight off microbes 326 00:27:48,160 --> 00:27:51,040 that might invade their nest, 327 00:27:51,040 --> 00:27:53,920 but the developing embryos don't. 328 00:27:55,280 --> 00:27:59,040 It's a battle of bug versus bird. 329 00:27:59,040 --> 00:28:03,360 Microbes might get inside the egg through its pores 330 00:28:03,360 --> 00:28:06,480 and consume the developing embryo within. 331 00:28:08,560 --> 00:28:12,120 But the eggs have a special protection. 332 00:28:12,120 --> 00:28:15,120 It's know as S-A-M, SAM. 333 00:28:16,480 --> 00:28:20,640 The letters stand for Shell Accessory Material 334 00:28:20,640 --> 00:28:24,720 and it's a microscopic protective layer that all eggs have, 335 00:28:24,720 --> 00:28:27,360 whether or not they are laid near water. 336 00:28:29,080 --> 00:28:32,440 This is the egg's first line of defence. 337 00:28:38,040 --> 00:28:40,160 And this is the second. 338 00:28:42,080 --> 00:28:47,080 This colourless substance is 339 00:28:47,080 --> 00:28:51,000 one of nature's most remarkable and mysterious materials. 340 00:28:51,000 --> 00:28:54,160 It's the albumen, that acts as a barrier, 341 00:28:54,160 --> 00:28:57,240 both biological and physical. 342 00:29:03,480 --> 00:29:07,080 To a microbe, travelling through the albumen to the yolk is like 343 00:29:07,080 --> 00:29:10,200 a human trying to walk across a desert - 344 00:29:10,200 --> 00:29:13,440 there's nothing to sustain life. 345 00:29:13,440 --> 00:29:18,120 But albumen contains lots of other things. 346 00:29:18,120 --> 00:29:23,400 Over 100 antimicrobial proteins have been identified in it so far 347 00:29:23,800 --> 00:29:27,680 and it seems likely that many more remain to be discovered. 348 00:29:27,680 --> 00:29:31,320 It might be hard for us to grasp that the white we see 349 00:29:31,320 --> 00:29:36,560 in our chicken eggs at breakfast is such a miraculous defence system. 350 00:29:38,720 --> 00:29:43,120 But it's the egg's way of defending itself against microbes 351 00:29:43,120 --> 00:29:47,960 that would, given half a chance, consume the developing embryo. 352 00:29:53,880 --> 00:29:57,080 Protected by albumen, and nourished by the yolk, 353 00:29:57,080 --> 00:30:00,840 the embryo continues to grow. 354 00:30:00,840 --> 00:30:04,200 As it does so, it generates water. 355 00:30:06,240 --> 00:30:08,520 We do this too when we eat, 356 00:30:08,520 --> 00:30:11,200 and we get rid of at least some of such water 357 00:30:11,200 --> 00:30:13,720 as vapour when we breathe. 358 00:30:13,720 --> 00:30:16,080 The chick does something similar 359 00:30:16,080 --> 00:30:19,840 and water vapour diffuses through the pores in the shell. 360 00:30:22,040 --> 00:30:26,840 The loss of this water creates a space at the blunt end of the egg. 361 00:30:28,960 --> 00:30:33,880 As the embryo develops, so the air space increases, 362 00:30:33,880 --> 00:30:36,520 and the oxygen it contains will help to give 363 00:30:36,520 --> 00:30:40,320 the chick the energy it needs to help it crack the shell 364 00:30:40,320 --> 00:30:42,760 when it starts to hatch. 365 00:30:55,160 --> 00:30:59,840 The shells of eggs can be extraordinarily beautiful. 366 00:31:03,720 --> 00:31:06,960 Some are almost jewel-like. 367 00:31:16,600 --> 00:31:19,480 But colour isn't mere decoration. 368 00:31:19,480 --> 00:31:24,480 It can play a crucial part in the egg's survival system. 369 00:31:24,600 --> 00:31:27,600 Sometimes, it serves as camouflage. 370 00:31:29,280 --> 00:31:31,760 Sometimes, it prevents overheating. 371 00:31:33,600 --> 00:31:38,880 And, remarkably, colour can also act as a defence against a murderer. 372 00:31:46,400 --> 00:31:47,960 A cuckoo... 373 00:31:47,960 --> 00:31:50,240 ..in the Fens of East Anglia. 374 00:31:55,200 --> 00:32:00,160 The cuckoo never builds a nest or cares for its young. 375 00:32:04,160 --> 00:32:08,760 Instead, it tricks other species into accepting its egg 376 00:32:08,760 --> 00:32:12,360 and then raising its baby instead of their own. 377 00:32:16,440 --> 00:32:21,240 A female cuckoo takes an egg from a reed warbler nest. 378 00:32:21,240 --> 00:32:25,160 Within seconds, she's laid her own egg in its place. 