Warming temperatures in parts of Europe may be knocking the local cuckoos out of sync with some of their usual targets for illicit egg laying.
Cuckoos swoop in to the nests of other bird species and lay eggs. After hatching, a cuckoo chick kills its foster siblings and hogs the food for itself. In parts of Europe that have warmed since 1990, the common cuckoo, Cuculus canorus, appears to be shifting its targets away from bird species that either don’t migrate or that travel only a relatively short distance each year, says Anders Pape Møller, a researcher for the French national research organization CNRS who is based at the University of Paris South in Orsay. In these zones, an increasing proportion of cuckoo eggs end up in nests of long-distance migrants, Møller and his colleagues report in a paper posted online the week of September 13 in Proceedings of the Royal Society B.
What’s driving the shift, Møller says, is that climate change affects migration patterns differently depending on their scope. Cuckoos themselves migrate all the way from Europe to sub-Saharan Africa for the winter. By the time they return in spring, early warming has already pushed the year-round residents and short-distance migrants too far along in their nesting to be good cuckoo targets.
“This study clearly illustrates how climate change not only alters the timing of breeding in many species but also disrupts the exquisitely timed interactions between closely interacting species,” says Naomi Langmore, an evolutionary ecologist at the Australian National University in Canberra.
Figuring out the interplay of climate, cuckoos and their target species took a lot of data. Møller and his colleagues scoured museum and private collections of bird eggs, contacted various people who track birds and mined the scientific literature to assemble a database of 32,843 records going back to the 1850s of a common cuckoo depositing an egg in another bird’s nest. Fortunately, Møller says, “There was this mania for collecting cuckoo eggs.”
To analyze trends in egg laying, the researchers first looked at spring temperatures during the laying season by country from 1958 through 1989 and then at temperatures since 1990. Due to spotty temperature changes across Europe, springs have heated up more than 2 degrees Celsius in parts of Scandinavia since 1990 but have warmed little or not at all in parts of Eastern Europe.
In countries where spring temperatures have risen the most, cuckoos have preyed less since 1990 on 73 species of short-distance migrants,the researchers say. In contrast, the opportunistic birds have increased their attacks on 49 species that migrate longer distances. That’s less cuckoo persecution for birds such as European robins, dunnocks and meadow pipits, but more trouble for reed warblers.
This shift isn’t just a matter of short-distance migrant populations shrinking over the years, the researchers say. If anything, the long-distance migrants as a group have thinned out.
The trend is so strong, Møller says, that he wouldn’t be surprised if cuckoos that specialize in short-distance migrant species face extinction in a matter of decades in locations where warming is driving them out of sync with their hosts. The common cuckoos in Europe include about 20 specialist lineages, called host races, that match a particular target species in egg color and pattern.
In North America, the celebrity egg-sneaks are cowbirds. “I doubt that a similar process is going on in North America because the cowbird is itself a resident species in parts of its range and a short-distance migrant in the rest of its range,” says Stephen Rothstein of the University of California, Santa Barbara, who has studied cowbirds for decades.
Langmore and her colleagues are currently exploring what might be happening to cuckoo species and their hosts in Australia. “Mistiming is a worldwide trend,” she says.