Social Hierarchy of Scavenging Raptors on the Falkland Islands, Malvinas

Abstract

On the Falkland Islands (Malvinas), raptors historically were perceived as a threat to livestock, and consequently were widely persecuted through the mid-twentieth century. Conservation measures now minimize persecution and have facilitated increases in raptor populations, but the ecology of raptors on the Falkland Islands (Malvinas) remains poorly understood. We investigated social hierarchies within an assemblage of nonmigratory raptorial scavengers: Variable Hawk (Buteo polyosoma), Striated Caracara (Phalcoboenus australis), Southern Caracara (Caracara plancus), and Turkey Vulture (Cathartes aura jota). During thirty observation sessions of 30 min each, we recorded 1386 aggressive interactions involving one or more of these species. We found that Variable Hawks were aggressors in 98% (95% CI = 96–100%) of interactions with Striated Caracaras, 82% (69–95%) of interactions with Turkey Vultures, and 80% (72–88%) of interactions with Southern Caracaras. Southern Caracaras were aggressors in 100% of interactions with Striated Caracaras, and 90% (80–100%) of interactions with Turkey Vultures. Turkey Vultures were aggressors in 71% (61–82%) of interactions with Striated Caracaras. Within species, we found adult Southern Caracaras were aggressors in 78% (72–84%) of interactions with conspecific juveniles and 76% (68–85%) of interactions with conspecfic subadults. Adult Striated Caracaras were aggessors in 100% of interactions with conspecific juveniles and 97% (91–100%) of interactions with conspecfic subadults. Predicted patterns of size-based dominance typical of complex African and South American avian scavenger assemblages were not observed in the relatively simple assemblage of the Falkland Islands (Malvinas), but we did observe single-species groups of up to 83 Southern Caracaras and 42 Striated Caracaras.