Contrasting foraging strategies of seasonally segregated populations of the band-rumped storm-petrel at St Helena, South Atlantic

Allochrony can be a cause or consequence of speciation, either creating temporal reproductive isolation that reduces gene flow between diverging populations or reinforcing divergence that has already occurred through geographic isolation. The former appears to apply to band-rumped storm-petrels (Hydrobates castro) at some breeding sites, where there are genetically differentiated hot and cool season breeding populations. It is unclear, however, whether seasonally segregated but genetically similar populations retain the same habitat preferences or whether divergence in foraging behaviour is associated with the process of allochronic speciation. We quantified the foraging distribution of band-rumped storm-petrels at St Helena, the largest known breeding colony in the South Atlantic at which hot and cool season breeders do not appear to be genetically differentiated. Fifty-four GPS tags were deployed on experienced breeders across two hot and two cool breeding seasons. We compared foraging trip parameters, foraging effort and examined whether environmental (oceanographic and atmospheric) conditions and habitat selection varied between seasonal populations. Long foraging trips lasted up to 9 days and involved travel distances of up to 3,285 km. The trip durations and distances were similar between the two seasonal populations, but directions differed markedly, resulting in pronounced differences in at-sea distributions. Adults breeding in the cool season foraged across ~ 619,000 km2 southeast of St Helena selecting warmer waters (~ 23.1 ± 0.7 °C). In the hot season, adults used a similarly sized area (~ 600,000 km2) to the southwest, but selected cooler waters (~ 21.2 ± 0.4 °C) even though overall conditions at unused but available locations were warmer (~ 23.7 ± 0.7 °C) than in the cool season (~ 20.6 ± 0.5 °C). Seasonal differences in oceanographic conditions likely force hot season breeders to select cool nutrient-rich waters, whereas cool season breeders may select wind or temperature conditions that minimise travel or thermoregulatory costs. This clear segregation in foraging range and habitat selection suggests that the divergence in at-sea distributions between two genetically similar seasonal breeding populations may contribute to allochrony and ultimately to sympatric speciation in the band-rumped storm-petrel at St Helena and elsewhere.