Multi‐Decadal Changes in Co‐Occurrence of Migrating Landbirds Are Associated With Species‐Specific Changes in Phenology and Abundance

ABSTRACT Human‐induced changes to the climate and environment have precipitated dramatic declines in abundance and shifts in plant and animal phenologies. These changes have been especially pronounced for migratory species that rely on numerous geographic locations throughout the year. Migratory bird species are notable in the number of species that have experienced both declines in abundance and shifts in phenology over the past 50 years, although the magnitude and direction of changes vary considerably across species. The community‐level impacts of species declines and phenological shifts have been explored in stationary communities, but we know little about the effects of these changes on species relationships during migration seasons when species may interact in ways that influence their route, timing, or success of the journey (e.g., through competition or access to information about resources). Therefore, we assessed the extent to which co‐migrating bird communities have changed over time, and whether changes in species co‐occurrence are associated with changes in abundance or shifts in migration timing. We used over 700,000 records of birds captured at five long‐term migration monitoring stations in eastern North America and found that pairwise species co‐occurrences have changed by as much as 40% over the past 50 years. Changes in co‐occurrence were consistently associated with species‐specific changes in phenology and sometimes associated with changes in abundance. Overall, stopover communities at three sites have significantly changed over the past few decades. Numerous and dramatic changes in co‐occurrence could be affecting the types and frequencies of interspecific interactions like competition and the exchange of social information, transforming the journeys of migratory birds in innumerable ways that could be altering their timing, energy, and safety.