Niche dynamics suggest ecological factors influencing migration in an insectivorous owl.

Seasonal migration is a widespread phenomenon undertaken by myriad organisms, including birds. Competing hypotheses about ultimate drivers of seasonal migration in birds contrast relative resource abundances at high latitudes ("southern home hypothesis") against avoidance of winter resource scarcity ("dispersal-migration hypothesis"). However, direct tests of these competing hypotheses have been rare and to date limited to historical biogeographic reconstructions. Here we derive novel predictions about the dynamics of individual niches from each hypothesis and provide a framework for evaluating support for these competing hypotheses using contemporary environmental and behavioral data. Using flammulated owls (Psiloscops flammeolus) as a model, we characterized year-round occupied niche dynamics using high-resolution global positioning system tracking and remote-sensed environmental data. We also compared occupied niche dynamics to counterfactual niches using simulated alternative nonmigratory strategies. Owl occupied mean niche was conserved among seasons, whereas niche variance was generally higher during migratory periods. Simulated year-round residents in Mexico would have experienced putatively more productive niches than migrants. These findings provide ecological support for the "dispersal-migration" hypothesis in which winter resource scarcity is the primary driver of migration rather than summer resource abundances.

Evidence of postbreeding prospecting in a long-distance migrant.

Organisms assess biotic and abiotic cues at multiple sites when deciding where to settle. However, due to temporal constraints on this prospecting, the suitability of available habitat may be difficult for an individual to assess when cues are most reliable, or at the time they are making settlement decisions. For migratory birds, the postbreeding season may be the optimal time to prospect and inform settlement decisions for future breeding seasons.We investigated the fall movements of flammulated owls (Psiloscops flammeolus) within breeding habitat after fledglings had gained independence and before adults left for migration. From 2013 to 2016, we trapped owls within a breeding population wherein all nesting owls and their young have been banded since 1981. We used stable isotopes in combination with mark-recapture data to identify local individuals and differentiate potential prospecting behavior from other seasonal movements such as migration or staging.We commonly captured owls in the fall-predominantly hatch-year owls-that were not known residents of the study area. Several of these nonresident owls were later found breeding within the study area. Stable isotope data suggested a local origin for virtually all owls captured during the fall.Our results suggest that hatch-year flammulated owls, but also some after-hatch-year owls, use the period between the breeding season and fall migration to prospect for future breeding sites. The timing of this behavior is likely driven by seasonally variable costs associated with prospecting.Determining the timing of prospecting and the specific cues that are being assessed will be important in helping predict the extent to which climate change and/or altered disturbance regimes will modify the ecology, behavior, and demographics associated with prospecting.