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
“Once hatched, the chick faces multiple threats, including cold, starvation and predators”
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. 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 cliff-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 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 sufficient food for all chicks to be reared successfully.