Contrasting migratory ecology of two threatened and allochronic storm-petrels breeding in the Mexican Pacific

ABSTRACT: Migration is an essential life stage in many species, but is little understood in some groups, e.g. storm-petrels. Considering that storm-petrels reside in non-breeding areas for over half of their lifespan, identifying these areas is a priority for conservation efforts. Townsend’s Hydrobates socorroensis and Ainley’s storm-petrels H. cheimomnestes are 2 threatened sister species, breeding allochronically on Guadalupe Island (Mexican Pacific), for which migratory patterns are unknown. In this article, we describe the non-breeding areas of these 2 species, assess artificial light events recorded by geolocators, and describe the birds’ daily activity patterns. We deployed geolocators from 2021 to 2023 and modeled migratory routes using SGAT. We successfully tracked 7 Townsend’s and 4 Ainley’s storm-petrels over their non-breeding period. Townsend’s storm-petrels were found to travel to the south of the Baja California Peninsula and spent most of the time in Mexican waters, while Ainley’s storm-petrels migrated toward Hawaii and spent most of the time on the high seas. For the Townsend’s storm-petrels, 16.1% of their core areas are in protected waters, whereas for Ainley’s storm-petrel, only 0.7% of the core areas are protected, and 0.8% of those areas are recognized as key biodiversity areas (KBAs). Further, our findings indicate that both species are mainly nocturnal, making them highly susceptible to the impacts of light pollution; we detected 6 artificial light events. Our findings also support the hypothesis that divergence in the migration patterns between allochronic populations could be a crucial factor in sympatric speciation, which seems likely in seasonal environments like the northern Pacific.