Birds

There is a wealth of data on bird phenologybird phenology, particularly on the timing of spring migration and the timing of breeding. Over recent decades, there has emerged a large and growing literature examining how bird phenology is shifting over time and in response to climate change, with consequences for populations, communities, and ecosystems. Here we describe the mechanisms underlying the timing of spring and autumn migration and breeding times, and explore how those drivers interact with recent environmental changes to shift bird phenology across the seasons. We highlight findings that leverage data from a wide range of sources, including historical field notes, long-term systematic sampling schemes, and a growing abundance of regional to global citizen science records, satellite measurements, and meta-analyses. There is abundant evidence that spring migration and breeding are shifting earlier, particularly in response to the direct and indirect effects of warming temperatures, while the drivers, trends, and climate responses of autumn migration are more complex and varied across species and locations. As researchers continue to combine novel phenological data sets and modeling approaches with the natural history of birds, we will see continued improvements of year-round phenological predictions and breakthroughs in our understanding of the risks associated with shifting spring and autumn migration and breeding times.