379 00:32:26,800 --> 00:32:28,760 It's slightly larger, 380 00:32:28,760 --> 00:32:32,280 but its colour exactly matches that of the reed warbler's. 381 00:32:34,120 --> 00:32:37,440 Professor Nick Davies from the University of Cambridge is 382 00:32:37,440 --> 00:32:39,680 the world's leading cuckoo expert. 383 00:32:41,440 --> 00:32:45,680 And he knows it isn't easy being a killer and a thief. 384 00:32:47,720 --> 00:32:49,680 It's actually a crazy thing to do, 385 00:32:49,680 --> 00:32:52,120 it's such hard work looking for host nests. 386 00:32:52,120 --> 00:32:55,320 I think if I was a bird, I'd just be an honest worker 387 00:32:55,320 --> 00:32:57,480 and raise my own young. 388 00:32:57,480 --> 00:33:02,480 Nick tests how important colour is by placing a wrongly coloured egg 389 00:33:02,600 --> 00:33:07,160 into the reed warbler's nest and seeing how the reed warbler reacts. 390 00:33:19,440 --> 00:33:24,080 The reed warbler immediately senses something isn't quite right. 391 00:33:28,680 --> 00:33:32,200 She starts to destroy the new egg. 392 00:33:34,320 --> 00:33:38,280 If you give reed warblers a blue egg or a white egg or a brown egg - 393 00:33:38,280 --> 00:33:40,520 very different from their own green eggs - 394 00:33:40,520 --> 00:33:42,080 they throw them out, 395 00:33:42,080 --> 00:33:45,440 but if you give them a green egg matching their own eggs, 396 00:33:45,440 --> 00:33:47,960 in other words mimicking what the cuckoo actually does, 397 00:33:47,960 --> 00:33:50,520 the reed warblers tend to accept that. 398 00:33:50,520 --> 00:33:53,960 So the cuckoo's egg has to match the reed warbler's eggs in colour 399 00:33:53,960 --> 00:33:56,200 if the cuckoo's got to get its egg accepted. 400 00:33:56,200 --> 00:33:59,040 So these very simple experiments show that this egg mimicry 401 00:33:59,040 --> 00:34:02,040 by the cuckoo is a crucial part of their trickery. 402 00:34:05,640 --> 00:34:08,760 There are several races of cuckoos in Britain, 403 00:34:08,760 --> 00:34:11,000 each with a distinctive egg 404 00:34:11,000 --> 00:34:15,320 that matches the colour of its particular host's eggs. 405 00:34:18,760 --> 00:34:23,000 But new research has shown that some cuckoos' forgery skills 406 00:34:23,000 --> 00:34:25,840 are increasingly being put to the test. 407 00:34:34,560 --> 00:34:37,000 Thousands of miles from the Fens, 408 00:34:37,000 --> 00:34:42,200 across the Atlantic, Princeton University. 409 00:34:42,400 --> 00:34:45,000 Here, one of Nick's former colleagues, 410 00:34:45,000 --> 00:34:49,840 Dr Mary Caswell Stoddard, is continuing cuckoo egg research. 411 00:34:49,840 --> 00:34:52,440 To really understand what's going on, 412 00:34:52,440 --> 00:34:55,920 Cassie and her team created a computer program 413 00:34:55,920 --> 00:34:58,760 that analyses colours and patterns on an egg 414 00:34:58,760 --> 00:35:01,120 in the same way as a bird might see them. 415 00:35:03,200 --> 00:35:06,800 It's very important to take a bird's-eye view 416 00:35:06,800 --> 00:35:10,120 when asking a question about egg mimicry, 417 00:35:10,120 --> 00:35:15,160 and that's because birds have very different vision than humans do. 418 00:35:15,160 --> 00:35:19,040 Birds are seeing a much more richly coloured world 419 00:35:19,040 --> 00:35:20,560 than we humans are. 420 00:35:22,040 --> 00:35:26,040 Cassie's computer program has revealed that the duel 421 00:35:26,040 --> 00:35:29,840 between a cuckoo and its victims is much more sophisticated 422 00:35:29,840 --> 00:35:31,520 than anyone previously thought. 423 00:35:35,000 --> 00:35:39,880 We were astonished to find that some hosts have evolved 424 00:35:39,880 --> 00:35:44,800 highly recognisable pattern signatures on their eggs 425 00:35:44,800 --> 00:35:47,800 in response to cuckoo mimicry. 