Army ant swarm raids in Neotropical montane forest are attended by diverse flocks of foraging birds that can include residents year-round and overwintering Nearctic migrants. We asked whether migrants and residents affect each other's ability to forage at army ant raids. We quantified variation in raid attendance by three guilds of birds: wintering migrants, regular and obligate raid attending residents (ant-following residents), and facultative raid attending residents. To test whether wintering migrants and residents were negatively associated at raids, we collected data on raid attendance in four adjacent life zones in the Tilarán Mountains of Costa Rica and in different seasons (when migrants were present and absent). We first compared the guilds' raid attendance among life zones. There was little geographic overlap of migrants and ant-following residents at raids, and raid attendance frequencies were strongly correlated with the estimated local abundances of these bird guilds in each life zone. We then analyzed resident bird flock size and species makeup in the life zones where migrants attended raids most often (Premontane and Lower Montane Wet forests). If migrants affected raid attendance by facultative resident birds, we expected resident numbers or species richness at raids to decrease with migrant presence. Resident flock size and species richness did not differ between times of year with and without migrants, and species identities differed little between seasons. Furthermore, resident flock composition in migrant presence season was similar at raids with and without migrants. We conclude migrants had no measurable effect on resident bird army ant exploitation. Migrants were smaller on average and less likely than residents to capture prey from the ground at raids. These body size and foraging substrate differences between residents and migrants may mitigate inter-guild competition for food at raids.
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