Failure to advance migratory phenology in response to climate change may pose a significant threat to a declining Nearctic-Neotropical songbird

Populations of long-distance migrants that breed in seasonal habitats can be significantly impacted by climate change. We examined the migratory and breeding phenologies of the cerulean warbler (Setophaga cerulea), a declining long-distance Nearctic-Neotropical migrant that breeds in deciduous forests of Indiana. Our primary objectives were to determine temporal trends in cerulean warbler migratory timing, and to identify climate variables that explain variation in this species' migratory and breeding phenologies. We reviewed trends in cerulean warbler first arrival to Indiana from 1982 to 2019, and compared them to several explanatory climate variables: spring temperature, growing degree days (GDD), North Atlantic Oscillation (NAO) index, and Oceanic Niño Index (ONI). We also compared the timing of cerulean warbler first lay dates from 2012 to 2019 with the aforementioned climate variables and annual spring precipitation. Cerulean warblers exhibited a minimal advance in first arrival timing (≤4 days in 38 years). Arrival timing was best predicted by GDD and a null model, but trends in GDD indicate that spring warming in Indiana has advanced by a greater margin, approximately 14 days. Climate variables did not predict first lay timing better than a null model. Springtime in Indiana is occurring earlier, but cerulean warblers are advancing their migratory timing to a much smaller degree. This failure to adapt may have a detrimental effect on warbler populations if it results in an asynchronization of important biological timings between them and their prey. Further studies of cerulean warbler breeding and prey phenologies are necessary to determine how climate change is impacting this species' reproductive success.

Post-fledging habitat use in a declining songbird

Background

Fledglings of many mature forest-dependent Neotropical songbirds move from mature forest habitats into areas of thick vegetation such as regenerating clearcuts. The Cerulean Warbler (Setophaga cerulea), a Neotropical migratory songbird, is a species of conservation concern across its range and it is listed as endangered in Indiana. This species has declined faster than any other species of wood-warbler in North America. Most prior research on Cerulean Warblers has examined the breeding biology, but there are no data on habitat use by fledgling Cerulean Warblers. Our research aimed to determine where fledgling Cerulean Warblers dispersed after they left their nest, but before they migrated to their wintering grounds.

Methods

Since 2007, Cerulean Warbler breeding populations have been monitored in Yellowwood and Morgan–Monroe state forests in southern Indiana as part of a 100-year study called the Hardwood Ecosystem Experiment. To identify habitats used by fledgling Cerulean Warblers, we captured by hand or mist-nets, adult and juvenile Cerulean Warblers once young had fledged from a nest. We attached radio-transmitters to individuals and tracked each bird daily using radio-telemetry. Radio-telemetry data were collected from May to July 2015–2017, and microhabitat data on fledgling locations and random locations were collected during the same years in the month of July.

Results

Fledgling presence, when compared to random non-use sites, was positively correlated to presence of grapevines, greater vertical vegetation density, and greater ground and canopy cover. Fledgling presence was negatively correlated with white oak abundance, aspect, basal area, and the abundance of mature trees that Cerulean Warbler adults use for nesting.

Conclusions

Our study is the first to demonstrate that Cerulean Warbler fledglings occupy habitats that are characterized by specific habitat components. Fledgling sites were located in areas with high vegetation density, such as clusters of grapevine, which provided cover from predators. Identifying Cerulean Warbler habitats throughout the breeding season can better inform natural resource personnel on how to manage forests to meet the habitat needs of this rapidly declining migratory songbird.

Feather corticosterone levels are related to age and future body condition, but not to subsequent fitness, in a declining migratory songbird

In migratory species, breeding and non-breeding locations are geographically separate, yet the effects of conditions from one stage may carry over to affect a subsequent stage. Ideally, to understand the mechanisms and implications of 'carry-over effects', one would need to follow individuals throughout the year, quantify potential environmental causal factors and physiological mediators during multiple life-history stages, and measure downstream fitness. Owing to current limitations of tracking technology, this is impossible for small, long-distance migrants, so indirect methods to characterize carry-over effects are required. Corticosterone (CORT) is a suspected physiological mediator of carry-over effects, but when collected from blood it provides only a physiological snapshot at that point in time. When extracted from feathers, however, feather corticosterone (CORT_f) provides a measure of responses to stressors from previous, and longer, time periods. We collected feathers grown during two life-history stages (post-breeding and subsequent wintering) from individuals of two age classes of a rapidly declining migratory songbird, the cerulean warbler (Setophaga cerulea), on their breeding grounds and quantified CORT_f concentrations. We then monitored reproduction and survival of individuals and analysed relationships among CORT_f and age, body condition and future fitness. Compared with older males, second-year males had higher CORT_f concentrations during both stages. When controlling for age and year, body condition at capture was positively related to CORT_f concentrations from winter (especially for older birds). However, we found no relationships between CORT_f and fitness (as defined by reproduction and survival). Thus, elevated CORT may represent a beneficial physiological response (e.g. hyperphagia prior to migration), particularly for certain life-history stages, and may mediate the condition in which individuals transition between stages. But for those birds that survive migration, subsequent fitness is likely determined by more recent events and local conditions (i.e. on breeding grounds), which have the potential to counteract conditions from the winter.