For breeding birds, timing is everything. Most species have just a narrow window to get the food they need to feed their brood—after spring’s bounty has sprung, but before other bird species swoop in to compete. Now, a new study suggests that as the climate warms, birds are not only breeding earlier, but their breeding windows are also shrinking—some by as many as 4 to 5 days. This could lead to increased competition for food that might threaten many bird populations.
Birds typically time their breeding to cues signaling the start of spring, so that their chicks hatch when food like plants and insects is most abundant. But global warming has pushed many species to breed earlier in the year; that effect is especially prominent at higher latitudes, where temperatures are rising faster than near the equator. Few studies, however, have examined how climate change affects the duration of breeding windows, which closely track the number of chicks born each year as well as overall population trends.
To find out how the length of breeding periods has changed over time, a team led by Maria Hällfors, an ecologist at the University of Helsinki, analyzed an extensive data set from amateur ornithologists coordinated by the Finnish Museum of Natural History. The data set spans from 1975 to 2017 and includes the nesting records of 73 species and more than 820,000 birds from a 1000-square-kilometer area in Finland’s boreal forests. Each year, trained volunteers placed uniquely numbered rings around the legs of newly hatched chicks to track their movements and survival. Because chicks had to be a certain size to get a ring, the researchers were able to use the timing of the tagging to work out when each chick had hatched—and therefore when breeding had occurred.
On average, the beginnings and ends of the breeding periods are occurring earlier in the year, Hällfors and colleagues report today in the Proceedings of the National Academy of Sciences. However, the ends are shifting back faster than the beginnings, resulting in an average breeding window that is 1.7 days shorter in 2017 than it was in 1975. During that same period, Finland’s average temperature rose by 0.8oC, suggesting many bird species are actively responding to changing temperatures, Hällfors says.
“It’s good for the species if it’s able to follow the optimum conditions as the climate changes,” she says. However, the shorter breeding windows mean more birds are breeding earlier in the season—a risky time for chicks’ survival, especially if the weather turns suddenly cold. In addition, because many late-season species are shifting their breeding windows up, that could mean more competition for food and nesting sites early on, leaving some chicks to go hungry. Although the researchers were unable to tease out overall population trends from their data set, Hällfors expects these shifts will have a large impact on bird numbers, with some species outcompeting others.
Lucyna Halupka, an ecologist at the University of Wrocław, calls the study “a very important paper” because it’s one of the few to measure the breeding period duration. For 2 decades, she says, many scientists studying birds and climate change have looked only at the earliest, median, or mean laying dates for specific groups of birds. However, she cautions that because the study is limited to Finland, the findings may not apply universally; future studies should examine how breeding seasons move in other regions where the effect of climate change is different. They should also try to determine how shifting breeding windows affect population sizes, she says.
For Hällfors, the new findings illustrate the power of long-term data sets. “Imagine the bird-ringing ornithologists in the 1970s,” she says. “They probably couldn’t have imagined that their data would be used in 2020 to look at climate change.” It’s also a valuable addition to other ongoing climate change research, says conservation biologist Stuart Butchart of BirdLife International. “Many people still think of climate change as a problem that’s going to arise in the future,” he says. “This is another study showing that entire communities of species have already shown substantial responses to climate change over recent decades.”