Brood Parasites in Birds

The Common Cuckoo is probably the most widely recognised brood parasite in the bird world. Its familiar call may have been heard across the country, but its numbers have fallen dramatically in recent times. It lives in the open fields across Europe and Asia and migrates to Africa in the winter.

More than 100 species have been recorded to be hosts to Cuckoo eggs. The process is quite simple; the female cuckoo flies into the nest of a bird such as the Meadow Pipit, Dunnock or Pied Wagtail and within 10 seconds, lays an egg then flies off. One female may visit in the region of 50 nests during a single breeding season, leaving behind her egg-shaped gifts.

The cuckoo chick then hatches before the chicks of the host species and promptly pushes the other eggs out of the nest. It does this because it will be significantly larger than the chicks of the host bird and needs to make sure it gets all of the food the host parents can provide. For example, at 14 days old, the cuckoo chick is three times the size of an adult Reed Warbler. Yet for some reason, these hosts continue to feed a chick which visually cannot be their own until it is old enough to fledge.

Killer parasites

Nor are cuckoos the only brood parasites who kill the host chicks. The Honeyguide is an African bird who lives on the wax produced in bee colonies as well as waxworms, the larvae of the waxmoth. They are known to lead humans to bee colonies as once a hive is open, they will keep returning to fee on it.

Honeyguides lay their eggs in the nests of different birds depending on the exact species, from barbets and woodpeckers to white-eyes and warblers. They eject the host eggs from the nest or use their thin, hooked beak to puncture the eggs and kill the chicks.

Gentler parasites

Not all species of brood parasites are as destructive to the host nest as the cuckoo. In the world of bird-keeping, there are a group of birds called Whydahs who parasite smaller waxbill birds. For example, the Pin-tailed Whydah is an impressive looking little bird, 12cm in length with a further 20cm worth of tail. This black and white bird and his brown mate lay their eggs in the nest of the Common Waxbill. However, unlike the cuckoo, the waxbill eggs are untouched, and the little finches will raise the Whydah chicks alongside their own.

Similarly, the related indigobirds also use small finches who live in the same locations to raise their chicks. The Village Indigobird lays its eggs in the nests of Red Billed Firefinches while the Quailfinch Indigobird lays, unsurprisingly, in the nest of Quailfinches. These species do not destroy the host chicks and the two species are raised together.

Nest-sharing

Another type of brood parasitism occurs amongst duck species such as the Common Goldeneye. The Goldeneye is a medium-sized duck who lives in the forests of Canada, the US, Scandinavia and Russia. The parasitism in this species means that females will often lay eggs in other goldeneye nests and leave them to be raised by their adoptive family.

Similarly, the Black headed Duck from north Chile, Paraguay and northern Argentina is known as the Cuckoo Duck for its habit of laying its eggs in other bird’s nests. It will lay in other duck species such as the Rosy-billed Pochard, nests of coots, even gulls and birds of prey. However, there is no harm to the host nest, the duck eggs are incubated for around 21 days then the chicks hatch, are independent within a few hours and leave the nest to fend for themselves.

Playing by the Numbers

The Brown headed Cowbird, to the other extreme, seems to be happy to lay her eggs in any nest she comes across. This bird is smaller than most of its family and has a finch-like beak and head. The cowbird lays the majority its eggs in the nest of small passerines such as the House Finch but has been known to lay in the nests of hummingbirds right up to raptors. In fact, there are documented cases of 220 host species being used by this bird. The cowbird is then fed at the expense of the host chicks but not always with success, depending on the natural diet of the host bird. In the instance of the House Finch, this is an unsuccessful pairing because the finch eats a seed based diet but the cowbird needs live food.

Even if the cowbird chicks are fed by the host birds, there can still be complications arising from the pairing. Studies have shown that American Redstart nests with cowbirds in them suffer higher rates of predation as the cowbirds make more noise begging than the redstart chicks would do.

The host species do fight back on some occasions with a variety of responses, but this too can have a negative effect. In what is called ‘mafia behaviour’ the cowbirds return to the nest and ransack it to force the birds to make a new one, where they lay their eggs again. Or the cowbirds destroy the eggs of the host.

Conclusion

Like all life, some species of birds have developed what they think of as a sure-fire strategy to guarantee their genes are continued in the next generation. Some, unfortunately, have developed a process which ends in the destruction of the host eggs or chicks or the running of the host nest if things don’t go well. Others have developed a symbiotic relationship where the chicks are raised alongside the hosts own with no adverse effects. Finally, some species use the host nest as a kind of incubator where the chicks develop then go about their own way in the world. Not all strategies work all of the time and to us, may seem harsh or lazy, but evolution has taught these birds that this is the best method for them, and they keep working at it.