Climate change could put food supply for migratory birds at risk

Canada warbler. Photo credit: Ric McArthur (Flickr)
Augusta Dwyer
12 May 2018

BONN, Germany (Landscape News) — Along with rising sea levels and disrupted weather patterns, climate change could also have a potentially harmful impact on migratory birds. That finding comes from a new study carried out by ornithologists at Cornell University, recently published in Ecology Letters.

The study used computer models of climate change predictions in the Western Hemisphere and compared them to weekly population estimates for 77 species of migratory birds that fly between North America and Central and South America. The scientists based those estimates on 13 years of observations from the eBird citizen-science database, an online program run by the Cornell Lab of Ornithology.

They also examined three climate variables from the computer modeling: minimum and maximum surface temperatures at two meters above the ground, and surface precipitation rates.

Previous studies have looked at novelty – deviation from historical climate norms – year by year, said Frank La Sorte, a research ecologist at the Cornell Lab of Ornithology and lead author of the paper.

“This is the first study that looks at novelty seasonally,” he said. “We are seeing that novelty actually shows some interesting dynamics when you look at it from one season to the next. And the dynamics carry interesting implications for birds and other species that would not be obvious just looking at annual summaries.”

According to the paper, there was greater deviation from novelty in the birds’ tropical non-breeding grounds than in their temperate breeding grounds in the North.

However, the paper states: “contrary to expectations, highly novel climates also occurred within temperate regions during the transition from breeding to autumn migration.”

The study saw “a pronounced peak occurring when juveniles are leaving the nest and preparing to embark on their first migratory journey, which may adversely affect their chances of survival.”

“This is a critical period, when juveniles are leaving the nest and both adults and juveniles are preparing for the long migration journey south,” said La Sorte.

Change in the autumn climate could threaten plants as well as insect communities. “So if insect resources are changed in terms of abundance, or species composition that could affect their ability to fulfill the basic requirements during that period, which might reduce survival,” he explained.

The implications are especially dire for eastern bird species. They need to both bulk up on insects to prepare for the long flight, and take advantage of the autumn weather conditions.

“They have to travel further, and many of them will cross the Gulf of Mexico or the Atlantic Ocean,” said La Sorte, “So they require either mild or supportive winds to make that journey efficiently. Birds in the West winter in Mexico. They don’t have to travel as far and they don’t have to cross any large bodies of water.”

If eastern bird species need more time to build up their fat layers, they will lose out on those generally favorable atmospheric conditions for flight. “The longer the wait, the worse the weather is going to get,” he said.

At greatest risk are juveniles preparing for their first migratory journey in the autumn, he added. “If they are adversely affected, that could affect the overall population size. In the long term, that could be detrimental.”

Having pinpointed locations and times when migratory bird populations face the greatest challenges from climate change, the scientists hope that information from their study will help identify when and where conservation efforts will have the greatest benefit.