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

BALZA, ULISES; LOIS, NICOLÁS A.; PIZZARELLO, GIMENA; LOPEZ, MARÍA EUGENIA; LOIS?MILEVICICH, JIMENA; RAYA?REY, ANDREA

IBIS

WILEY-BLACKWELL PUBLISHING, INC

Año: 2023

0019-1019

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 raptorsare relatively threatened species that rely on simplified food webs and usually co-occurwith invasive species. Here, we studied a population of Striated Caracara Phalcoboenusaustralis in Isla de los Estados (Argentina) to understand the relative contribution of foodavailability, topographical features and territoriality to breeding parameters. Caracaras inthe area depend on the short seasonal (i.e. pulsed) availability of seabirds for feedingtheir chicks. On the other hand, the recent introduction of non-native ungulates mayhave led to a suboptimal breeding environment by reducing Caracara nest-site availabil-ity within the seabird colonies via disturbance of the grassland. Using data from sevenbreeding seasons we analyse the abundance, the proportion of breeding individuals, thespatial structure of breeding events and the population-level breeding performance,including chick sex ratio, to study whether the breeding ecology of this species is stillshaped by marine food web pulses or could be associated with invasive herbivore distur-bance. The breeding adults represented only 6–15% of the population. The spatial pat-tern of nest-sites did not differ from random arrangement after considering theheterogeneous distribution of food availability. Territory identity accounted for most ofthe variation in clutch size and productivity, and penguin availability explained part ofthe variability in territory occupancy and hatching rate. The chick sex ratio was biasedtowards females and varied according to brood size, with the more productive territoriesproducing 91% of males. The breeding cycle of this population is still mainly dependentupon the Rockhopper Penguin Eudyptes chrysocome food pulse and presents mixed evi-dence of being disrupted by invasive herbivores. The studied factors contributed differ-entially to the output of each reproductive stage, with territory identity accounting formost of the variation. Our results suggest that individual-level, long-term data could berelevant to understanding the dynamics of this species in the Fuegian archipelago, andthus a broad-scale research programme is warranted to assess the conservation status andpropose future management scenarios for this species.