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.

Wilson’s Warbler
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.

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.