426 00:35:47,800 --> 00:35:52,560 So it's not just that cuckoos have evolved the ability to match 427 00:35:52,560 --> 00:35:55,040 the colour and pattern of host eggs, 428 00:35:55,040 --> 00:35:59,320 hosts are also fighting back against cuckoo mimicry 429 00:35:59,320 --> 00:36:04,560 by evolving patterns they can easily recognise on their own eggs. 430 00:36:05,000 --> 00:36:10,040 Some hosts are evolving ever more complex patterns on their own eggs 431 00:36:10,040 --> 00:36:10,080 to make mimicking them much harder. 432 00:36:10,080 --> 00:36:12,600 to make mimicking them much harder. 433 00:36:14,160 --> 00:36:19,360 This is similar to the way in which a bank might insert watermarks 434 00:36:20,000 --> 00:36:24,880 on their dollar bills to make life more challenging for counterfeiters. 435 00:36:33,400 --> 00:36:37,240 So, egg patterns, it seems, can change. 436 00:36:38,840 --> 00:36:42,360 And as new technology reveals ever more ways 437 00:36:42,360 --> 00:36:45,920 in which we can understand how birds see their eggs, 438 00:36:45,920 --> 00:36:48,760 who knows what more will be discovered? 439 00:36:54,080 --> 00:36:58,560 And the same is true of the one important characteristic of the egg 440 00:36:58,560 --> 00:37:00,800 that we have not yet examined... 441 00:37:02,840 --> 00:37:04,880 ..its shape. 442 00:37:06,520 --> 00:37:09,200 Describe something as being egg-shaped 443 00:37:09,200 --> 00:37:12,720 and it's usually a chicken's egg that we have in mind. 444 00:37:18,040 --> 00:37:23,280 But eggs are extremely varied both in size, shape and colour. 445 00:37:23,760 --> 00:37:27,920 Each has doubtless been developed for a particular reason. 446 00:37:29,280 --> 00:37:32,680 Although it's not always clear what that is. 447 00:37:34,120 --> 00:37:37,200 And there's one egg with a shape so extreme 448 00:37:37,200 --> 00:37:40,560 that it has long been one of ornithologists' great mysteries. 449 00:37:43,080 --> 00:37:46,920 It's produced by a bird called the guillemot. 450 00:37:46,920 --> 00:37:52,120 And if we want to solve the puzzle of this curiously conical egg, 451 00:37:52,640 --> 00:37:55,240 then here is a good place to start. 452 00:37:59,720 --> 00:38:02,680 The Welsh island of Skomer. 453 00:38:02,680 --> 00:38:06,240 It's home to one of the largest guillemot colonies in Britain. 454 00:38:08,880 --> 00:38:12,560 Professor Tim Birkhead is a leading ornithologist 455 00:38:12,560 --> 00:38:16,800 who has been studying the birds that breed here for over four decades. 456 00:38:21,280 --> 00:38:26,120 And in the last few years, he's turned his attention to their eggs. 457 00:38:34,240 --> 00:38:37,840 Guillemots lay these fabulous eggs. 458 00:38:37,840 --> 00:38:41,720 The guillemot egg shape has, for years, been recognised 459 00:38:41,720 --> 00:38:43,360 as the most extreme. 460 00:38:43,360 --> 00:38:47,080 It's more pointed, more extreme than any other species 461 00:38:47,080 --> 00:38:49,880 and that's been a puzzle for a long time. 462 00:38:51,760 --> 00:38:55,160 Many people have put forward theories to explain it, 463 00:38:55,160 --> 00:38:57,800 and one egg collector came up with a suggestion 464 00:38:57,800 --> 00:39:00,680 that was beguilingly simple. 465 00:39:00,680 --> 00:39:02,320 It's based on the fact 466 00:39:02,320 --> 00:39:04,960 that guillemots don't lay their eggs in nests, 467 00:39:04,960 --> 00:39:09,160 but instead balance them precariously on cliff ledges. 468 00:39:13,440 --> 00:39:16,720 This is a fake guillemot egg. 469 00:39:16,720 --> 00:39:20,160 When it's spun, it rotates like a top. 470 00:39:22,200 --> 00:39:25,040 This, the egg collector's theory went, ensures 471 00:39:25,040 --> 00:39:28,120 that if an egg is accidentally knocked 472 00:39:28,120 --> 00:39:32,600 it will spin on its axis rather than roll off a narrow cliff ledge. 473 00:39:33,920 --> 00:39:37,400 The egg, he said, would rotate in the breeze, 474 00:39:37,400 --> 00:39:39,200 you know, which is just ludicrous. 475 00:39:39,200 --> 00:39:42,360 You know, the egg is so heavy it couldn't possibly rotate. 476 00:39:42,360 --> 00:39:45,920 What he was doing was using a museum egg which was empty, 477 00:39:45,920 --> 00:39:49,680 but, you know, a real guillemot egg is full of yolk or an embryo. 