Wildlife

Global warming shrinks bird breeding windows

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 examined a data set spaning from 1975 to 2017 which 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. 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.

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