Use of burned grasslands by migrating birds and the effects of prescribed fire on earthworm prey abundance

Fire is a critical ecological disturbance in grasslands worldwide, including in the U.S. Great Plains, where fire was once widespread. Each spring, millions of birds, including many declining grassland species, migrate through this region. Despite the importance of high-quality stopover sites for avian populations, little is known about how migrating birds interact with fire, including their potential use of recently burned areas as migratory stopover locations and how burning affects prey resources. We investigated how the entire community of migrating grassland birds responds to fire and how fire affects earthworms with context to prey availability for birds. We quantified the response of migrating birds to fire using bird survey data collected during spring migration in Osage County, Oklahoma, USA, in a region managed intensively with prescribed fire. We evaluated the influence of time-since-fire on abundance of all passage migrants combined, as well as on densities of 7 bird species on two temporal scales: years-since-fire, and days-since-fire at recently burned sites. Additionally, we investigated a potential mechanistic link between fire and prey availability for one fire-associated bird, the American Golden-Plover, by measuring post-fire dynamics of the earthworm community concurrent with spring migration. We examined how near-surface earthworm abundance varied with respect to days-since-fire at recently burned sites, as well as earthworm species composition at these sites. Migrating birds overall were most abundant in recently burned grasslands, and 6 of 7 species analyzed had highest densities in sites burned in the same season. Three species of birds, including American Golden-Plover, occurred in highest densities immediately (<10-14 days) after fire. Earthworms were most abundant near the soil surface in this same temporal window (<10-14 days since fire). Earthworm species composition consisted primarily of native earthworms, but with some non-native presence. Further research into resources used by birds in recently burned areas is needed to provide full mechanistic understanding of the links between fire and habitat use in this system. We conclude that fire is an essential but overlooked component of the stopover ecology of migrating birds in the Great Plains, potentially providing important prey resources for several species of grassland birds.