Migratory birds complete the stages of the annual cycle (i.e., winter, migration, breeding, molt) in areas thousands of kilometers apart and events in one location can influence subsequent events. Understanding these “carry-over effects” is fundamental to identifying seasonal variation in mortality and limits on breeding output, both of which are important components of effective conservation and management strategies. Researchers are beginning to grasp the impact of carry-over effects from winter to breeding events, however less progress has occurred in understanding carry-over effects across the rest of the annual cycle. For example, studies on carry-over effects from breeding to molt or winter are infrequent. Additionally, we still lack an understanding of how winter to breeding carry-over effects experienced by adults impact the post-fledging period, one of the most limiting times for juvenal survival and recruitment. Studying carry-over effects in migratory birds presents an enormous challenge, as many complete their annual cycles in disparate places and can be difficult to track from one season to the next, especially small (<20g) songbirds. However, we can begin to fill knowledge gaps on carry-over effects through the use of intrinsic markers (i.e., measures of physiological state) and environmental tracers (e.g., stable carbon isotopes) from tissues (e.g., feathers, claws). These techniques can span the annual cycle, and when combined with breeding metrics expand our knowledge of carry-over effects to inform conservation strategies.
The overall objective of my dissertation is to fill gaps in our knowledge of carry-over effects in order to inform conservation initiatives, and specifically how they function in a species of conservation concern, the Prothonotary Warbler (Protonotaria citrea). The Prothonotary Warbler is a Neotropical migrant songbird that breeds in forested wetlands across eastern North America and winters in Central and South America. They currently face threats across the annual cycle from habitat loss, habitat alteration, and global climate change. To achieve this objective, I collected extensive breeding data (i.e., arrival date, breeding effort, success, and post-fledging survival) on a population of Prothonotary Warblers, located in central Ohio, from the Spring of 2015 through 2019. Additionally, in the winter of 2017 and 2018, I traveled to Panama to document winter habitat dynamics.
Beginning with the winter stage, I assessed within-patch survival and site persistence over a six-week period as the dry season progressed using radio-tagged birds in central Panama. Additionally, I compared age and sex ratios, metrics of condition (i.e., breast muscle, fat and condition), and feather molt among habitats. Site persistence was highest in wet/mangrove habitat compared to dry/non-mangrove habitat as seasonal drying intensified; however, the probability of survival did not differ among habitats. There was no difference in condition or fat among habitats however birds in dry/non-mangrove habitat were more likely to be young males, have less breast muscle, and higher intensity body feather molt. Finally, I provide evidence for a partial prealternate molt in Prothonotary Warblers, where at least some body feathers are replaced. This study is one of the first to demonstrate habitat-dependent movement in a non-territorial overwintering migrant songbird, and highlights the need to conserve intact, mature mangrove and lowland wet forests.
Next, I tested whether carry-over effects from winter habitat impact arrival timing, breeding phenology, and ultimately limit post-fledging survival through breeding phenology. I first validated the use of stable carbon isotopes (sampled from claw clippings) as a measure of habitat moisture (thereby quality) across a range of winter habitats in Panama and Colombia. Using this information, I then assessed the direct and indirect correlations between winter habitat, arrival timing, reproduction, and post-fledging survival. I found evidence for an interaction between winter habitat quality and climatic conditions, highlighting the importance of validating stable carbon isotope-habitat relationships as well as incorporating annual variation into studies of carry-over effects. Winter habitat quality influenced male Prothonotary Warbler arrival timing to breeding sites and indirectly predicted breeding outcomes and post-fledging survival, thus adding to the growing body of literature on carry-over effects from winter to breeding.
Finally, I investigated carry-over effects between breeding events, molt, and winter habitat. I examined the relationship between breeding, energetic effort and time investment, and physiological state, nutritional condition (i.e., feather growth rate) and stress (i.e., feather corticosterone levels) during post-breeding molt. I then tested whether nutritional condition and stress impact feather quality (i.e., feather area density) or winter habitat use (measured through stable carbon isotopes) during the spring pre-migratory period. Overall, I found that breeding events correlated with nutritional status and stress at the time of molt, and that they subsequently predict feather quality and future winter habitat quality. However, these relationships varied between years, sexes, and in some cases age, and may be impacted by environmental conditions and/or individual quality. In male Prothonotary Warblers, high breeding effort or investment may have consequences for feather quality and winter habitat use that impact spring arrival on the breeding grounds. Females with more intense breeding effort may not experience reductions in feather quality, however they may settle in lower quality winter areas in years with moist, cool conditions.
Combined, these results highlight the importance of understanding connection across the annual cycle, as events in one stage can impact outcomes in later stages and can be mediated through variation in climate. As anthropogenic climate change and deforestation continue to alter landscapes in the Neotropics, it is likely that the repercussions will be observed on the breeding grounds of many Neotropical migratory birds. This makes it critical that conservationist act now to preserve vital habitats for these species across the annual cycle, but especially in the Neotropics. For the Prothonotary Warbler specifically, impacts from climate change across their wintering grounds (projected warming and reduced precipitation) may be particularly pronounced as they are tied to moisture across the annual cycle and my results indicate that carry-over effects from both the winter and breeding seasons can have costs in subsequent stages. Additionally, changes in breeding productivity could cause consequences in subsequent seasons that carry-over to the next breeding season.