Foraging and migratory ecology of tropicbirds (Phaethontidae)

[eng] In polar and temperate regions, strong seasonality in environmental conditions often drives animal phenology, resulting in population-wide synchrony in the timing of critical biological events such as breeding and migration. The association between phenology and seasonality is less pronounced in tropical systems, where environmental conditions remain relatively constant throughout the year. As a result, many species exhibit asynchronous or year-round breeding patterns. This leads to the question of whether these species respond to seasonal changes, which may remain present albeit to a lesser extent, and what shapes their phenology. To assess the effects of seasonality on tropical species, we investigated the foraging and migratory ecology of the Red-billed Tropicbird (Phaethon aethereus), a poorly studied pantropical species that breed year-round in Cabo Verde, between 2017 and 2024. Along four chapters, this thesis presents novel insights into how tropicbirds cope with seasonal changes in resource availability and environmental conditions using a combination of biologging (GPS, GLS-immersion loggers, time-depth recorders (TDR), and accelerometry), nest monitoring, and diet analyses. In the first chapter, we used auxiliary biologging data from immersion loggers, TDR and accelerometry to evaluate and semi-supervise Hidden Markov Model to classify tropicbirds' behaviors at sea based on tropicbird GPS tracks. We found that although overall classification accuracy greatly improved with semi-supervision, these models failed to capture tropicbird the foraging state, and give a word of caution on using these models to classify behaviors in other opportunistic foragers. Building on the methodological insights of this first chapter, the second chapter of this thesis focuses on the effects of seasonality on the foraging behavior of tropicbirds during the breeding season. In this chapter, we found seasonal patterns in foraging behavior, occupancy, and diet, which affected fitness metrics. We relate these patterns to ...