By Melissa Gaskill
Texas Climate News

Some two billion birds migrate over the Gulf Coast each spring. The highest traffic occurs in Texas, with as many as 26,000 birds per kilometer of airspace each night, 5.4 times the number detected along the coast from Louisiana to Florida.

Several recent studies suggest that climate change could make the trip tougher for many of these birds, with shifts in the arrival of spring affecting their ability to fuel up for migration.

Both spring and fall migrations could be affected by changes in food availability and other conditions along the route and at the birds’ ultimate destinations, and in fall, birds could face stronger headwinds.

Spring now arrives earlier in 76 percent of all U.S. National Wildlife Refuges, according to a study in PLOS One, which could put birds out-of-sync with food availability and appropriate seasonal changes during their journey.

Researchers based this finding on published data of first appearance of leaves and flowers of deciduous trees from 496 National Wildlife Refuges and four major North American bird migratory routes between 1901 and 2012. The study also found that Canadian breeding sites for the whooping crane (Grus americana) have shown significant advances in the arrival of spring, but their wintering sites in Texas have not. The cranes and other long-distance migrants rely on specific conditions at widely spaced wintering and breeding habitats and so may be especially sensitive to changes in timing of seasonal transitions.

“Spring is not coming way earlier in southern Texas, as it is in the northern areas,” said lead author Eric Waller with the U.S. Geological Survey in California.

Migration decisions

“The concern is that if the birds make spring migration decisions based on conditions in Texas, they may arrive in boreal Canada past peak conditions, which could affect their food supply and reproductive fitness. There is a lot of spatial and temporal variability to climate change, and the effects of that need to be understood in terms of managing for species, particularly endangered ones like the whooping crane.”

In the journal Global Change Biology, scientists report shifts in the earliest seasonal movement of some migratory species. Combining data from weather radar stations and from citizen scientists on eBird, they documented a 1.5-day per decade advance in when birds first begin spring migrations. Peak activity timing hasn’t changed, though, which may indicate most birds have yet to adapt, a cause for concern.

“Typically, people look at the magnitude of warming, but it’s hard to judge how species respond to magnitude alone,” said co-author Frank La Sorte, a research ecologist at Cornell Lab of Ornithology. “The idea here is to provide historical context and look at change at specific locations and times of year. Migratory birds have a complex life cycle, with each stage connected to the next, and change at any stage can have effects across their entire life cycle.”

He added that more on-the-ground, citizen-scientist observations are needed to provide ecological context to climate projections and show how birds actually respond.

La Sorte is also an author on a paper in Ecology Letters that projects avian migrants encountering novel climates – those varying from what they are used to – across the majority of their annual cycle. Most at-risk, La Sorte said, would be migrants that fly long distances requiring large fuel reserves and juvenile birds undertaking their first migration – a hazardous journey in the best of conditions.

“Novelty implies uncertainty, and uncertainty implies chaos,” La Sorte said. “Conditions [that are] different from historical create chaos. Birds have never experienced these before. There is little year-to-year variation in the tropics, and birds there evolved for things to stay the same, so even a small change is significant. It’s hard to say how it will turn out, but it is likely to be detrimental across ecosystems. That in itself is something to be concerned about.”

Spring in Texas remains fairly stable, but migration requires many factors to line up through space and time, said Richard Kostecke, associate director of conservation for The Nature Conservancy of Texas.

“With conditions shifting, you see that start to uncouple and potentially make it harder for many birds. We are already seeing mismatch with some short-term migrants not moving as far south because winter farther north is milder. Texas may not see the numbers of winter birds we once did.”

More winter Texans?

On the other hand, he added, some species may opt to winter in south Texas rather than continuing on to Mexico and Central America.

Evidence suggests that birds in Europe and North America already are moving their breeding ranges north. Some species that migrate short distances are simply staying put. “The long-distance ones are sort of committed to making that leap, though,” said La Sorte.

Climate change also may affect migration by altering wind patterns. In another Global Change Biology paper, Cornell researchers used data from 143 weather radar stations to estimate the altitude, density, and direction birds took during spring and autumn nocturnal migrations over several years. They compared that with wind data from 28 different climate change projections in the most recent report from the Intergovernmental Panel on Climate Change (IPCC). Together, the data predict headwinds in autumn in central and eastern parts of the continent, making southern nocturnal migrations more difficult.

“The low-level Gulf stream will become stronger overall, especially in spring,” La Sorte said. “The large majority of birds migrating in spring actively take advantage of the jet stream. But in autumn, birds flying south have to contend with it. Juveniles in particular will find these conditions more challenging. In spring, birds take a straight shot north, but in autumn, they have to wait for weather to be favorable and are more selective about nights that they fly.”

Migrating birds also face other global climate change factors, including shifts in temperatures, rainfall, and land cover, and even alarming declines in the numbers of insects.

While migration has evolved in the past as climates changed, scientists warn that birds may not be able to keep pace with the current rate of change.

“If birds can find other habitat to move into, they have the ability to adapt,” said Kostecke. “We need to maintain stepping stones of habitat throughout the migration route, and build resilience into those systems. That is key to their survival. Much of this is very unpredictable. There are winners and losers with many of these changes, and when all is said and done, it may look quite different than it does now.”

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Melissa Gaskill, an independent journalist based in Austin, is a contributing editor of Texas Climate News.

Image credit: Tom Benson, Flickr