Bird migratory patterns change with the climate

A study tracking hundreds of bird species across the US over more than two decades has found they are altering their migration patterns in response to the changing climate, which is shifting timing of peak abundance for seeds, fruit and insects.

Published in the journal Nature Climate Change, the authors say it’s the first to investigate critical questions about birds and accelerated climate change on a continental scale.

“Bird migration evolved largely as a response to changing climate,” says senior author Andrew Farnsworth from Cornell University. “It’s a global phenomenon involving billions of birds annually.”

But how they adapt to the escalating weather extremes – and resulting shifts in timing of food resources – has been unclear, and nearly impossible to analyse on a systemic scale across time, space and species.

This has important implications for ecosystems that benefit from migrating birds for pest control, pollination, seed dispersal and food sources for other wildlife. In turn, the birds need food and habitat resources to refuel on their journey.

Using 24 years of weather surveillance data on nocturnal bird migration from the National Oceanic and Atmospheric Administration, the researchers created a new tool to process the data using machine learning. 

The instrument, “MistNet”, uses computer vision techniques to distinguish birds from rain, a major limiting factor for biologists until now. It also enabled the researchers to estimate flying speed and movements of migrating birds.

They sampled more than 4000 spring and autumn nights from around 13 million radar scans from 1995 to 2018 and investigated changes in peak migration across years and latitude.

Results showed that during spring, the birds were more likely to fly over certain stops earlier than they would have 20 years ago and that the timing was connected to temperature: the greatest changes occurred in regions of higher latitude that are warming more quickly.

Changes were also most apparent in the western flyway, where the largest number of species migrate the shortest distances. The authors suggest this could reflect their ability to respond to altered resource availability more quickly than species that migrate over long distances.

In autumn the migration shifts were more variable, which surprised lead author Kyle Horton, from Colorado State University, although migration tends to be more difficult to discern during those months when there is no competition for mates.

“In the spring, we see bursts of migrants, moving at a fairly rapid pace, ultimately to reach the breeding grounds,” he explains. “However, during the fall, there’s not as much pressure to reach the wintering grounds, and migration tends to move at a slower, more punctuated pace.”

The altered flight patterns don’t necessarily mean the migrating birds are keeping up with climate change though, he adds, as even subtle shifts in resources could impact the health of migratory birds.

Next, they plan to include Alaska in analyses, where climate change is having more serious impacts – already, migratory bird populations have been in free fall there due to habitat degradation, over-exploitation and poaching.