478 00:39:49,680 --> 00:39:51,040 It can't do that. 479 00:39:53,240 --> 00:39:54,920 If you watch guillemots, 480 00:39:54,920 --> 00:39:58,480 not that many eggs actually roll off the ledges 481 00:39:58,480 --> 00:40:01,520 so we started to think about what other possible explanations 482 00:40:01,520 --> 00:40:04,160 there could be for why the eggs are this shape. 483 00:40:07,640 --> 00:40:11,360 To investigate the mystery of the guillemot egg shape, 484 00:40:11,360 --> 00:40:14,560 Tim had to get his hands dirty and investigate 485 00:40:14,560 --> 00:40:18,080 another aspect of the guillemot's breeding environment. 486 00:40:24,200 --> 00:40:26,800 So this is a rather typical guillemot colony - 487 00:40:26,800 --> 00:40:31,080 almost sheer cliffs, birds breeding on tiny ledges, 488 00:40:31,080 --> 00:40:33,480 often, though, at incredible densities, 489 00:40:33,480 --> 00:40:37,160 up to 20 pairs per square metre. 490 00:40:37,160 --> 00:40:41,040 Such crowding, Tim noticed, meant that the birds were 491 00:40:41,040 --> 00:40:44,680 inevitably covering their eggs, and those of their neighbours, 492 00:40:44,680 --> 00:40:49,680 in something that may look like dirt but is actually rather nastier. 493 00:40:55,840 --> 00:40:57,920 Because guillemots don't make any nests, 494 00:40:57,920 --> 00:41:01,960 their ledges become covered in what is technically known as faeces. 495 00:41:01,960 --> 00:41:05,240 And when it's wet, this turns into an absolute pig farm, 496 00:41:05,240 --> 00:41:07,680 I mean, you can see my hands are covered now. 497 00:41:07,680 --> 00:41:10,280 And, you know, the birds are incubating, 498 00:41:10,280 --> 00:41:12,320 alternating wet and dry weather, 499 00:41:12,320 --> 00:41:16,000 so that muck is continually being heated up, cooled down 500 00:41:16,000 --> 00:41:19,160 and, you can see here, covering the egg. 501 00:41:23,040 --> 00:41:26,760 Now, there are, as we've seen, at least two defences 502 00:41:26,760 --> 00:41:29,200 that should prevent microbes in the muck 503 00:41:29,200 --> 00:41:31,600 from infecting the developing chick - 504 00:41:31,600 --> 00:41:35,080 first the shell and then the albumen. 505 00:41:36,880 --> 00:41:41,200 But could nature also have evolved an unusually shaped egg 506 00:41:41,200 --> 00:41:43,360 as an additional protection? 507 00:41:45,240 --> 00:41:47,440 I kind of started thinking about this 508 00:41:47,440 --> 00:41:51,520 and noticing that it's this pointed end of the egg 509 00:41:51,520 --> 00:41:53,720 that becomes the most covered in this muck 510 00:41:53,720 --> 00:41:58,360 because that's the one that's lying, like that, on the ledge in the muck. 511 00:41:58,360 --> 00:42:01,480 That made me think that maybe the pointed shape is actually 512 00:42:01,480 --> 00:42:05,200 an adaptation for coping with that filth on the ledge. 513 00:42:05,200 --> 00:42:08,160 Because, as I say, when the egg is sitting there, 514 00:42:08,160 --> 00:42:11,240 it's this big end here, you can see, is relatively free, 515 00:42:11,240 --> 00:42:12,840 and that's where the airspace is 516 00:42:12,840 --> 00:42:15,480 and that's where the chick is going to be breathing from. 517 00:42:15,480 --> 00:42:18,720 So I think this shape of the egg may be related to the fact 518 00:42:18,720 --> 00:42:21,800 that these birds breed in such an unusually dirty environment. 519 00:42:25,240 --> 00:42:28,480 Who would have thought that the most likely explanation 520 00:42:28,480 --> 00:42:32,440 of the guillemot's pointed egg was its droppings? 521 00:42:33,880 --> 00:42:36,600 But that's what Tim's research has shown, 522 00:42:36,600 --> 00:42:41,080 that this shape helps the blunt end, where the chick's head is, 523 00:42:41,080 --> 00:42:43,640 to remain relatively free of filth 524 00:42:43,640 --> 00:42:46,760 so that it's easier for the chick inside to breathe. 525 00:42:48,600 --> 00:42:51,680 But could there be other factors at work as well? 526 00:42:56,320 --> 00:42:59,880 Back in the Princeton University labs, 527 00:42:59,880 --> 00:43:04,000 Cassie Stoddard is also pondering on the mysteries of egg shape. 