Why Does a Small Bird Feed a Chick Twice Its Size? An Arms Race It Is Losing

The intruder hatches earlier than the host eggs, which is achieved partly by the parent laying late in the host's cycle and partly by the embryo developing unusually fast, having begun before the egg was even laid. Within hours of hatching, and while still blind and naked, it manoeuvres each remaining egg or chick onto a hollow in its back and heaves it over the rim of the nest, repeating until it is alone. It then solicits food with a call that acoustically resembles a whole brood rather than one chick, and with a gape that presents an exaggerated stimulus, both of which drive the hosts to feed it at a rate far beyond what their own brood would receive.

Several adaptations make the deception work:

• •Egg mimicry, with different lineages laying eggs matching different host species
• •Very fast laying, taking seconds rather than the minutes a normal laying takes
• •Removal of one host egg, keeping the count unchanged
• •Thick eggshells that survive the laying and resist being punctured
• •Early hatching, so the intruder is established before competitors
• •Begging signals tuned to exceed what host chicks produce

Hosts are not passive and the situation is better described as a contest in progress. Many host species do reject foreign eggs, and rejection rates vary enormously between populations, being high where parasitism is common and long established and low where it is recent or rare, which is exactly what an arms race predicts. Rejection carries costs, since a host that becomes too discriminating risks ejecting its own eggs by mistake, and the calculation depends on how likely parasitism is. Once an egg has hatched, rejection becomes far rarer, and the leading explanation is that the chick's signals exploit responses the host cannot easily override without endangering its own young in other circumstances.

The European cuckoo has declined sharply across much of its range, and the reasons illustrate how a parasitic species is exposed to problems from several directions at once. Its hosts have declined, reducing the nests available. The caterpillars it feeds on have declined with changes in land management. Timing has shifted, since the birds migrate on a schedule set by conditions in Africa while their hosts now breed earlier in response to warming in Europe, so the parasite can arrive after the host's laying period has begun or passed. Its wintering grounds in central Africa have been degraded. Satellite tracking of individual birds since 2011 has mapped the migration in detail and identified where losses concentrate, which is on the return crossing of the Sahara.

This strategy has evolved independently at least seven times among birds and appears elsewhere too. Cowbirds in the Americas parasitise a very wide range of hosts and do not eject the host brood, instead outcompeting it. Honeyguides in Africa hatch with hooked bill tips used to kill the host chicks directly. Some ducks lay in the nests of other ducks, which is a milder version within a species. Several fish and insects do comparable things, including a catfish whose young are brooded in the mouths of cichlids and eat the cichlid young. What the cases share is that raising offspring is expensive, so exploiting somebody else's investment is a route that natural selection finds repeatedly.

The intruder hatches early, heaves the remaining eggs and chicks over the nest rim while still blind, and then begs with a call resembling a whole brood. Egg mimicry, seconds-long laying and thick shells get it there. Hosts reject foreign eggs at rates that track how long parasitism has been established locally, which is what an arms race predicts, and the strategy has evolved independently at least seven times.