Wintering Together: Do Migrants Impact Residents? A Literature Review.

Year after year, billions of long-distance migratory birds join year-round resident tropical birds during the overwintering period (dry season in the northern tropics) marked by harsh environmental conditions. The overlapping habitat use and apparent resource overlap during this time suggest potential interspecific competition between residents and migrants. Previous studies have explored the effects of such competition on migrants, but the impact on residents has been neglected. This review critically evaluates and summarizes the findings in the literature about interactions between migratory and resident birds in tropical ecosystems, using the necessary and sufficient conditions recommended by Dhondt to assess interspecific competition. This review reveals that interspecific territoriality, consistent aggression (interference competition), and alterations in foraging behavior and microhabitat impacted resident birds. High dietary overlap observed in some species pairs suggests potential exploitation competition, although our understanding of year-round residents' diets and the full scope of these interactions remains limited. The complexity of these relationships underscores the need for comprehensive research to disentangle seasonal effects from competitive pressures and to assess impacts on resident fitness, distribution, and abundance. Inconsistent methodologies and inadequate spatio-temporal scales have hindered a clear understanding of this phenomenon. We emphasize the importance of long-term studies, including observations before and after migrant arrival at wintering grounds, to detect shifts in resource availability and foraging niches. Given climate change's influence on migration patterns and food resources, elucidating these biotic interactions is crucial for developing effective conservation strategies for resident tropical avifauna. Particularly, as many resident birds exhibit high levels of endemism, higher sedentarism, and specialized foraging habits, they may be more vulnerable to ecological changes than migrants, who are more flexible, generally occupying broader foraging niches. This review highlights critical knowledge gaps and proposes directions for future research to enhance our understanding of migrant-resident bird dynamics in tropical ecosystems.