Eggs, the perfect life capsules
Varying in colour and size, birds’ eggs provide protection and nourishment, perfectly adapted to ensure the survival of their tiny occupants
LYING IN A round cup of grass, straw, small twigs and other plant material are four pale blue, speckled eggs. Just over 1in (30mm) long, the tiny eggs belong to a pair of blackbirds. The female bird will incubate them for approximately two weeks. When there is no parent bird sitting on them, the eggs are hidden from predators, tucked away in their nest in a hedge, or branches of a tree. Inside its protective cradle, the chick has all it needs to thrive before hatching. It is the perfect life support capsule. Taking flight Unlike mammals, which give birth to live young, birds are oviparous. They produce eggs which are fertilised internally and then laid. Leaving the young to develop outside the body would seem to be a dangerous strategy, one inherited from their dinosaur forebears. But, for birds, it has more than paid off. “Laying eggs is an effective and efficient means of reproduction for birds in every terrestrial habitat on earth,” says Douglas Russell, senior curator of birds’ eggs and nests at the Natural History Museum (NHM) in London. “Birds are so successful at what they do, that retaining eggs in their body would offer them no real advantages. There would, however, be one big disadvantage. Flight would be much more difficult.” Laying eggs gives female birds the freedom to fly and, with no restrictions of weight, the ability to produce more offspring. Making an egg It takes approximately 24 hours for a female bird to produce an egg, with most laying one a day until their clutch is complete. The process starts with a reproductive cell, known as an oocyte, in the ovary. This combines with protein and fat transported from the liver to create a yolk, the growing embryo’s food supply. Bound by a thin membrane, the yolk also contains vitamins, minerals and antibiotics to protect against harmful bacteria. The yolk, which has genetic material on its surface, is released into the infundibulum, the first stage in its journey down the reproductive tract. If sperm are present, it will be fertilised here. If not, the unfertilised egg will continue anyway. “The bird has no way of telling whether or not an egg is fertilised,” says Douglas. “This might seem inefficient, but mating happens regularly, so it is actually a successful strategy. If the egg fails, a bird will eat it and reabsorb the nutrients.” As it passes through the infundibulum, twisted strands of protein, called chalazae, begin to appear at each side of the yolk. When fully formed, these anchor the yolk in the centre of the egg, and ensure the developing embryo is the right way up. After approximately 30 minutes, the egg reaches the magnum, where layers of thick and then thin albumen are formed around it. The albumen cushions the embryo, as well as providing water, protein and more nutrients. Now the egg moves down to the isthmus, where an inner and an outer shell membrane are laid down, again preventing bacteria from reaching the yolk. As the egg cools after being laid, these two membranes separate to create an air sac, usually at the blunt end of the egg. The hatching chick uses this to breathe. The final stage lasts approximately 20 hours, and takes place in the uterus, or shell gland. This is when the hard outer shell, which gives the egg its shape, is laid
down, along with its colours and markings. Bigger eggs generally have thicker shells. Made of calcium carbonate, the shell protects the embryo. It also donates calcium to it as it grows, so that the shell is thinner by the time the chick hatches. Pores on the surface of the shell allow oxygen to pass to the developing embryo, and waste carbon dioxide and water to diffuse out. In many birds, the shell is coated in a thin, waxy layer, which stops dust and bacteria from entering. No one currently knows exactly how this works, or why the eggs of some birds, notably pigeons and doves, do not have it. The completed egg now travels down to the cloaca, and is laid almost immediately. After approximately an hour, the whole process starts again. Sometimes, however, the timing can go awry, and two yolks can be released from the ovary in quick succession. These may both be encased within the same shell. Double-yolked eggs are believed to occur in one in 1,000 chickens’ eggs.
How many?
The number of eggs in a clutch varies enormously. As a general rule, small birds tend to lay more eggs than larger ones. This is thought to be a reflection of lifespan, with naturally short-lived species, or those more vulnerable to predators, laying more eggs than longer-lived birds. Like most songbirds, the robin usually lays four to six eggs in a clutch, but many seabirds lay only one. The ground-nesting grey partridge may lay up to 24. The number of clutches laid each year also varies. This is related to the parents’ ability to raise their chicks successfully and is often related to food supply. “Blue tits lay a single clutch of eight to 12 eggs between April and June,” says Douglas. “Their young feed mainly on insect larvae, so it makes sense to lay eggs in early summer, when food is most abundant. But other birds, like starlings and chaffinches, have a more varied diet and constant supply of food, and usually have two or more clutches a year.” If a first clutch falls victim to predators, or is otherwise lost, most birds will lay a replacement brood. This is particularly true if it is early in the season. One notable exception to this is the golden eagle, which rarely replaces its single clutch of two, or sometimes three eggs if this is destroyed.
