Spring chicks
In hedgerows and on trees and cliff ledges across the country, the eggs of wild birds are hatching and new chicks are emerging – trembling, hungry and vulnerable. How do baby birds survive?
Tim Birkhead explores an annual spring miracle
The image of a fluffy little chick emerging from a hen’s egg after popping open the ‘lid’ is a sugary exaggeration of what really happens.The truth is no less remarkable: simply escaping the egg is the first potentially lethal challenge of many in the young life of birds.
A bird’s eggshell has to be strong enough to support the weight of the incubating parent, but not so robust that the chick cannot get out. How is this tricky compromise achieved? The answer is through a clever evolutionary trick in which the calcium-carbonate crystals that make up the eggshell are arranged like the wedge-shaped bricks of an arched stone bridge, such that they can be safely pushed down upon (by the incubating parent), but when pushed from the inside, are less strong, allowing the chick to escape.
“Once hatched, the chick faces multiple threats, including cold, starvation and predators”
The eggshells of small songbirds — like our robin — are somewhat leathery, and the chick has to cut its way out, using its egg tooth, a conspicuous hardened tip of the bill. This sounds easier than it probably is. Inside the shell, the chick has to rotate several times, all the while chipping away at the shell. Then, by pushing with its feet and neck, it can lift the top, the big end of the egg, and push itself free from the shell.
Once out of the egg, the chick faces multiple threats, including cold, starvation and predators, to name but three. Survival strategies vary wildly between species. Just hours after hatching, the chicks of some bird species can run about, feed themselves and even fly, while others hatch blind and helpless and are dependent on the parents for weeks or months.
LIFE AFTER HATCHING
Some small songbirds go from hatching to fledging in as little as 10 days, followed by a week or two of being fed by one or both parents. At the other end of the spectrum, fulmar chicks spend two months on the nest being fed by both parents, and after they fledge they are totally independent, relying on their vast store of body fat for the first days or weeks. Guillemot chicks leave their cli -ledge breeding site at about three weeks of age, unable to fly, and are taken to sea and fed by their fathers until they become independent after about three months.
The egg tooth of a hatching domestic chicken; 
newly hatched robins waiting for food; 
a fluffy, ‘precocial’ (ready to go) pheasant chick
The robin chick emerges, blind, and is covered with just a few wisps of down. It and its nest-mates are totally reliant on the parent birds to brood it and keep it warm and feed it during the 14 days it takes to fledge. At the other extreme, the pheasant chick hatches with a dense covering of insulating down, strong legs, feathered wings and its eyes open.
Once its down covering has dried, assisted by brooding by its mother, it is able to run around and feed itself and, after a few days, it can fly.
Robin chicks rely on the safety of the carefully hidden nest their parents have constructed to protect them from predators such as weasels, stoats, magpies and cats. They keep their food-begging calls to a minimum to avoid detection. The recently hatched pheasant chick, in contrast, spends most of its time on the ground, reliant on a combination of its beautifully camouflaged plumage and parental vigilance to avoid being seen and caught be predators.
BOLDLY DRESSED
The chicks of some birds are anything but camouflaged and the recently hatched chicks of coots and moorhens have startlingly coloured heads – bright skin and bright down feathers. They can afford not to be camouflaged because as soon as they hatch, they leave the nest and take to the water, where they are relatively safe from predators. Why they are coloured in this way is still a mystery.
Within a brood of coot chicks, the brightness of their head colours varies, possibly reflecting their quality. The parent birds preferentially feed the brightest since they are perceived as the ‘best’ and because there is rarely suffcient food for all chicks to be reared successfully.
BABY NAME GAME
Young birds have been given a variety of names, including some that relate to their age: hatchling (meaning recently hatched), nestling (within but not ready to leave the nest), fledgling (ready to leave or fly from the nest, where fledge means ready to fly). The suffix ‘-ling’, which dates to the 1300 or 1400s, means ‘belonging to’ and has been used for particular birds: a gosling being something that belongs to a goose, a duckling belongs to a duck. In principle, we could have any ‘ling’: pheasantling, parrotling, swallowling, bustardling… but what would be the point? The term puffing to denote a young puffinn is a recent invention and has been adopted, it seems, because both the name and bird are cute, but a young puffin (pictured) does not belong to a puffin, it belongs to a puffin, so strictly speaking this should be a puffinnling. Other terms for the young of familiar birds include cygnet (a diminutive of cygnus, meaning a swan), a term dating to the 1400s, when mute swans became the property of the British crown. Eaglet and owlet are medieval names for a young eagle or owls, respectively. An eyas is a nestling falcon (from nyas, meaning nest), and squab — an ancient term of unclear origin — refers to a young pigeon.
