Variation in nest site occupancy and breeding outcome in a threatened Subantarctic raptor

ULISES BALZA; NICOLAS LOIS; GIMENA PIZZARELLO; MARÍA EUGENIA LOPEZ; JIMENA LOIS-MILEVICICH; ANDREA RAYA-REY

IBIS

WILEY-BLACKWELL PUBLISHING, INC

Lugar: Londres; Año: 2023

0019-1019

Breeding outcome in birds can be influenced by both intrinsic and extrinsic factors, and these may contribute differentially to each stage of the breeding cycle. Island raptors are relatively threatened species that rely on simplified food webs and usually co-occur with invasive species. Here, we studied a population of Striated Caracara (Phalcoboenus australis) in Isla de los Estados (Argentina) to understand the relative contribution of food availability, topographic features and territoriality to breeding parameters. Caracaras in the area depend on seabirds for feeding their chicks, and the recent introduction of invasive non-native ungulates may have reduced caracara nest site availability due to direct disturbance of the grassland. Using data from seven breeding seasons we analyse the abundance, breeding propensity, the spatial structure of breeding events and the population-level breeding performance, including chick sex-ratio, to study whether the breeding ecology of this species is still shaped by marine food web pulses or if it could be associated with invasive herbivore disturbance. The breeding adults in the population represented only 6-15% of the population. The spatial pattern of nest sites did not differ from random arrangement after taking into account the colony distribution of the Striated Caracaras’ main prey, the Southern Rockhopper Penguin (Eudyptes chrysocome). Territory identity accounted for most of the variation in clutch size and productivity, while penguin availability explained some of the variability in early stages of the breeding cycle . The chick sex ratio was biased towards females and varied according to brood size, with the more productive territories producing 91% of males. The breeding cycle of this population is still mainly dependent upon the Rockhopper Penguin pulse and presents little evidence of being disrupted by invasive herbivores. The studied factors contributed differentially to the output of each reproductive stage, with territory identity accounting for most of the output . Our results suggest that individual-level, long term data and information from outside the study area could be relevant to the dynamics of this species in the Fuegian archipelago, and thus a more broad-scale research program is warranted to understand the conservation status and future management scenarios of this species.