Warming up
Incubation keeps the eggs at a constant temperature for a sustained period, ensuring the chicks develop properly. Although in many species the male takes a turn incubating the eggs, the female usually undertakes most of the work. Its mate’s role is to provide the food. Changes in hormone levels mean feathers drop out of the female’s abdomen, the tissue swells, and blood vessels expand. This creates a warm area, known as a brood patch, where the eggs come into direct contact with the skin. In rare cases like the dotterel, where the male bird alone incubates the eggs, it will also develop a brood patch. Few birds sit on the nest continually. Most will leave for short periods. The adult turns the eggs regularly to ensure an even temperature. Turning also prevents the
contents sticking to the inside of the shell. Almost all songbirds, waders and game birds wait until all their eggs have been laid before incubating them together. This is known as synchronous incubation, and the chicks hatch within hours of each other. This is important for birds like mallards, whose ducklings must be able to leave the nest together. On the other hand, some birds, including owls and falcons, undergo asynchronous incubation. They start to incubate earlier eggs before the last ones are laid. This means that the chicks hatch at different times. As a result, the older ones will be bigger and more dominant than the younger ones. “If food is plentiful, there’s no issue, as all the chicks will be fed,” says Douglas. “But if food is scarce, asynchronous incubation ensures the bigger ones at least will have a chance of survival. They may even eat their siblings.” Incubation periods last from as little as 11 days for the yellow wagtail to six weeks or more for the mute swan. In some species, such as the coot or plover, the chicks are well developed when they hatch. They can leave the nest almost immediately. Able to feed and defend themselves relatively quickly, they require little care from their parents. These are known as precocial birds. Their eggs are particularly nutritious, with large yolks, and tend to be incubated for longer. Altricial birds, such as the house sparrow or wren, have young that are a lot more vulnerable. They may be born with
their eyes closed and with only downy feathers. Their parents must provide warmth and food until they can fly. These birds have a relatively short incubation period, with smaller eggs and yolks.
Shapes and sizes
What is thought of as egg shaped, like the oval hen’s egg, is the most common. But there are others. Birds such as owls lay round eggs. At the other extreme are the long, pear-shaped eggs of the guillemot. Others fall between these poles. Some are gently rounded at one end and pointed at the other, others more elongated and rounded at both. “Shape has a purpose, and there are probably many reasons for the different ones,” says Douglas. “The pointed eggs of waders fit neatly under the bird in a circle, so that there is maximum shell surface in touch with the parent and its warmth. The general ovoid egg shape is easily laid, very strong and can withstand considerable physical force, especially considering the fragility of the shell itself. The shape of egg that is most advantageous depends on the circumstances of breeding and incubation.” Although the theory has been challenged, many believe the pyriform shape of the guillemot egg evolved as it forces the egg to roll in an arc. This stops it from falling off the cliff ledge. The smallest birds’ eggs in the UK are laid by the common firecrest, found in parts of south-east England. At just ½in long, (13.5mm), they are slightly smaller than those of the more widespread goldcrest. The largest eggs belong to the mute swan, approximately 4½in by 3in (112.5 x 73.5mm). “A couple of centuries ago, a mute swan’s egg would have been beaten in size by the sadly now extinct great auk,” notes Douglas. “We have six eggs of this enigmatic species in the NHM collections, each measuring approximately 4¾in by 3in (118 x 78mm).”
Colouring in
Egg collecting was once a popular hobby, precisely because of the wonderful range of colours and markings of birds’ eggs, even within the same species, or from an individual bird. Colour is laid down in the shell gland, and some is an integral part of the shell, rather than being deposited on its surface. Two groups of pigments may be added. Biliverdins create a range of blue-green and violet-grey colours. Protoporphyrins give rise to yellows, reds and browns. Dots, streaks and swirls appear as the egg is
rotated through the shell gland, an action which also smooths its surface. In some eggs, the marks are so fine that they look like writing. For this reason, the yellowhammer is also known as the scribbling lark, or scribe. Highly nutritious, birds’ eggs are eaten by many animals. Those birds that lay in holes in the ground, or deep in crevices tend to have white eggs. They are hidden from the sight of most predators, and there is no need to spend energy colouring the white of the base eggshell. Birds which nest in more easily accessible spaces, such as bushes or hedgerows, tend to have coloured and speckled eggs for better camouflage. Those nesting on open ground, like the stone curlew, have heavily camouflaged eggs, with lots of blotches and streaks. But there are exceptions. “There are a few birds that lay coloured eggs in out-of-sight locations, so camouflage cannot account for all colouring,” says Douglas. “Birds have four photo-receptors, as opposed to the three of humans. They can also see in the UV spectrum. They may well perceive the colours of their eggs differently from us, and may also be able to pick out other markings that we cannot see. This may open up new possibilities as to why eggs are the colours they are. “Studies on the great tit found birds laid more spotted eggs in colder weather, and when breeding in areas where there were a lot of other birds. So environmental conditions seem also to play a part. “As with the shape of eggs, we still have a long way to go before we can fully understand their colours and markings. This is why they are so fascinating. There is so much more to learn.”