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.
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.
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.