Climate change is bringing spring earlier to three-quarters of the United States' federal wildlife refuges and nearly all North American flyways used by migratory birds, a shift that threatens to leave them hungry as they are preparing to breed, new research shows.

The spring green-up of the landscape brings an abundance of insects, the prime food for many migratory birds. If warm weather comes too early, tardy birds might find fewer insects to eat, the scientists found.

Birds that migrate particularly long distance are at even greater risk because of how physically depleted they are at the end of their journeys.

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The researchers from the U.S. Geological Survey and the University of Arizona, writing in the journal PLoS One, followed the onset of spring in 496 national wildlife refuge sites.

They analyzed the timing of the first blooms and first leaves of the season over the past century, then compared the timing during two periods: from 1901 to 2012 and the more recent period of 1983 to 2012, when the effects of human-caused climate change became more pronounced in the environment.

They found that spring in the more recent period came earlier to 76 percent of all wildlife refuges. Further, warmer weather arrived extremely early in nearly half the refuges, especially those along the Pacific coast and in the Mojave Desert, northern Great Plains and upper Midwest.

Northern Latitudes Warming Faster

North American migratory bird flyways extend from the Arctic to southernmost Mexico and are divided into four North-South bands: the Pacific, Central, Mississippi and Atlantic. The study found that spring is arriving earlier in all of the flyways, and that in all but the Pacific temperatures are also warming up faster in the northern latitudes than in the southern.

Those differences increase the risk of nutritional mismatches and deficits that could affect the overall health of bird populations. For example, birds traveling to breeding grounds in the north might find the insect populations have passed their peak because spring came early and progressed rapidly, said Eric K. Waller, a USGS scientist and co-author of the paper.

At the same time that their food supplies might be reduced, they also could face new threats brought on by global warming, such as diseases, invasive species and droughts, the authors said.

Can Migrating Birds Adapt?

It remains unclear whether migratory species can adapt as quickly as they need to in order to survive. The researchers found, for example, that blue-winged warblers have been arriving earlier at their breeding areas in the northeastern U.S. and Canada, but their shift still lags behind the green-up of vegetation in those areas. Whooping cranes, an endangered species, haven't changed their spring or fall migration timing by much at all.

"Bird species that are unable to advance their overall migration timing have already suffered declines," the authors said, "while those with certain behavioral characteristics (e.g. longer migration distances) or specific habitat requirements may also be susceptible to mistimed arrivals."

Previous studies indicate that some migratory birds are adapting to seasonal shifts driven by climate change. Research shows that some species are arriving earlier in the spring and leaving later in the fall, but those studies also echoed the USGS research that birds traveling longer distances are particularly vulnerable to low food availability because of early spring.

The researchers said they hope the study can help guide wildlife refuge managers as they try to assist migrating birds.