Nonbreeding Habitat Occupancy and Population Processes: An Upgrade Experiment With a Migratory Bird

Abstract

Evidence is accumulating that winter habitats occupied by migratory birds produce differences in individual condition that can carry over into subsequent stages of the annual cycle. Despite strong observational evidence, experimental work is needed to strengthen support for this hypothesis. We experimentally upgraded individual American Redstarts (Setophaga ruticilla) from low‐quality second‐growth scrub habitat to high‐quality mangrove forest habitat by permanently removing behaviorally dominant, primarily adult males from mangrove, allowing females and immature males from scrub to colonize vacated territories. Prior to the manipulation, upgraded and control redstarts had stable‐carbon isotope values in their blood indicative of scrub habitat occupancy and were comparable in body mass. Relative to control birds that overwintered exclusively in scrub, upgraded redstarts incorporated mangrove isotopic signatures, maintained body mass from winter to spring, departed earlier on spring migration, and returned at a higher rate in the following winter. Furthermore, insect biomass on upgrade territories was significantly greater than on control territories, suggesting food availability as a proximate mechanism underlying gradients of nonbreeding habitat suitability. Findings here demonstrate that winter habitat occupancy can be an important determinant of individual performance in migratory birds. Restricted access to food‐rich winter habitats may limit survival of females and immature males, an outcome that could be an important driver of population structure and dynamics.

Multi-colony tracking reveals spatio-temporal variation in carry-over effects between breeding success and winter movements in a pelagic seabird

“…Powerful approaches exist, in particular manipulative experiments (Studds & Marra 2005, Legagneux et al 2012, Catry et al 2013, Schultner et al 2014 or long-term longitudinal measurements whereby individuals act as their own controls (Daunt et al 2014). Our approach to investigating the temporal structure in the strength of carry-over effects at multiple colonies provides an additional method of distinguishing be tween genuine carry-over effects and crossseasonal correlations (our third hypothesis).…”

“…Quantifying the relative importance of extrinsic and intrinsic effects has proved challenging in correlative studies, since they are confounded (individuals may vary both in performance and environments experienced). Two approaches have been used to date to tease apart these effects: experimental manipulation in one season as a causal test of carry-over effects (Studds & Marra 2005, Legagneux et al 2012, Catry et al 2013, Schultner et al 2014) and longitudinal measurements in individuals across a range of environmental conditions whereby they act as their own controls (Daunt et al 2014). Studies that investigate the temporal structure in the strength of seasonal interactions in multiple populations provide a third option.…”

Non-breeding season habitat quality mediates the strength of density-dependence for a migratory bird

“…Redstart habitat quality influences numerous measures of bird performance in both sexes. Individuals in high-quality habitats are typically in better physical condition [39,40] and depart earlier on spring migration [39,41] at the end of the non-breeding period than individuals in lower quality habitat. Annual survival probability is also related to overwinter habitat quality [36,42].…”

Manipulating individual state during migration provides evidence for carry-over effects modulated by environmental conditions

“…Studds & Marra [15] showed that individuals upgraded (i.e. those that maintained their body mass compared with controls) by a removal experiment were able to depart earlier on migration and had greater return rates the following winter than controls.…”