On hatching, a cuckoo chick will push the eggs and young of its host out of the nest and pester the duped foster parents until they exhaust themselves feeding it. Nor does the female cuckoo take any interest in the subsequent fate of her abandoned offspring.
Our cuckoo doesn’t visit North America but the Land of the Free has its own avian conman. The brown-headed cowbird, no relation of the cuckoo, dupes other birds into raising her young. She’s more of an ordinary decent criminal however, not wantonly cruel to her victims. This black finch-like bird is the size of a starling. Only males have the brown head.
Flocks, mixed with other species, roam the countryside. According to Matthew Louder of the University of Illinois, an expert on cowbirds, she’s a sophisticated operator.
Money-launderers use a legitimate business to disguise their ill-gotten gains. Cowbirds do likewise. In spring, a female seeks out the nests of other birds. Waiting until the owners are away, she lays one, or more, eggs in each nest. The host’s eggs aren’t harmed; the aliens being passed off as members of the family.
Unlike the cuckoo, which targets a particular species, the cowbird doesn’t specialise; its speckled eggs have been found in the nests of over 220 kinds of bird. Warblers and sparrows are the usual victims but tiny hummingbirds, aggressive hawks and everything in between, may be duped.
Being had by a cowbird isn’t as disastrous as falling victim to a cuckoo; the host parents are just conned into treating each interloper as one of their own.
The owners sometimes notice that an egg is alien. When they do so, they either throw it out or abandon the nest. To insure against such losses, a cowbird lays up to 36 eggs in a season. Generating that many eggs is a full-time occupation; this is not a lazy bird.
A study by the Florida Natural History Museum in the 1990s found cowbirds may become violent when an egg is dumped, wrecking the potential host’s nest and destroying her eggs. This retaliatory Mafia-esque behaviour is no mere temper tantrum — it forces the victims to build a new nest which the cowbird can try to parasitise again.
Louder and colleagues have taken the study of cowbird behaviour a stage further. They constructed special nest-boxes which attract warblers. Many of those taking up residence were duly parasitised by cowbirds. Researchers removed cowbird eggs from some nests and left them in others, tracking the fledging rates of the broods.
A cowbird’s involvement with a nest, they found, doesn’t cease when the eggs are laid; she continues to keep tabs on it, monitoring how her youngsters are progressing.
When laying again, later in the season or the following spring, she returns to the nests from which her young have successfully fledged, in this case those in which the researchers had allowed her eggs to remain. Ones, from which researchers had removed the alien eggs, were avoided.
But cowbirds not only pay attention to nests with their own eggs in them, they seem to keep an eye on nests other cowbirds are using. If warblers were successful in raising any cowbird young, they were marked down for re-parasitising. If they failed, they were avoided.
The results show that cowbirds don’t lay willy-nilly in any nest they chance upon. They adopt a much more sophisticated approach. Theirs is the avian equivalent of an “information society”. Gathering data on nesting performance within seasons and from year to year, they are able to maximise their reproductive success and avoid wasting valuable eggs. For the brown-headed cowbird, knowledge is power. Matthew Louder et al. A generalist brood parasite modifies use of a host in response to reproductive success. The Royal Society Proceedings B 2015.