528 00:43:06,080 --> 00:43:10,120 But not just that of the guillemot's egg. 529 00:43:10,120 --> 00:43:15,000 She is exploring how egg shape has evolved among birds worldwide. 530 00:43:17,640 --> 00:43:22,880 We analysed the shapes of almost 50,000 eggs from digital images. 531 00:43:26,920 --> 00:43:31,000 And these eggs represented 1,400 species, 532 00:43:31,000 --> 00:43:34,040 about 14% of all birds. 533 00:43:35,320 --> 00:43:39,880 Eggs can be round or conical, or anything in-between. 534 00:43:41,920 --> 00:43:46,280 And Cassie was intrigued to discover that there is a link 535 00:43:46,280 --> 00:43:49,960 between egg shape and a bird's flying ability. 536 00:43:51,600 --> 00:43:53,800 The stronger a bird's flight, 537 00:43:53,800 --> 00:43:56,680 the more elliptical and pointed its egg will be. 538 00:43:57,720 --> 00:43:59,440 But why? 539 00:44:00,960 --> 00:44:04,560 Well, our best guess at the moment is that 540 00:44:04,560 --> 00:44:09,560 in order for a bird to maintain a streamlined body plan 541 00:44:09,560 --> 00:44:13,480 it can't lay an egg that's too wide across 542 00:44:13,480 --> 00:44:16,320 cos this would disrupt the streamlined nature 543 00:44:16,320 --> 00:44:19,040 of a flying bird's body. 544 00:44:19,040 --> 00:44:23,880 And so one solution to this potential problem may have been 545 00:44:23,880 --> 00:44:28,400 to lay an egg that's more pointy, more elliptical, 546 00:44:28,400 --> 00:44:33,200 cos then a bird can still pack a large volume into an egg 547 00:44:33,200 --> 00:44:36,840 without it disrupting the birds' body plan. 548 00:44:38,280 --> 00:44:42,560 The bigger an egg, the more nutrients it can hold for the chick. 549 00:44:42,560 --> 00:44:45,200 But if the birds are to fly efficiently, 550 00:44:45,200 --> 00:44:48,640 their bodies can't accommodate a wide, bulky egg. 551 00:44:48,640 --> 00:44:51,520 A long, pointed egg reduces that problem. 552 00:44:53,320 --> 00:44:58,560 Case closed, you might say, on the mystery of the avian egg shape. 553 00:44:59,800 --> 00:45:02,360 But you'd be wrong. 554 00:45:02,360 --> 00:45:06,840 Well, we're certainly not closing the book on egg shape. 555 00:45:06,840 --> 00:45:10,360 There is still so much to discover. 556 00:45:10,360 --> 00:45:13,640 There's much more to be done to really understand 557 00:45:13,640 --> 00:45:18,640 why birds lay eggs that come in such a variety of shape. 558 00:45:20,720 --> 00:45:25,720 For Tim, too, the explanation of egg shape is far from complete. 559 00:45:27,040 --> 00:45:30,280 He thinks that future research will show 560 00:45:30,280 --> 00:45:34,800 that the demands of incubation are also major influences. 561 00:45:37,600 --> 00:45:41,040 Although people like the idea of a single factor 562 00:45:41,040 --> 00:45:44,880 explaining a phenomenon, it might be that several different factors 563 00:45:44,880 --> 00:45:49,720 might all work together to help to shape the evolution of eggs. 564 00:45:49,720 --> 00:45:51,760 As any good scientist knows, 565 00:45:51,760 --> 00:45:54,240 what you understand is going on in the world 566 00:45:54,240 --> 00:45:56,480 is what we call the truth for now 567 00:45:56,480 --> 00:45:59,120 because, probably, somebody will come along later 568 00:45:59,120 --> 00:46:02,600 with some new evidence, we'll find a different kind of truth. 569 00:46:14,160 --> 00:46:19,400 An egg, whatever its shape, is an excellent life-support system. 570 00:46:20,640 --> 00:46:24,920 But paradoxically, its success will ultimately depend 571 00:46:24,920 --> 00:46:28,400 on the ease with which it can be broken. 572 00:46:28,400 --> 00:46:31,680 The time comes when a chick must break free. 573 00:46:33,480 --> 00:46:37,320 Some species invested time building up large yolks. 574 00:46:38,400 --> 00:46:40,640 Their chicks will emerge fully feathered 575 00:46:40,640 --> 00:46:42,600 and ready to search for food. 576 00:46:45,440 --> 00:46:48,080 Others have not made that investment. 577 00:46:48,080 --> 00:46:50,320 They will have to spend their energies 578 00:46:50,320 --> 00:46:54,720 over the next few weeks feeding naked and defenceless chicks. 