Breeding earlier because of climate change may put birds at risk


Researchers at Cornell University have been tracking the reproductive success of tree swallows for the last 30 years, finding they have mated about three days earlier per decade over that time. Photo by David Chang van Oordt/Cornell University

Sept. 29 (UPI) -- As the climate warms, spring temperatures are arriving earlier and earlier, causing some animals to alter their breeding and migrational patterns.

Research published this week in the journal PNAS suggests some birds may be putting themselves at risk by mating and breeding earlier in the year, when they are more likely to face inclement weather events -- and a lack of food.

"We actually can look at it empirically for our specific study location using really great long-term weather records collected near Ithaca, N.Y., that start in 1893," study co-author Conor Taff told UPI in an email.

"What we see is that springs have gotten warmer overall in that time period, but the date of the last major cold snap has not changed," said Taff, postdoctoral fellow in behavioral ecology at Cornell University.

To better understand how birds are adapting to shifts in seasonal weather patterns, researchers turned to a 30-year effort to track the nesting success of tree swallows living near Ithaca.

In addition to reproduction data, researchers analyzed fluctuations in insect abundance over the last 25 years, as well as climate patterns stretching back 100 years.

Their analysis showed tree swallows have been mating roughly three days earlier every decade for the last 30 years. The climate and insect data showed fledglings born earlier were at greater risk of exposure to inclement weather and a subsequent reduction in food availability.

Lots of bird species have altered their breeding schedules in response to changes in food availability, researchers found.

"For example, a lot of migrant birds depend on gleaning caterpillars and bugs from tree branches," Taff said. "Those food sources don't become available until after the spring leaf-out. With climate change, leaf-out happens earlier, so food is available earlier and many bird species have changed their timing to match that."

However, leaves and caterpillars can usually withstand a few cold days. The latest research showed cold spells have a much more significant impact on the availability of flying insects, the main food source for tree swallows.

"Even a day of cold weather can drastically reduce the amount of flying insects available," Taff said. "Those prey species might just be delayed and emerge a few days later when it warms up, but that doesn't help the swallows if their nest has already failed."

If climate change slows and stabilizes, scientists might expect birds to adapt to the risks posed by an earlier breeding season, but researchers worry birds and other animals simply can't keep up with the accelerated pace of human-caused climate change.

For many challenges, individual variability among birds and other animals ensures that some prove more resilient to newfound challenges than others. But research suggests that when it comes to cold weather, even the most resilient birds are no match.

"We know that this kind of between-individual difference is important and describes the variation that selection can operate on, but the effect of a bad cold snap in these populations is so strong that basically all the birds who happen to be at a vulnerable stage are impacted," Taff said.

Taff and his research partners are working to more closely analyze the affects of temperature and food availability on breeding success among swallows.

"We have several lines of investigation going on now that look at how early life conditions, including the temperature when nestlings are in the nest, influences behavior and performance throughout the lifetime," Taff said.