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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. 

Global migrations of diverse animal species often converge along the same routes, bringing together seasonal assemblages of animals that may compete, prey on each other, and share information or pathogens. These interspecific interactions, when energetic demands are high and the time to complete journeys is short, may influence survival, migratory success, stopover ecology, and migratory routes. Numerous accounts suggest that interspecific co-migrations are globally distributed in aerial, aquatic, and terrestrial systems, although the study of migration to date has rarely investigated species interactions among migrating animals. Here, we test the hypothesis that migrating animals are communities engaged in networks of ecological interactions. We leverage over half a million records of 50 bird species from five bird banding sites collected over 8 to 23 y to test for species associations using social network analyses. We find strong support for persistent species relationships across sites and between spring and fall migration. These relationships may be ecologically meaningful: They are often stronger among phylogenetically related species with similar foraging behaviors and nonbreeding ranges even after accounting for the nonsocial contributions to associations, including overlap in migration timing and habitat use. While interspecific interactions could result in costly competition or beneficial information exchange, we find that relationships are largely positive, suggesting limited competitive exclusion at the scale of a banding station during migratory stopovers. Our findings support an understanding of animal migrations that consist of networked communities rather than random assemblages of independently migrating species, encouraging future studies of the nature and consequences of co-migrant interactions. 

Commentary on companion article, “Loss of flockmates weakens winter site fidelity in golden-crowned sparrows (Zonotrichia atricapilla),” 10.1073/pnas.2219939120. 

ABSTRACT Migratory birds catabolize large quantities of protein during long flights, resulting in dramatic reductions in organ and muscle mass. One of the many hypotheses to explain this phenomenon is that decrease in lean mass is associated with reduced resting metabolism, saving energy after flight during refueling. However, the relationship between lean body mass and resting metabolic rate remains unclear. Furthermore, the coupling of lean mass with resting metabolic rate and with peak metabolic rate before and after long-duration flight have not previously been explored. We flew migratory yellow-rumped warblers ( Setophaga coronata ) in a wind tunnel under one of two humidity regimes to manipulate the rate of lean mass loss in flight, decoupling flight duration from total lean mass loss. Before and after long-duration flights, we measured resting and peak metabolism, and also measured fat mass and lean body mass using quantitative magnetic resonance. Flight duration ranged from 28 min to 600 min, and birds flying under dehydrating conditions lost more fat-free mass than those flying under humid conditions. After flight, there was a 14% reduction in resting metabolism but no change in peak metabolism. Interestingly, the reduction in resting metabolism was unrelated to flight duration or to change in fat-free body mass, indicating that protein metabolism in flight is unlikely to have evolved as an energy-saving measure to aid stopover refueling, but metabolic reduction itself is likely to be beneficial to migratory birds arriving in novel habitats.