579 00:46:57,840 --> 00:47:02,120 But how do the chicks break out from the cramped confines of the egg? 580 00:47:03,320 --> 00:47:06,200 How can the shell that's been strong enough to protect the chick 581 00:47:06,200 --> 00:47:09,440 from the outside world be also weak enough 582 00:47:09,440 --> 00:47:11,720 to allow the chick to break it? 583 00:47:19,960 --> 00:47:23,840 The first breath of fresh air outside the egg. 584 00:47:26,080 --> 00:47:29,080 A captive-bred jungle fowl chick emerges. 585 00:47:34,480 --> 00:47:37,800 It's the climax of the egg's existence. 586 00:47:37,800 --> 00:47:40,440 CHICK TWEETS 587 00:47:43,960 --> 00:47:47,120 The shell may look the same as when the egg was laid, 588 00:47:47,120 --> 00:47:51,440 but out of sight it's been changing. 589 00:47:51,440 --> 00:47:53,320 It's been getting thinner. 590 00:47:54,960 --> 00:47:57,920 The chick has been absorbing calcium from the shell 591 00:47:57,920 --> 00:48:03,120 into its own bones, making itself stronger and the shell weaker. 592 00:48:04,440 --> 00:48:08,680 Not only that, but it also used the shell's calcium to create 593 00:48:08,680 --> 00:48:11,520 a tool to help it break free. 594 00:48:12,600 --> 00:48:17,840 A hard, jagged tip on the end of its beak, an egg tooth. 595 00:48:18,600 --> 00:48:21,760 The chick couldn't have broken free without it. 596 00:48:21,760 --> 00:48:25,880 Even so, it can still take hours, sometimes days, 597 00:48:25,880 --> 00:48:28,400 to hammer its way out of a shell. 598 00:48:30,400 --> 00:48:35,520 This egg and this newly hatched little chick are part of a clutch 599 00:48:35,720 --> 00:48:40,920 that was laid on the ground between 21 and 26 days ago 600 00:48:41,240 --> 00:48:43,880 and they are just now hatching. 601 00:48:43,880 --> 00:48:48,320 This one is only about half an hour old. 602 00:48:48,320 --> 00:48:52,600 And this one is just beginning to peck its way out. 603 00:48:52,600 --> 00:48:56,360 And as they do, they communicate with one another. 604 00:48:56,360 --> 00:49:01,560 And the sound of this little chick encourages that unhatched chick 605 00:49:01,720 --> 00:49:04,000 to break its way out of the egg 606 00:49:04,000 --> 00:49:09,200 so that within an hour or so the whole clutch is hatched 607 00:49:09,600 --> 00:49:13,720 and then they can run away as a little group and find safety. 608 00:49:18,280 --> 00:49:21,600 But what of the other woodland birds I was watching? 609 00:49:29,280 --> 00:49:31,960 The tits are breaking out of their shells 610 00:49:31,960 --> 00:49:34,200 into the British spring. 611 00:49:37,600 --> 00:49:40,720 They are naked, blind and hungry. 612 00:49:44,600 --> 00:49:48,320 But outside, the woods are filled with food. 613 00:49:48,320 --> 00:49:52,320 And the parents' careful timing has paid off. 614 00:49:57,600 --> 00:50:02,240 They took account of the weather and changed their behaviour and won. 615 00:50:05,320 --> 00:50:09,560 Each element of the egg combined to create new life - 616 00:50:09,560 --> 00:50:11,560 from the nutritious yolk 617 00:50:11,560 --> 00:50:14,440 to the defensive albumen 618 00:50:14,440 --> 00:50:16,680 to the protective shell. 619 00:50:17,800 --> 00:50:22,960 Nature's most perfect life-support system has served its purpose, 620 00:50:23,600 --> 00:50:27,280 broken by the life that it sustained. 621 00:50:31,840 --> 00:50:36,520 Every new arrival is a confirmation of the complex efficiency 622 00:50:36,520 --> 00:50:39,240 of a seemingly simple egg. 623 00:50:44,800 --> 00:50:46,960 As with all forms of life, 624 00:50:46,960 --> 00:50:49,640 what we see are the success stories - 625 00:50:49,640 --> 00:50:52,040 the adaptations that work. 626 00:50:52,040 --> 00:50:56,200 So it's little wonder that we think of eggs as being perfect. 627 00:50:57,720 --> 00:51:00,600 But of the 10,000 different species of birds 628 00:51:00,600 --> 00:51:02,880 that exist in the world today, 629 00:51:02,880 --> 00:51:06,920 there are still hundreds whose eggs have never even been described. 