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

BALZA ULISES; LOIS, NICOLÁS; PIZZARELLO, GIMENA; LOPEZ MARÍA EUGENIA; LOIS-MILEVIVICH JIMENA; 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 may26 contribute differentially to each stage of the breeding cycle. Island raptors are relatively threatened27 species that rely on simplified food webs and usually co-occur with invasive species. Here, we studied a28 population of Striated Caracara (Phalcoboenus australis) in Isla de los Estados (Argentina) to understand29 the relative contribution of food availability, topographic features and territoriality to breeding30 parameters. Caracaras in the area depend on the short seasonal (i.e., pulsed) availability of seabirds for31 feeding their chicks. On the other hand, the recent introduction of non-native ungulates may have led to a32 suboptimal breeding environment by reducing caracara nest site availability within the seabird colonies33 via disturbance of the grassland. Using data from seven breeding seasons we analyse the abundance, the34 proportion of breeding individuals, the spatial structure of breeding events and the population-level Review Copy35 breeding performance, including chick sex-ratio, to study whether the breeding ecology of this species is36 still shaped by marine food web pulses or if it could be associated with invasive herbivore disturbance.37 The breeding adults in the population represented only 6-15% of the population. The spatial pattern of38 nest sites did not differ from random arrangement after considering the heterogeneous distribution of food39 availability. Territory identity accounted for most of the variation in clutch size and productivity, while40 penguin availability explained part of the variability in territory occupancy and hatching rate. The chick41 sex ratio was biased towards females and varied according to brood size, with the more productive42 territories producing 91% of males. The breeding cycle of this population is still mainly dependent upon43 the Rockhopper Penguin food pulse and presents mixed evidence of being disrupted by invasive44 herbivores. The studied factors contributed differentially to the output of each reproductive stage, with45 territory identity accounting for most of the variation. Our results suggest that individual-level, long term46 data could be relevant to understanding the dynamics of this species in the Fuegian archipelago, and thus47 a broad-scale research program is warranted to assess the conservation status and propose future48 management scenarios for this species