Over the last 40 years, some songbirds have been shifting their arrival time at important stopover sites on long migratory routes, raising questions about how well they can adapt to changing environments as temperatures rise.

For a study published March 2 in the Journal of Avian Biology, researchers from Calvin University in Grand Rapids, Michigan and St. Edward’s University in Austin analyzed bird-banding data of nine different migrating songbird species from two stopover sites near Kalamazoo, Michigan, along with temperature data for the same locations and time period.

They found that between 1975 and 2015, temperatures at these sites rose by 2 degrees Celsius on average, as hermit thrushes, ruby-crowned kinglets and myrtle warblers simultaneously arrived at these feeding locations later and later. At the same time, gray-cheeked thrushes, Swainson’s thrushes and Tennessee warblers arrived earlier.  

“Any time you see changes in these systems, which we're seeing with rising temperatures and seeing some changes in behavior, there's potential implications on the survival of birds,” said coauthor Darren Proppe, research director of the Wild Basin Creative Research Center at St. Edward's University. 

In recent years, ornithologists have found that birds are migrating up to two days earlier every decade due to rising temperatures. Over the course of migration, birds need to time their arrival at stopover sites properly to ensure they’ll be fit for the long haul. They aim to arrive when food is most plentiful: when insects are leaving their larval phase or when bushes just begin to bear fruit. 

A migrant may stay at a stopover site for about three or four days, and by the time it leaves it should have bulked up by a few grams to a half-gram, depending on the species — a practice similar to carb-loading before a marathon.

Rising temperatures may not only affect how birds migrate, but how plants and insects bloom or hatch. If a bird arrives too soon or too late to a stopover site, it may not have enough food to fuel up for the remaining journey, and may not reach its final destination.

To understand how well these songbirds were adapting to shifting arrival times, the researchers also examined how much mass had been added by each of the 102,000 birds at each stopover site. To their surprise, they found that migrants had been reliably gaining the weight they needed, regardless of any temperature change at the stopover site. 

“We were happy to find that the stations were still functional,” Proppe said. “But I think the question is also, ‘What is that threshold at which they can no longer make these adjustments?’” 

Without additional research on how insects and fruit-bearing plants are handling climate change, Proppe and his colleagues can only hypothesize about why these birds seem to be thriving despite their shifting arrival times. They may have changed their diet to include more of one food source over another, while continuing to be fit enough to finish their migration journey.

Still, while his current study shows a discernible level of adaptation in migrating songbirds, Proppe believes the environment can only change so much until an animal can no longer adapt.

“We do know that animals adapt,” Proppe said. “They do change with their systems to some extent, but if plants shift to a point at which they're outside that window of adaptability, then we start to see some changes in populations.”

In future studies, Proppe hopes to conduct similar experiments with different birds in other parts of the world, not only using banding data but tracking entire routes using satellite geolocation technology, to see how speeds and routes may vary over an entire migratory pathway due to changing environments. Even though this study shows that birds can adapt to some degree of temperature change, Proppe doesn't know how much they can take and over what time period.

“Just because it didn't happen at this point doesn't mean it couldn't happen if this change continues,” Proppe said. “If climate continues to warm, we might see shifts that reach a threshold at which we see a detrimental effect on birds. It's certainly something we need to be looking out for."

The study, “Avian migrants encounter higher temperatures but continue to add mass at an inland stopover site in the Great Lakes region,” published March 2 in the Journal of Avian Biology, was authored by Schuyler D. VanTol, Calvin University; Carolyn R. Koehn, Calvin University and University of Nevada, Las Vegas; Darren S. Proppe, Calvin University and St. Edward’s University; and Rich Keith and Brenda Keith, Kalamazoo Valley Bird Observatory.

Correction: An earlier version of this article incorrectly listed the first name of coauthor Darren Proppe, the name of Calvin University and the location of the university. The errors have been corrected.