Dark and Light Black-capped Petrels Breed in Different Places and Months
Satellite tracking reveals that dark and light forms breed in different mountains and on different schedules.
A seabird with two identities
New satellite tracking has provided the strongest evidence yet that the two recognised forms of the Black-capped Petrel Pterodroma hasitata - the dark form and the light form - breed in different regions of Hispaniola and on different schedules. The findings help explain the species’ strong genetic structure and may illuminate an early stage of speciation in one of the Caribbean’s most threatened seabirds.
The team tracked two males captured at sea in the western North Atlantic, one of each form. Both birds migrated south to Hispaniola, but they settled in different mountain regions and visited potential nest sites more than a month apart. These results align with earlier suggestions that colour forms differ markedly in both geography and timing of breeding.
The Black-capped Petrel, locally known as the Diablotin, is globally Endangered and suffers ongoing pressures from deforestation, forest fires and predation by introduced mammals. Understanding whether the two forms represent distinct breeding populations is now considered essential for conservation planning.
Different mountains, different months
The light-form petrel visited the central Dominican Republic in early October, strongly indicating breeding activity in Valle Nuevo National Park. In contrast, the dark-form petrel reached south-eastern Haiti and showed activity near the La Visite escarpment from mid to late November.
The two probable nest sites lie roughly 180 km apart across the highlands of Hispaniola. The difference in timing - more than five weeks - is consistent with earlier evidence from moult patterns and observations at sea.
These contrasting breeding schedules - known as allochrony - can act as potent isolating mechanisms, reducing opportunities for interbreeding and enabling population divergence.
A hidden breeding season revealed
Because both petrels were tracked with solar-powered satellite transmitters, the researchers used drops in battery voltage and gaps in satellite communication to infer when birds were underground in burrows. Combined with location estimates, these patterns identified likely periods of nest attendance even when the birds were not transmitting.
This approach has proved particularly valuable for species that nest in dense montane forest, where direct access to breeding sites is extremely difficult. For the Black-capped Petrel, where only around 140 burrows are currently known, such insights are a major boost to conservation science.
Conservation implications
The study reinforces the view that the light and dark forms of the Black-capped Petrel behave as separate breeding populations with distinct schedules and landscapes. If confirmed widely, these differences would mean that threats and management needs may also differ between forms.
Breeding sites in both Haiti and the Dominican Republic remain vulnerable to forest loss, fire, and disturbance. Accurate knowledge of migration timing, burrow attendance and connectivity to at-sea foraging areas will be key to protecting both forms.
Although only two petrels yielded full tracking data, the approach demonstrates the potential for future work to uncover undiscovered colonies and to deepen understanding of a seabird still shrouded in mystery.
November 2025
Read the full paper here
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