CHICK TALES
1. GREYLAG GOOSE
Like other waterfowl, the greylag goose produces ‘precocial’ chicks, ready to ‘go’ soon after hatching, but also dependent on their parents, whom they follow assiduously. They are preprogrammed to follow the first thing they see on hatching (usually a parent), a process called ‘imprinting’.
2. BLUE TIT
This common garden visitor takes advantage of nest boxes we provide and, fitted with a camera, we can watch as the eggs are laid, incubated and hatch. The blue tit lays the largest clutch – up to 16 eggs – of any bird.
The blind and helpless chicks are brooded for the first few days and kept warm by the well-insulated nest. Fed prodigious numbers of caterpillars, the chicks grow rapidly and fledge after three weeks.
“The chicks ‘peep’ to each other from within the eggshell even before the first hole is made”
3. MALLARD DUCK
Young mallards leave the nest within hours of hatching. All birds lay only one egg each day, so to ensure all her chicks hatch and leave the nest together, the mallard starts to incubate only after the last egg is laid. The chicks ‘peeping’ –e ectively talking – to each other from within the eggshell even before the first hole in the shell is made further enhance synchrony of hatching.
4. MUTE SWAN
Among the heaviest of all flying birds, the mute swan weighs about nine kilos and lays a clutch of four to 10 eggs, each of which weighs about 340 grams. On hatching, the cygnet – the archetypal ‘ugly duckling’ – is covered in grey down, but with its eyes open and ready to take to the water. The cygnets are protected by both parents for three or four months and attain the white plumage of their parents only after a year.
5. GOLDEN EAGLE
The Highlands’ most celebrated bird of prey lays two eggs but often only one chick fledges. If food is abundant, both chicks are reared, but if food is short, the weaker chick is allowed to starve, or in some cases is killed by their older sibling. This seemingly harsh tactic ensures that, overall, an eagle will rear more o spring during its lifetime than they would otherwise.
6. KITTIWAKE
Our only cli -nesting gull and its chicks have evolved to sit tight in the nest and not run around like the chicks of ground-nesting gulls. The kittiwake’s one or two (sometimes three) chicks hatch covered in white down and with their eyes open, but they are utterly dependent on the parents to both brood and feed them in the nest. They fledge after about 35 days.
7. GOLDCREST
Regarded as Britain’s smallest bird, at around five grams, the goldcrest lays a large clutch of 10 to 12 eggs, despite its size. Birds use an area of featherless skin on the belly called the brood patch to warm eggs. The goldcrest’s brood patch is not large enough to make contact with so many eggs at once, so the female gets round this by pumping blood into her legs and uses the heat in them to warm her eggs. At hatching, the tiny, naked, helpless chicks weigh about 0.6 grams, less than the weight of a standard paperclip.
8. GUILLEMOT
This chocolate-brown and white seabird lays a single brightly coloured egg, the pointed shape of which helps to keep it stable, and less likely to roll, on the bare rock ledge on which it is laid. Incubation takes 32 days, and the chick requires two or three days to break out of the top of the thick shell. By the time it hatches, the chick and parents have learned to recognise each other’s voices.
9. MANX SHEARWATER
Chicks ‘imprint’ on their parents as a guide to who to breed with when they later mature, but young birds also ‘imprint’ on their surroundings as a guide for where to breed. Many Manx shearwaters return to their natal colony – their place of birth – to breed, and learn where that is by taking a fix from the stars as they sit outside their burrow in the dark before fledging.
10. CUCKOO
It is well known that the cuckoo will lay and abandon its egg in the nest of another species. Once hatched, the cuckoo chick will oust its nest mates – the o spring of its foster parents – to avoid sharing food.
The young cuckoo typically hatches first, because its egg requires a shorter period of incubation. Despite being blind and naked, the hatchling cuckoo has very strong legs and a small hollow on its back into which it manoeuvres the host’s egg or chick and then, pushing with its legs, flips them over the side of the nest.
Tim Birkhead was scientific adviser to the BBC’s Wonder of Song (iPlayer), and is emeritus professor of zoology at the University of Sheffield. His research has focused on promiscuity in birds, and on birds’ eggs. His latest book is Birds and Us (Penguin, £25).