DON CORLEONE, eat your heart out. Around the world, parasitic birds such as cuckoos and cowbirds are displaying the same combination of guile, ruthlessness and brutality as any self-respecting Godfather in order to 鈥減ersuade鈥� other birds to accept their eggs and raise their chicks.
In the 1990s, Anders Pape M酶ller of the University of Copenhagen in Denmark and colleagues discovered that great spotted cuckoos (Clamator glandarius) run a mafia-style protection racket against magpies living in Andalusia, Spain. If a magpie rejects a cuckoo egg laid in its nest, the cuckoo promptly returns to destroy the magpie鈥檚 own eggs or kill its chicks (New 杏吧原创, 2 December 1995, p 21).
Not to be outdone, American cowbirds, which are not related to cuckoos, employ an even more forceful tactic against warblers. 鈥淭he cowbird has much more sophisticated predatory behaviours than we thought,鈥� says Jeff Hoover of the Illinois Natural History Survey in Champaign, who has been monitoring brown-headed cowbirds (Molothrus ater) for four years in swamps around the Cache river in southern Illinois.
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Wild prothonotary warblers (Protonotaria citrea) are compliant with cowbirds and are not known to recognise or eject cowbird eggs. But Hoover and Scott Robinson wanted to find out what would happen if they did. To mimic this, they provided artificial nests for 180 pairs of warblers, waited for the cowbirds to cuckold them, then selectively removed the cowbird eggs.
The cowbirds soon retaliated, returning to the nest to eat or destroy the remaining warbler eggs. What鈥檚 more, warblers that had laid their eggs too early for the cowbirds to cuckold them didn鈥檛 escape retribution either. Cowbirds would routinely eat or trash these more developed eggs to force the warblers to lay again. They would then spy on the warbler parents, find out where they were nesting anew and sneak in to lay an egg at exactly the right time.
This process, which the researchers call 鈥渇arming鈥�, enables the cowbirds to effectively manufacture a new breeding opportunity for themselves. On average, 20 per cent of warbler nests were farmed in this way, and the cowbirds parasitised 85 per cent of the rebuilt nests. Hoover says the overall effect is that the cowbirds bully the warblers into 鈥渁n evolutionary state of acceptance鈥�.
What鈥檚 more, it鈥檚 better for the warblers if they comply. On average, they raise three of their own chicks when they also support a cowbird chick, but raise just one of their own if a cowbird egg has been rejected (Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.0609710104).
鈥淭his has been a controversial area, that cuckoos and cowbirds might have protection rackets to get their own eggs protected,鈥� says Becky Kilner, a researcher on cuckoos at the University of Cambridge. She says the study provides the best evidence yet that parasites destroy hosts鈥� nests, and that hosts ultimately benefit by complying. Hoover is now tagging and filming wild cowbirds to see in more detail how they act in a natural environment.