630 00:51:08,880 --> 00:51:12,360 When it comes to the most perfect thing in the universe, 631 00:51:12,360 --> 00:51:15,960 there's still much magic and mystery to explore. 632 00:51:35,520 --> 00:51:38,960 Collecting birds' eggs, of course, is now illegal, 633 00:51:38,960 --> 00:51:41,600 but it wasn't when I was a boy, 80 years ago, 634 00:51:41,600 --> 00:51:45,080 and I too had a collection of birds' eggs. 635 00:51:45,080 --> 00:51:49,320 And I blew them simply by taking a pin and making a hole in one end 636 00:51:49,320 --> 00:51:52,880 and then one in the other and then blowing it. 637 00:51:52,880 --> 00:51:58,120 But professional egg collectors were more thorough and neat about it. 638 00:51:59,280 --> 00:52:02,120 They started with a little needle 639 00:52:02,120 --> 00:52:07,200 which they created a small hole like that 640 00:52:07,200 --> 00:52:12,440 and then they used one of these and enlarged the hole. 641 00:52:14,520 --> 00:52:19,560 And then you take a little glass pipette like this... 642 00:52:31,040 --> 00:52:34,840 And there it is, the egg is blown. 643 00:52:34,840 --> 00:52:38,680 And this is what the collectors collected. 644 00:52:38,680 --> 00:52:43,920 The empty, dead, lifeless outer cover of an egg. 645 00:52:46,840 --> 00:52:50,040 By discarding the most essential part of an egg, 646 00:52:50,040 --> 00:52:53,680 collectors were able to preserve their fragile shells. 647 00:52:58,600 --> 00:53:01,640 Here, at the Natural History Museum in Tring, 648 00:53:01,640 --> 00:53:06,120 there are more than a million eggs from historic collections. 649 00:53:11,800 --> 00:53:16,840 It's quite right that amateur egg collecting has been made illegal. 650 00:53:17,080 --> 00:53:20,480 But that doesn't mean that we shouldn't ask questions 651 00:53:20,480 --> 00:53:24,200 of egg collections that are already in museums. 652 00:53:24,200 --> 00:53:28,240 And some of the answers they provide are very surprising. 653 00:53:30,280 --> 00:53:33,120 It was by studying museum collections 654 00:53:33,120 --> 00:53:38,200 that scientists discovered exactly why some populations of birds, 655 00:53:38,320 --> 00:53:39,840 like the song thrush, 656 00:53:39,840 --> 00:53:43,680 had been in decline ever since the Industrial Revolution. 657 00:53:43,680 --> 00:53:48,520 They found that the eggshells had been getting thinner. 658 00:53:48,520 --> 00:53:53,040 The birds had been unable to get the calcium they needed. 659 00:53:53,040 --> 00:53:58,280 And that was because they normally got that calcium from snail shells, 660 00:53:58,760 --> 00:54:02,320 and acid rain had killed the snails. 661 00:54:05,000 --> 00:54:08,640 Again, in the 1960s, historic eggs exposed 662 00:54:08,640 --> 00:54:11,400 another calcium-related crisis, 663 00:54:11,400 --> 00:54:14,360 this time when populations of top predators 664 00:54:14,360 --> 00:54:18,000 like raptors, owls and herons suddenly crashed. 665 00:54:20,440 --> 00:54:25,360 It was measurements made on eggshells in museum collections 666 00:54:25,360 --> 00:54:28,240 that showed only too clearly 667 00:54:28,240 --> 00:54:33,400 that the reduction in thickness of the eggshell coincided exactly 668 00:54:34,000 --> 00:54:37,080 with the introduction of DDT. 669 00:54:38,320 --> 00:54:41,640 Half a millimetre of calcium, 670 00:54:41,640 --> 00:54:45,000 that's the difference between life and death. 671 00:54:47,440 --> 00:54:51,920 So what more can we learn from eggs? 672 00:54:51,920 --> 00:54:56,120 Well, deep in the basement of the Natural History Museum in London, 673 00:54:56,120 --> 00:54:59,640 one of the most important eggs in the world is going to be 674 00:54:59,640 --> 00:55:04,520 gently interrogated to see if it will surrender any of its secrets. 675 00:55:05,960 --> 00:55:10,960 This is the oldest egg in the museum's collection, 676 00:55:11,120 --> 00:55:15,840 and it's about to be placed within one of the most advanced 677 00:55:15,840 --> 00:55:19,080 and latest pieces of technical equipment here. 678 00:55:22,200 --> 00:55:27,240 What makes this egg so special is that it was laid by a great auk, 679 00:55:27,400 --> 00:55:32,320 a giant sea bird that became extinct in the 19th century. 