Ecological segregation of two superabundant, morphologically similar, sister seabird taxa breeding in sympatry

Prions Pachyptila are the most abundant seabirds in the Southern Ocean and comprise two main groups: those with and without bill lamellae to filter zooplankton. With few exceptions, each breeding location supports at most one species from each of these groups. However, Gough Island supports two morphologically very similar, filter-feeding species: broad-billed P. vittata and MacGillivray’s prions P. macgillivrayi. To understand how these two species co-occur in sympatry, we compared the foraging ranges, habitat selectivity, trophic segregation and moult schedules of these species using combined geolocation-immersion loggers. After breeding, both species showed a well-defined westward migration prior to moulting. Moult lasted 11–19 weeks and was significantly longer in MacGillivray’s than broad-billed prions. Moulting birds occurred in specific areas within the Argentine Basin, with little overlap between the two species. Habitat analysis revealed species-specific preferences, in particular sea surface temperature. Activity patterns also differed; MacGillivray’s prions spent more time in flight, which indicates a more active foraging strategy, relying less on filter feeding. Stable isotope ratios (δ¹⁵N) in flight feathers were greater in MacGillivray’s prion, which is consistent with its less specialized bill morphology resulting in feeding at a higher trophic level. Inter-specific spatial segregation was observed for most of the tracking period, in large part because broad-billed prions breed roughly 3 months earlier than MacGillivray’s prions. At Tristan da Cunha, 250 km farther north, where only broad-billed prions breed, they departed, moulted and returned significantly later (15–17 days) than conspecifics from Gough Island, providing evidence for character displacement in sympatry with MacGillivray’s prion.