Food selection of two Neotropical austral migrant birds during the breeding season in the Espinal ecoregion

Food availability can be a limiting factor for nest site selection in birds. We evaluated the diet of two migrant Tyrant flycatchers, the Vermilion Flycatcher (Pyrocephalus rubinus) and the Fork-tailed Flycatcher (Tyrannus savana) in relation to food availability at their nesting sites. The study was carried out during the 2016-2017 breeding season in an open semiarid forest in La Pampa Province, Argentina. We determined the availability of prey by sampling arthropods using a sweep net at 20 Vermilion Flycatcher nest sites, at 23 Fork-tailed Flycatcher nest sites, and at 31 control sites. We collected and analyzed fecal samples from 14 Vermilion flycatchers and from 15 Fork-tailed flycatchers. Prey were identified to the level of Order. Vermilion flycatchers consumed eight orders of insects and one of arachnids. Fork-tailed flycatchers consumed seven orders of insects and one of gastropods. In total, 50 and 83 prey items were consumed by Vermilion flycatchers and Fork-tailed flycatchers, respectively. Both species consumed mostly coleopterans. To determine food availability, we collected 1,260 arthropods at Vermilion Flycatcher nest sites, 1,337 at Fork-tailed Flycatcher nest sites and 1,753 at control sites. Food availability did not significantly differ between nest sites of both species and control sites. Correlation between diet and food availability was significant and positive for Vermilion Flycatcher, but there was no correlation between diet and food availability for Fork-tailed Flycatcher. In the Espinal ecoregion, Vermilion flycatchers consumed the most available insects in the environment and Fork-tailed flycatchers selected predominantly coleopterans and orthopterans, and in a lower number other prey items, such as freshwater gastropods.