Foraging and nonbreeding segregation between allochronic populations of <i>Hydrobates (Oceanodroma) castro</i>

Abstract Geographical variation is a well-established characteristic in many seabird species, largely examined in morphometric traits and primarily during the breeding season. However, comparable variation also occurs during the nonbreeding period, particularly in relation to spatio-temporal oceanographic conditions. The Band-rumped (or Madeiran) Storm-Petrel Hydrobates castro complex represents a widely distributed taxon occurring across both the Atlantic and Pacific Oceans, with unresolved taxonomy. Because local adaptation to distinct oceanographic conditions in spatially segregated populations may promote ecological differentiation, such information can help elucidate lineage divergence in closely related taxa. In this study, we deployed miniaturized geolocation loggers (GLS) on 35 individuals from 4 North Atlantic populations to investigate nonbreeding movements, migratory routes, and areas of spatial occupancy. Specifically, we aimed to characterize phenological schedules at high temporal resolution, identify principal foraging zones during both breeding and nonbreeding periods, and quantify spatio-temporal overlap among taxa. We found near-complete spatial segregation between breeding and nonbreeding ranges in the H. castro from 4 different populations breeding at 2 localities in the north-east Atlantic, each including a summer and a winter-breeding population. These populations remained effectively allopatric throughout the annual cycle. These findings are discussed in the context of ongoing taxonomic debates within the H. castro species complex.