The warming climate appears to be encouraging a cross-continental exchange of brood parasites near the Bering Sea.
Reports of Common and Oriental Cuckoos — species that are difficult to differentiate from each other but are unlike any North American birds — have been on the rise in Alaska for the last few decades. This past June, for example, a Common Cuckoo was found near Sitka, in southeastern Alaska, more than 1,700 miles east of the western Aleutians, where most sightings occur.
“Considering how sparse the observational coverage of Alaska is, these records suggest that Common Cuckoos could already be breeding in North America and using a novel migration route,” writes Vladimir Dinets, a psychology professor at the University of Tennessee Knoxville, in the September issue of the Journal of Field Ornithology.
Similarly, says Dinets, Brown-headed Cowbirds appear to be crossing the Bering Strait into Russia, posing a threat to native Asian birds.
Cuckoos and cowbirds don’t make their own nests. Instead, they lay eggs in the nests of other birds. Species that haven’t evolved with parasitic birds cannot defend their nests against the intruders and often suffer population declines when they arrive.
Dinets suggests that the arrival of the Eurasian cuckoos may be bad news for native North American birds. He and his colleagues added fake cuckoo-like eggs to American Robin nests and fake cowbird-like eggs to nests of Common Redstart, a widespread Eurasian species. The robins accepted most cuckoo eggs, while only about half of the redstarts accepted cowbird eggs, suggesting that they have better defenses against brood parasites.
American Robins’ natural defenses against cowbird eggs are likely to fail against invasive cuckoos, Dinets writes, because cuckoos, unlike cowbirds, can mimic the egg color of their hosts. He expects other native species will fare just as poorly against the Eurasian invaders.
Read the paper
Vladimir Dinets, Peter Samaš, Rebecca Croston, Tomáš Grim, and Mark E. Hauber. Predicting the responses of native birds to transoceanic invasions by avian brood parasites. Journal of Field Ornithology, Vol. 86, No. 3 (2015). PDF.
A version of this article was published in the December 2015 issue of BirdWatching.
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