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Avian Brood Parasites Are About to Have Their Adaptability Tested

The future of parasitic birds, which lay their eggs in other nests, is totally dependent on their hosts' ability to adjust to climate change.

Imagine it’s spring in the year 2048. A Common Cuckoo returns from Africa to her breeding grounds in Europe, just as her ancestors have for thousands of years—except that lately the species has arrived earlier.

When the cuckoo, which depends on other birds to rear her young, finds a mate and then searches for a “host” nest to sneak her eggs into, she is out of luck: The potential foster parents arrived even earlier and it’s too late for her to invade their nest. Climate change has rescheduled both the host’s and the parasite’s breeding dates, and the two are no longer in sync.

The cuckoo saves time and energy through its unconventional parenting method of forcing chick-rearing duties onto another bird. But this behavior, known as brood parasitism, also makes it vulnerable to environmental upheavals that affect its hosts. If a host species changes its breeding behavior—arrives at nesting sites earlier, for example, or begins nesting in a new location—the parasite must keep up. If it doesn’t, there may be no place to re-home its offspring, and without any parenting skills, it may fail to reproduce.

While avian parasites and hosts have endured climate swings before, never have they faced such rapid warming. Scientists’ early insights into how they may fare as climate change progresses suggest that many, though not all, of these species are about to have their adaptability tested. Some hosts are moving to new areas or breeding sooner as global temperatures warm, according to recent research, and their parasitic dependents are starting to show signs that they might struggle to keep up.

Research published last year in Nature Communications suggests that some brood parasites may fare better than others. Generalists that hedge their bets by dropping eggs into the nests of multiple host species may have an advantage—Brown-headed Cowbirds, for instance, can colonize the nests of hundreds of species. In an unstable climate, distributing eggs among a variety of hosts may ensure that if one set of adoptive parents fails to breed due to poor conditions, eggs raised by a more resilient host may still survive. Specialists, such as the Channel-billed Cuckoo, tightly linked to just a few host species may struggle to adapt. They have put all their eggs in one basket, literally, so their future relies on their hosts’ success adjusting to climate change.

In the study, by researchers at the University of Illinois at Urbana-Champaign, evolutionary biologist Nick Antonson and three other scientists examined environmental conditions for 84 species of avian brood parasites as well as for their hosts. The analysis found that parasites that live in areas where temperatures fluctuate widely tend to use more host species than those that inhabit stable climates. That seems to suggest, Antonson says, that fluctuating climates in the past could have spurred avian brood parasites to seek out a wider selection of hosts—an adaptation that could help these generalists weather climate change this time around. Specialists, on the other hand, prefer stability—including predictable temperature. That could make them more vulnerable to population declines as temperatures spike. “When you specialize in one species for a long time, it may be hard to jump to another species,” Antonson says.

“Parasites are completely dependent on their hosts,” says Iliana Medina Guzmán, a biologist at the University of Melbourne in Australia who was not involved in the study. “Species that target one species of host, those species can be much more affected than those that have different types of hosts.”

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Publication Date: 06/29/2021
Photo credits: FLPA/Alamy
Editor: By April ReeseReporter, Audubon Magazine