A large-scale analysis of bird migrations in the contiguous United States confirms what ornithologists and amateur birders already suspected: Overall, birds’ seasonal long-distance flights are happening earlier than they did a quarter of a century ago.

This shift is probably due to higher temperatures, which have risen on average around half a degree Celsius per decade, researchers report December 16 in Nature Climate Change. Looking back over 24 years, the researchers found that warmer seasons often predicted earlier migrations. One reason for this could be that birds may rely on a variety of cues, including temperature and length of day, to sync their flights with the availability of food and nesting-friendly conditions.

Previous studies of individual species have shown that some birds are migrating earlier in the year. But “that you can see these kinds of shifts at a broad scale… it’s a striking statement about how powerful these impacts of climate change can be,” says Andrew Farnsworth, a migration ecologist at Cornell University.

Farnsworth and his colleagues collected data from 13 million radar scans taken on 2,115 spring nights and 2,152 fall nights by 143 weather surveillance stations across the continental United States from 1995 to 2018. On radar maps, groups of migrating birds show up as circular blobs, whereas storms and precipitation look more like irregular patches. While the researchers couldn’t identify specific species from the radar blobs, the breadth of the data meant that their analysis encompassed hundreds of species.

K. Horton/Colorado State Univ. Wilson’s Warblers (Cardellina pusilla) pass through every state in the lower 48 as they migrate to Canada and Alaska.

The researchers found that, in the spring, birds migrated half a day earlier on average each decade. The effect is most noticeable around 45⁰ N latitude (roughly the border separating Montana and Wyoming), where the average was about 1.5 days earlier. There, the peak of northward migration — meaning that half the birds had passed overhead — occurred around May 10 in the mid-1990s, but had moved closer to May 5 by 2018.

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The trend toward earlier migration was similar, but weaker in fall, when birds’ departures south are often grounded by rainstorms and young birds are added to the migration mix. The researchers didn’t see a strong link between changing temperatures and migration timing in the fall.

Shifts in phenology, or how organisms respond to seasonal cycles, can throw the delicate synchrony of the migration system out of whack (SN: 3/4/03). For example, birds may arrive at their northern breeding grounds before the spike in insect populations and find little to eat, or get there late and miss the feast. In worst-case scenarios, young birds might starve, or bird populations may dwindle, says Nathan Senner, a population ecologist at the University of South Carolina in Columbia who wasn’t involved in the work.

Some species may adapt or migrate to cope with temporal mismatches caused by warmer average temperatures (SN: 7/11/14). But “they may not be changing fast enough to keep up with the resources they rely on,” says study coauthor Kyle Horton, an ornithologist at Colorado State University in Fort Collins.