Temperature variability reduces nesting success

Many songbirds are nesting earlier in spring because of warmer temperatures brought about by climate change. But the shift brings another danger that is especially deadly for nestlings: greater exposure to temperature variability in the form of cold snaps and heat waves. Such extremes result in more nest failures. These findings come from a Cornell Lab of Ornithology study just published in the journal Nature Communications.

"When we talk about temperature changes, the focus is mostly on averages," said co-lead author Conor Taff, a researcher in Cornell University's Ecology and Evolutionary Biology Department. "But all creatures, including humans, interact with weather conditions right in the moment, not with long-term averages. Even a one or two-day period when it's really cold or really hot can be incredibly challenging even if the average temperature hasn't changed. Changing temperature averages and temperature variability are two different components of climate change."

To understand how temperature variability might affect nesting success, the researchers analyzed 300,000 breeding bird records submitted to the Cornell Lab's NestWatch project between 1995 and 2020. They pinpointed the coldest three-day day period and the hottest three-day period for each one of the nests and then looked at whether those values predicted lower nesting success. Success was measured by how many nestlings survived to fledge.

"We found that 16 of the 24 species we studied had reduced reproductive success when a cold snap occurred during the incubation or nestling stages," Taff said. "Eleven of 24 had reduced success when a heat wave occurred during the breeding season. Aerial insectivores were the most sensitive to temperature extremes, especially cold."

The vast majority of birds feed insects to their young, regardless of their final diet, and cold snaps reduce insect availability. If these episodes occur when nestlings are most vulnerable, they can trigger a mass die-off. During a cold snap, adult birds may move away to find survivable conditions which leaves eggs and nestlings exposed to cold and lack of food.

"It's the nestlings that really get hit hard because they can't regulate their own body temperature yet," said co-author Ryan Shipley, a Cornell University Ph.D. student at the time of the research. "Nestlings also grow at an exponential rate during the first week or two of life and if insect activity drops because of a cold snap, the young birds likely won't survive."

Taff and Shipley also examined 100 years of weather data to see if there have been changes in the timing of cold snaps and heat waves during the March through August breeding season in the United States and Canada. Although they found no clear pattern in the timing of temperature extremes, they do note that it's getting warmer everywhere.

"Even if nestlings somehow manage to survive a cold snap or heat wave, there may still be long-term consequences affecting the overall health of the birds," notes Shipley. "We're only looking at a brief snapshot during early life and cannot measure long-term health in an unbanded wild population."

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Cracking the code: Genome sequencing reveals why songbirds are larger in colder climates

Scientists have unlocked the genetic basis underlying the remarkable variation in body size observed in song sparrows, one of North America's most familiar and beloved songbirds. This discovery also provides insights into this species' capacity to adapt to the challenges of climate change.

The study, published today in Nature Communications, used genomic sequencing to successfully pinpoint eight genetic variants, or DNA mutations, largely responsible for the nearly threefold difference in body size observed across the song sparrow range from Mexico to Alaska. For instance, song sparrows that live year-round in the Aleutian Islands can be up to three times larger than their counterparts in the coastal marshes of California.

Katherine Carbeck, the study's first author and a PhD candidate in the faculty of forestry, University of British Columbia, explains that body size varies predictably in many species that inhabit vastly different climatic conditions, aligning with "Bergmann's rule" which states that organisms in cooler climates tend to be larger as an adaptation to regulate body temperature.

"The existence of 'locally adapted' populations implies that natural selection has shaped the genetic makeup of song sparrow populations across their range, enabling individuals to survive and reproduce in drastically different climatic conditions," said Carbeck. "However, the genetic mechanisms underlying Bergmann's rule have remained elusive until now."

Whole-genome sequencing cracks the code

Carbeck and colleagues from the Cornell Lab of Ornithology, University of Alaska and Ouachita Baptist University used the power of whole-genome sequencing to decode the entire song sparrow genome and unlock its secrets.

They combed through genetic samples from the two largest song sparrow subspecies that live year-round in the Aleutian Islands, as well as two smaller subspecies: one that breeds in Alaska but migrates to warmer sites in winter, and one that lives year-round on the B.C. coast, where the Pacific Ocean maintains comparatively mild winter weather.

Their comparison of the larger and smaller-bodied subspecies revealed several candidate genes associated with body mass. By characterizing these candidates, they identified eight specific genetic variants closely linked to body mass -- aligning with Bergmann's rule.

Genetic diversity helps life adapt to climate change

The researchers suggest that revealing a genetic basis for Bergmann's rule helps us to understand how evolution, natural selection and climate have interacted throughout a species' history.

"Our results highlight the potential role habitat conservation plays in enabling the continued exchange of genes between populations -- which is important in the face of ongoing change," said Carbeck.

Dr. Jen Walsh, a study co-author and research associate at the Cornell Lab of Ornithology, added: "From a genomic perspective, identifying a small number of candidate genes with an apparently large impact on variation in body size is really interesting. The magnificent range of phenotypic diversity seen in song sparrows suggest they offer exciting opportunities to identify genes underlying a host of well-known and generally accepted eco-geographic rules."

Dr. Peter Arcese, a co-author and a professor in UBC's department of forest and conservation sciences, said the findings suggest a resilient future for these birds.

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