Parasitic cowbirds thrive with a less ruthless strategy than cuckoos
The secret to cowbird success is to join your nestmates in making noise, then hog the food
America’s brown-headed cowbird and the European cuckoo are the classic parasitic birds, laying their eggs in the nests of other bird species and leaving the chick-rearing to another parent.
But while a cuckoo hatchling thrives by muscling its host’s eggs out of the nest and hogging all the food, a new study by biologists from the University of California, Berkeley, and the University of Cambridge in England shows that cowbird chicks survive with a less ruthless strategy.
A cowbird chick instead joins its nestmates in a chirping chorus that brings in more food than one noisy cowbird chick could demand from its host parents. By eating more than its share, the researchers found, the cowbird chick actually grows faster when sharing the nest and food with two host chicks than it does when all alone in the nest.
“The cowbird alone is incapable of bringing in enough parental resources – basically food – to be able to grow optimally,” said Mark E. Hauber, a Miller Research Fellow in UC Berkeley’s Museum of Vertebrate Zoology. “When it has nestmates, the whole nest brings in more parental care, because there is more begging altogether, and so the parents attend the nest more. But the cowbird monopolizes the feeding attempts by the parents. In these experiments, instead of getting 33 percent of the feedings that a brood of two host chicks and one cowbird chick gets, the cowbird actually got over 50 percent of the feeding. So, it grew better than when it was living alone.”
Hauber, along with University of Cambridge biologists Rebecca M. Kilner and Joah R. Madden, published the findings in the August 6 issue of Science. Hauber will take a faculty position at the University of Auckland in New Zealand at the end of this year.
Though reviled by bird lovers, cowbirds have proved spectacularly successful, having expanded their range from the Midwest to the entire United States over the past 400 years. This is partly due to their preference for cattle and short grass, both of which have increased with deforestation and the rise of suburbs. Their parasitic ways have drawn the attention of biologists, who have published more than 3,000 papers on their behavior. Hauber is one of them, having studied cowbird parasitism of Eastern phoebe nests for some six years while obtaining his Ph.D. in biology from Cornell University in upstate New York, using the facilities of the Cornell Lab of Ornithology in Tompkins County.
Kilner and Madden, who have studied and written about cuckoo brood parasitism, approached Hauber last year with a plan to experimentally manipulate cowbird nests to see if they could determine why cuckoos and cowbirds use different strategies. Kilner had proposed that, unlike cuckoos, cowbirds suffer when alone in the nest without the begging assistance of nestmates. While cuckoos are far too big to share the nest and food with any other chicks, Kilner thought that her hypothesis could be tested in the cowbird, which is about the same size as its hosts – some 100 different insect-eating passerines or songbirds.
“Genetically, you don’t benefit from growing up with a nestmate who is not your own species, let alone your brother or sister,” Hauber said. “The question is, why don’t cowbirds kill their nestmates?”
He placed ads in local newspapers in search of backyard nests of the Eastern phoebe (Sayornis phoebe), a migrant flycatcher that is a common host of the brown-headed cowbird (Molothrus ater). From nearly 100 responses, he and his colleagues found 81 phoebe nests with eggs.
Kilner and Madden switched eggs among the nests to test various scenarios. They added a single cowbird egg to 20 of the nests, and after the chick hatched, they removed the host eggs from 10 of those. In the other 10 nests, after the cowbird hatched they placed two phoebe hatchlings of the same age or a day older. (Phoebe eggs hatch about five days later than cowbird chicks, after an incubation of 15 days versus 10 for the cowbird.) The remainder of the nests were used as sources of eggs for the experimental nests, as controls, or contained cowbird eggs that didn’t hatch.
They then monitored the nests with video cameras to determine how often each chick ate, and daily for nine days weighed each chick and measured its tibia length. At UC Berkeley, Hauber conducted DNA analyses of the chicks to make sure that male and female chicks were equally represented in the study.
The researchers found that by day eight, cowbird chicks raised with two phoebe chicks were, on average, 14 percent heavier than cowbird chicks raised alone. Also, phoebe parents brought food to nests with three hatchlings about four times each hour, versus 1.5 times per hour for a lone cowbird chick.
This strategy of sharing the nest to gain more resources appears to be successful generally among all the 100 or so species of birds parasitized by cowbirds, Hauber said.
“We found in a comparative analysis of 18 different host species that, if you look at the growth of the cowbird chick, it does best in hosts who have about 1.8, or approximately two, nestmates growing up together with the cowbird chick,” he said.
Kilner had previously shown that the survival of cowbird chicks was greatest in nests of those host species that have about two nestmates surviving alongside the cowbird.
The study notes that among the cowbird’s historical hosts, you typically find two eggs with a single cowbird egg. The phoebe is a new host, with an average clutch size of five eggs.
“This was a very experimental study, combining some specific predictions from the literature with a purely experimental approach,” Hauber said. “All the work was done in 45 days in April and May of 2003. It’s an $80 project if you don’t count living expenses, which is pretty good for science. All it took was a good idea, a couple of video cameras and lots of videotape.”
Hauber and his colleagues are pursuing other behavioral and evolutionary questions, such as what happens when two cowbird eggs are laid in the same nest, either by the same mother or by different mothers, and whether cowbird chicks ever shoulder host eggs out of the nest to obtain the optimal two nestmates. He also studies cowbird parasitism at Point Reyes on the coast just north of San Francisco and at Mono Lake in eastern California.
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