Brood parasitism of various sorts occurs in a wide range of animals including fish, insects and of course birds. Birds have been studied the most because people and birds perceive their worlds largely through sight and sound, so we can detect much of what’s important to birds. Insects and many other types of animals rely heavily on olfaction or chemical perception, something humans are not very good at. Our ability to tune into so much of what birds perceive explains why birds are such frequent subjects in biological studies, especially those involving ecology, evolution and behavior.

Birds show both facultative and obligate brood parasitism. In facultative parasitism a species either raises its own young or parasitizes the nest of another bird, often another member of their species. Parasitism of members of the parasite’s own species is called conspecific parasitism and can .be hard to detect without molecular markers. Obligate brood parasites are species that never raise their own young and always parasitize other species.

DNA studies indicate that obligate brood parasitism evolved independently 7 times in bird: in cowbirds with 5 species in North and South America, in honeyguides with 15 species in Africa and Asia, in 20 finch species in Africa, in one duck species in South America and separately 3 times in cuckoos. The cuckoos appear to have evolved brood parasitism twice in the Old World and once in in the New World. The two groups of Old World cuckoos are the typical parasitic cuckoos like the common cuckoo, a group with about 57 species and a much smaller group of 4 species. All species in the larger group push host eggs and young out of the nest while this ejection behavior is absent in the smaller group so that host young are often reared along with the cuckoo nestlings. The New World parasitic cuckoos consist of 3 uncommon species and there is only a limited amount of knowledge about them but one remarkable fact is that they have independently evolved the nestmate stabbing behavior that honeyguides have.

One of the most frequently asked questions about brood parasitism is how did it evolve? We don’t know precisely what led to about 1% of all birds species being obligate parasites. But we know that parasitic birds evolved from bird lineages that once had parental care because parasitic birds have related species with parental care.

You might think brood parasitism is easy, just lay your egg in someone else’s nest. But to be successful, a brood parasite must time its egg laying to occur during the host’s egg laying period, which is only a small fraction of the overall nesting cycle. If a hopeful parasite lays its egg too early, the host may be toss it out of the nest or cover it with nesting material if the host is still building its nest. If the parasite lays its egg after the host has finished laying its own eggs and is now incubating, the parasitic egg may not receive enough incubation to hatch. So, a potential parasitic bird must time its laying properly by predicting when the host will be laying eggs. Laying errors are likely at the beginning of a species’ evolution towards parasitism. Cowbirds evolved parasitism about 4-5 mil years ago and still make timing errors about 25% of the time when they lay eggs. So proper timing is a real challenge for a brood parasite.

One thing some brood parasites have in common is that they have unusual means of foraging. The most striking are the honeyguides, which got their name because they actually guide people to natural beehives. The honeyguides cannot break into to beehives on their own. But when they lead people to beehives, the people break into the hive to acquire honey. The surprising thing is what the honey guides feed on. It’s not honey but wax! Waxes are fairly indigestible, but honey guides have an intestinal flora that allows them to actually break down the wax and get nutrition from it. The unusual foraging that cuckoos show is that many species commonly feed on caterpillars with formidable defensive bristles. But cuckoos systematically bash bristly caterpillars on something and break off the bristles. Cowbirds are unusual because they follow livestock closely and feed on insects flushed by the livestock. Cattle and horses were not present in North America or South America until Europeans brought them there. Early explorers in North America remarked on how closely cowbirds followed bison. There were no bison in South America but there were lots of large mammals for cowbirds to associate with before the widespread extinctions that occurred at the end of the Pleistocene. Interestingly, it is likely that cowbirds once associated with proboscideans, elephants and their relatives, which were still present as recently as 15-20,000 years ago. So, it’s possible that unusual foraging methods pre-disposed some bird lineages to evolve brood parasitism because they interfered with the birds raising their own young (although that is pure speculation!).

However it evolved, brood parasitism shows a range of traits that parasites have developed as they have become better parasites. Besides, proper timing of their eggs another adaptation is a very short incubation period which allows the parasite to hatch before the host young and either kill them by pushing them out of the nest or outcompeting them because it is larger. Yet another adaptation is an unusually thick eggshell which keeps a parasitic egg from breaking if it happens to hit some of the host eggs. Thick eggshells may also benefit parasites by cracking some of the host eggs, which keeps them from hatching and reduces the amount of competition the parasitic nestling faces. Then of course there are a range of adaptations that parasites have evolved to neutralize defenses that hosts evolved such as egg mimicry to counter the egg recognition that some hosts possess.

We will probably never know for sure why brood parasitism evolved in certain bird groups and not others, but one thing that is certain is that avian parasite-host interactions provide an incredibly fertile area of research for evolutionary biologists.


The bird sounds featured in the animation were provided by the Macaulay Library at the Cornell Lab of Ornithology.

Credits for Sound Recordists:
Common Name - Scientific Name - Recordist

Eastern Phoebe - Sayornis phoebe - Ronan Pangie
Gray Catbird - Dumetella carolinensis - deborah grimes
SuperbFairywren - Malurus cyaneus - Steve McIntosh
Brown-headed Cowbird - Molothrus ater - Glen Chapman
Little Bee-eater - Merops pusillus - Peter Boesman
Little Bee-eater - Merops pusillus - Peter Boesman
Black-headed Duck - Heteronetta atricapilla - Bernabe Lopez-Lanus
Greater Honeyguide - Indicator indicator - Jennifer F. M. Horne
Lesser Honeyguide - Indicator minor - Jennifer F. M. Horne

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