Prey availability and habitat structure explain breeding space use of a migratory songbird

Abstract

Researchers have long recognized that the spatial distribution of animals relates to habitat requirements. In birds, despite recent advances in tracking techniques, knowledge of habitat needs remains incomplete for most species. Using radio telemetry, we quantified the relative space use of 37 Wood Thrush (Hylocichla mustelina) males, captured over 2 years (2013, 2014) on their breeding grounds in coastal Virginia. Following tracking, we collected data on prey availability (n = 370 plots) and habitat structure (n = 222 plots) within bird home ranges, and modeled bird utilization distribution with both sets of variables using mixed models. Our objectives were to (a) determine the relative importance of habitat structure and prey availability for bird use, (b) identify specific resources that related to bird utilization distribution, (c) test the hypothesis that soil moisture explained prey availability, and (d) evaluate models by determining whether model-identified conditions agreed with data at sites where Wood Thrushes were absent over the preceding 5 years. Of prey variables, high-use areas within bird home ranges were linked to higher biomass of spiders and worm-like invertebrates, which were strongly correlated with soil moisture. Of habitat structure variables, bird use related negatively to red oak (Quercus spp.) count and pine (Pinus spp.) basal area, and positively to forest canopy height, snag basal area, and number and species richness of trees, among others. Evaluation of 12 covariates in our best model revealed that 5 were significant, with conditions at bird absence sites congruent with our models. Goodness-of-fit tests revealed poor fit of the prey-only model, whereas the habitat-only model explained nearly 8 times the variation in bird use. The model utilizing both prey and structure covariates yielded only marginal improvement over the habitat-only model. Consequently, management objectives aimed at habitat improvement for the declining Wood Thrush should particularly consider habitat structure resources.