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

Abstract

Breeding outcome in birds can be influenced by both intrinsic and extrinsic factors, which 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, topographical features and territoriality to breeding parameters. Caracaras in the area depend on the short seasonal (i.e. pulsed) availability of seabirds for feeding their chicks. On the other hand, the recent introduction of non-native ungulates may have led to a suboptimal breeding environment by reducing Caracara nest-site availability within the seabird colonies via disturbance of the grassland. Using data from seven breeding seasons we analyse the abundance, the proportion of breeding individuals, 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 could be associated with invasive herbivore disturbance. The breeding adults represented only 6–15% of the population. The spatial pattern of nest-sites did not differ from random arrangement after considering the heterogeneous distribution of food availability. Territory identity accounted for most of the variation in clutch size and productivity, and penguin availability explained part of the variability in territory occupancy and hatching rate. 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 Eudyptes chrysocome food pulse and presents mixed 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 variation. Our results suggest that individual-level, long-term data could be relevant to understanding the dynamics of this species in the Fuegian archipelago, and thus a broad-scale research programme is warranted to assess the conservation status and propose future management scenarios for this species.