Birds and Surrogate Parents: Female Cowbirds Check Back

Cowbirds have historically been viewed as something like the world's worst avian parents, it seems: They lay their eggs in other birds' nests and let those other birds raise their young, for crying out loud. But researchers recently discovered that cowbirds return to take a look-see. This way the female cowbirds can assess which host nests were most successful, and avoid laying future eggs in those that failed.

"Cowbirds may be paying attention not only to their own reproductive success, but to other cowbirds' as well," Matthew Louder, study leader and Ph.D. student at the University of Illinois, said in a news release. "No one's ever suggested before that cowbirds or even other brood parasites pay attention to their own reproductive success."

The researchers noted that this act of brood parasitism, where one species leaves their young for another to raise, is not practiced by many. The cuckoo is one other bird that does this. However, such forced-surrogate nesting doesn't always work: Some of the chosen foster parents notice foreign eggs, and roll them out of their nests.   

"There were a lot of implications of that earlier work, and one of them was that cowbird females aren't abandoning their eggs in another species' nest; they're paying attention, to a certain point," Jeff Hoover, a Illinois Natural History Survey (INHS) avian ecologist, explained in a release.  "And so we wondered how long they continued to pay attention."

The researchers modified experimental nesting to exclude predators. They tested to see if the cowbirds decided to lay their eggs randomly or based on previous performances, meaning whether or not their young survived in that home. They discovered that the cowbirds chose to lay their eggs in prothonotary warblers' nests. From this the researchers were able to monitor the cowbird and the warbler nestlings.

"We try to discriminate between host success and cowbird success," Wendy Schelsky, the INHS biological surveys coordinator, said in the release. "The cowbirds might be selecting nests where young cowbirds succeed, but they might also prefer nests where the warblers are doing well, and not pay attention to cowbird survival."

Overall, the researchers found that the cowbirds laid their eggs in nests that had successfully hosted their young previously, and were less likely to return to nests that hadn't, according to the release.

"Our results mean that somebody's paying attention, and it makes the most sense that the female that's laying the eggs would be paying attention to her own reproductive success," Louder said in the release. "We think that other females are also paying attention."

Their findings were recently published in the Proceedings of the Royal Society B.