The effect of seasonality on the foraging behaviour and breeding success of a tropical marine top predator

• Foraging and breeding success of tropical seabird vary seasonally. • They foraged further during twilight to consume more squid in the dry season. • Higher nest occupancy, chick condition, and breeding success in the dry season. • Potential drivers: seasonal squid availability, nest-site suitability, foraging costs. In polar and temperate regions seasonality in environmental conditions is an important driver of animal phenology. In tropical systems, where the environment is relatively homogeneous year-round, the link between phenology and seasonality is weakened, and many species breed asynchronously or even year-round. This leads to the question of how these species adapt to seasonal changes, which remain present albeit to a lesser extent. To assess relationships between foraging plasticity and seasonal changes in oceanographic conditions and resource availability, we investigated the foraging ecology of the red-billed tropicbird ( Phaethon aethereus ), a poorly studied pantropical species that breeds year-round in Cabo Verde. From 2017 to 2022, we monitored tropicbird nests at three islands (Boavista, Sal and the Cima Islet) and GPS tracked 907 foraging trips from 329 adults to evaluate seasonality in nest occupancy, foraging patterns and efficiency, chick growth, chick and adult body condition, and breeding success, alongside links to seasonal changes in resource availability and environmental conditions. We found seasonal patterns in foraging behaviour, with individuals taking more distant and less sinuous foraging trips, using a larger core foraging area, spending more time foraging during twilight, and consuming more squid in the dry season (December-June), than in the wet season (July-November). Moreover, nest occupancy, chick body condition and breeding success were higher in the dry season. We suggest that the observed seasonal patterns are related to an increase in the availability of squid at the end of the dry season, and a decrease in the nest-site suitability and an increase in weather-related foraging costs in the wet season. Our results provide some of the first in-depth knowledge on the seasonal variation in foraging behaviour of a tropical seabird species, suggesting seasonality in tropical systems may be a stronger driver of the movements of top predators than previously thought.