Many birds around the world have responded to warming temperatures by advancing their breeding phenology, initiating and fledging nests earlier in warmer years. These shifts are primarily considered a means to maintain synchrony with insect availability in the spring. Meanwhile, poleward range shifts are thought to be the main mechanism by which birds track their optimal environmental conditions for survival and reproduction.
White-crowned Sparrow (Zonotrichia leucophrys) nestlings in California. Photo by Sam Roberts.
However, a study of bird communities in California that was recently published in PNAS suggests that phenology shifts can also serve to maintain environmental niches in the spring. The research team, led by Jacob Socolar, found that birds have advanced their breeding phenology between 5-12 days over the past century, essentially negating a 1 degree C temperature increase over the same time period. In other words, the birds have used phenological shifts to maintain a particular temperature niche, which may reduce the need for range shifts. Socolar's group also found that nesting success changes with temperature anomalies; in the warmer parts of a species' range, hot years are associated with low nesting success.
Socolar's team combined observations by Joseph Grinnell (pictured above, preparing specimens in the field) and other observers in the early 1900's with more recent observations, to examine phenology shifts over the past century. Photo from the Bancroft Library/University of California.
This study is an important step forward in understanding the mechanisms behind shifting phenology: birds that nest earlier encounter cooler temperatures and increase their nesting success. As Richard Primack pointed out in an Audubon article highlighting this study, the next important step is to identify the specific links between temperature and nesting success, which may include heat or drought stress, or even insect availability.
Amanda, Thanks for this post. It will be interesting to see if bird ecologists follow-up this suggestion and actually study how climatic variability affects the physiology of these migratory bird species. Richard
ReplyDeleteI agree, Richard! The physiological mechanisms for these 'early bird' shifts is an important next area of study. Earlier breeding times in birds is a widespread phenomenon in cities, a function of the urban heat island effect. Some mechanistic studies have tackled the question of how birds shift their timing of breeding, but it remains unclear what the key triggers are. See for example work in Phoenix, Arizona by Scott Davies & Pierre Deviche. Thanks for drawing my attention to this very cool study, Amanda!
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