680 00:55:32,320 --> 00:55:36,360 There are only 70 of these eggs left in the world. 681 00:55:37,400 --> 00:55:40,680 This is a 3D X-ray microscope 682 00:55:40,680 --> 00:55:44,120 and it will scan the entire structure of the egg. 683 00:55:44,120 --> 00:55:48,080 Whoever it was who collected this egg back in the 1700s 684 00:55:48,080 --> 00:55:51,200 could never have imagined something like this. 685 00:55:52,480 --> 00:55:55,720 The research is being led by Professor Birkhead. 686 00:55:57,520 --> 00:55:59,960 It's a unique opportunity for us 687 00:55:59,960 --> 00:56:04,960 to use this fantastic technology here to explore the egg's structure 688 00:56:04,960 --> 00:56:07,680 so we can infer something about the biology of the great auk 689 00:56:07,680 --> 00:56:10,880 that we don't know. It's very exciting. 690 00:56:13,000 --> 00:56:17,680 We know the great auk was a relative of the guillemot. 691 00:56:17,680 --> 00:56:21,520 But what no-one knows is whether they bred in a similar way - 692 00:56:21,520 --> 00:56:24,400 packed tightly together in faecal squalor 693 00:56:24,400 --> 00:56:26,800 like the birds we saw on Skomer. 694 00:56:28,600 --> 00:56:31,640 The guillemot's egg when it's resting on its cliff ledge, 695 00:56:31,640 --> 00:56:33,640 which is where it's incubated, 696 00:56:33,640 --> 00:56:36,760 the blunt end is raised up because of the shape of the egg. 697 00:56:36,760 --> 00:56:41,160 It's raised up out of the muck that guillemots typically breed in. 698 00:56:41,160 --> 00:56:45,000 And we think that the additional pores there is an adaptation 699 00:56:45,000 --> 00:56:49,120 to allow the embryo to breathe under these rather dirty conditions. 700 00:56:50,520 --> 00:56:55,240 The egg is a fragile link back to a vanished past. 701 00:56:55,240 --> 00:57:00,440 As you can see it's very beautiful, the patterning is just exquisite. 702 00:57:01,760 --> 00:57:05,360 The shape of the egg is similar to that of the guillemot's. 703 00:57:05,360 --> 00:57:07,800 But what about the pores? 704 00:57:09,160 --> 00:57:12,400 And now we can take a slice through the shell. 705 00:57:12,400 --> 00:57:14,560 So this is now a cross-section. 706 00:57:14,560 --> 00:57:17,960 You can see a pore here running from the outside surface of the egg... 707 00:57:17,960 --> 00:57:20,800 The results are a complete surprise. 708 00:57:20,800 --> 00:57:24,160 Interestingly, it turns out that great auk eggs 709 00:57:24,160 --> 00:57:27,160 are much more like the eggs of a razorbill 710 00:57:27,160 --> 00:57:30,800 than they are of a guillemot, so strikingly different. 711 00:57:30,800 --> 00:57:35,040 The razorbill is another sea bird relative of the great auk, 712 00:57:35,040 --> 00:57:39,320 but it breeds in clean nests that are spaced out. 713 00:57:39,320 --> 00:57:43,360 Their eggs don't need a high density of pores at the blunt end, 714 00:57:43,360 --> 00:57:47,680 unlike the guillemot egg that's adapted to dirty conditions. 715 00:57:47,680 --> 00:57:49,640 Razorbills don't have that problem, 716 00:57:49,640 --> 00:57:51,680 because they breed in very tidy nests, 717 00:57:51,680 --> 00:57:53,720 they defecate outside the nest, 718 00:57:53,720 --> 00:57:56,360 whereas guillemots breed in very messy circumstances. 719 00:57:56,360 --> 00:57:58,640 Amazing, amazing. 720 00:58:00,640 --> 00:58:05,640 So while the egg's shape suggests that the auk bred like a guillemot, 721 00:58:05,640 --> 00:58:08,440 its pore structure reveals that, in fact, 722 00:58:08,440 --> 00:58:12,240 it's likely to have bred like a razorbill. 723 00:58:12,240 --> 00:58:16,520 One ancient egg and one modern machine have revealed 724 00:58:16,520 --> 00:58:20,480 a tantalising glimpse into the life of an extinct bird. 725 00:58:23,600 --> 00:58:27,040 And as more technological innovations come along, 726 00:58:27,040 --> 00:58:30,880 who knows what other mysteries egg collections of the past 727 00:58:30,880 --> 00:58:32,880 could help us solve?