Are multi-brooded birds the current “winners” of climate change?

This blog post is provided by Abigail Kimmitt and Daniel Becker and tells the #StoryBehindthePaper for the paper “Plasticity in female timing may explain earlier breeding in a North American songbird“, which was recently published in the Journal of Animal Ecology. In their study, they explored breeding success of the dark-eyed junco from long term datasets, to see if birds respond to climate change in a plastic way, or if selection for changes to breeding timing are happening.

Timing life history events (such as breeding and migration) to match environmental conditions is critical for the survival and fitness of animals living in seasonal environments. So, warmer springs and environmental unpredictability associated with climate change creates a challenge for seasonally breeding animals. Fortunately, shifts in timing (e.g., earlier breeding) are one of the most documented responses to climate change, suggesting that animals can adapt to warming springs. Shifts in timing, however, might not always completely match the rapidly changing environment, such that some species might experience population declines as they are unable to keep up. Our understanding of how animals shift timing of life history events remains limited, as results are inconsistent, likely due to the variability in species’ biology and geography. Birds are a strong model for studying shifts in timing because of many long-term research studies, in which breeding populations are monitored for decades as part of research programs in evolution, behavior, and physiology.

In a recent paper in the Journal of Animal Ecology, we used 32 years of breeding data from a population of a common North American songbird, the Dark-eyed Junco, to ask 1) was the breeding phenology shifting in relation to increasing spring temperatures over time? And 2) if so, were these changes driven by phenotypic plasticity of timing or selection for earlier breeding in response to increasing spring temperatures? Phenotypic plasticity is when an animal can alter some aspect of its phenotype (e.g., behavior, size, hormone regulation) in response to changes in the environment. Therefore, if we found evidence of plasticity, this might be at the individual level, in which some females were more able to adjust their breeding timing in response to warmer springs than other females. While not mutually exclusive, our other hypothesis was that selection for earlier breeding might be allowing for microevolutionary change within the population. This means that individuals breeding earlier would produce more offspring and, in turn, have higher fitness. If timing of breeding was genetically inherited, this trait could be passed down to offspring, such that the population might evolve, or in this case, shift its breeding phenology in response to temperature.

A dark-eyed junco released after sampling at Mountain Lake Biological Station. (Photo credit: Alexander Becker)

Overall, initiation of the breeding season in the Dark-eyed Junco population, measured as the first egg laid in a female’s first nest, occurred earlier over the 32-year period and co-varied strongly with spring temperatures in April. Females that bred in three or more years also typically bred earlier when they experienced warmer Aprils, which is evidence of plasticity in timing. Interestingly, individuals did not exhibit many differences in how well they were able to adjust to warmer springs: females across the population were consistent in how much they changed their timing based on temperatures experienced. Females that bred earlier also had higher fitness, in which they had more offspring fledge within the breeding season. This pattern of selection favoring earlier breeding, however, was not associated with any temperature bracket across the breeding season. Other environmental variables, such as precipitation or selection on male reproductive timing (that in turn affects female timing), could explain the pattern we observe supporting selection for earlier breeding. While both plasticity and selection likely play a role in driving earlier breeding in this population, our findings show greater support for plasticity in breeding phenology in response to a warming climate. But this is not the case for all birds, as other scientists have found evidence in other species for evolutionary change in timing in response to climatic conditions. So, what might explain these findings in the Junco? Our study population, which resides in the Appalachian Mountains year-round, are not long-distance migrants. As a result, without the constraint of migration, they have a longer breeding season and are multi-brooded, as females can re-nest up to five times in a season, and pairs have been observed to fledge three successful nests of offspring. For a multi-brooded bird, reproductive success is likely based on the length of the breeding season as a longer breeding season would allow them to attempt to raise more broods within one season. Therefore, warmer springs may allow females to start breeding earlier and extend their breeding season. Temperature, however, might not act as a constraint on breeding for these multi-brooded birds, which is why we might not observe a relationship between temperature and selection for earlier breeding. We conclude that multi-brooded birds could be considered the current “winners” of climate change, as warmer springs does not cause mistiming of their breeding season (which is common in migratory species) but instead provides them with more breeding opportunities. To understand how females can adjust their breeding phenology in response to spring temperatures, a more extensive understanding of the physiological mechanisms underlying female reproductive timing is essential.

A junco nest with 4 eggs hidden under grasses in the bank. (Photo credit: Jessica Graham)
Author bios

Abigail is a postdoctoral researcher at the University of Michigan. Her research focuses on understanding how animals cope with changing environments, by studying the physiological mechanisms and evolutionary consequences of seasonal behaviors. She conducted this work in collaboration with colleagues at her graduate research institution, Indiana University. Learn more about her research at and @AbbyKimmitt on Twitter. 

Daniel is an assistant professor at the University of Oklahoma. His group focuses on the ecology and evolution of immunity and infectious disease in wildlife, with a particular affinity for bat and songbird systems. Daniel collaborated on this project following a postdoctoral fellowship at Indiana University. Learn more about his research at and @danjbecker on Twitter.            

Read the paper

Read the full paper here: Kimmitt, A. A., Becker, D. J., Diller, S. N., Gerlach, N. M., Rosvall, K. A., & Ketterson, E. D. (2022). Plasticity in female timing may explain earlier breeding in a North American songbird. Journal of Animal Ecology, 00, 1– 11.

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