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.”

Brood parasites with multiple hosts aren’t totally off the hook, though. A 2020 study found that the Lesser Cuckoo, which breeds mostly in South and East Asia, is shifting its nesting locations away from those of its 12 hosts in pursuit of favorable climate conditions. “They’re actually starting to spread out farther and farther apart from each other,” Antonson says. “Those brood parasites could be in real trouble.”

Other brood parasites are decoupling from their hosts through timing. A 2009 study in Biology Letters found that as it’s become warmer, Europe’s Common Cuckoos—a harbinger of spring across the region—are breeding earlier. However, 15 of their host species are shifting even more quickly and nesting even sooner. “When they come back from Africa [in the spring], it’s too late for the cuckoos to parasitize hosts,” says Mark Hauber, a University of Illinois Urbana-Champaign ecologist who co-authored last year’s Nature Communications study with Antonson and has studied avian brood parasites for many years. A 2011 study found that in regions with the biggest increase in spring temperatures, the Common Cuckoo’s hosts experienced less parasitism—further evidence that warming may be making it harder for some avian parasites to successfully reproduce.

As they move out of their typical territory, brood parasites may have to become inventive. In Australia, a large cuckoo species called the Pacific Koel appears to be moving south as its preferred temperatures shift with climate change. “They are encountering new hosts that they have never exploited before,” Medina says. “So now they have to come up with new exploiting tactics.”

Forming a full picture of how avian brood parasites and their hosts are responding to climate change will require more studies of specific bird populations, says Hauber. We need to get to the ground and figure out what individual parasites are doing.” To that end, he has studies underway to track certain species for signs of changing behavior, including one in his own backyard: the Brown-headed Cowbird. Hauber suspects the notoriously adaptable bird may be one of the few brood parasites to benefit from climate change. But as more data rolls in on avian brood parasites overall, “we think we’ll see more generalization and less specialization”—a trend that could help at least some of these birds weather the changes ahead, if they can adapt in time.