Autilio, Anna – Gang Behavior at Carcasses in Wintering Striated Caracaras (Phalcoboenus Australis) in the Falkland Islands. 2016.
The Striated Caracara (Phalcoboenus australis) or “Johnny Rook” is a highly social, scavenging raptor native to the Falkland Islands and Tierra del Fuego. The IUCN Near-Threatened status and highly restricted range of this bird have not allowed it to be a subject of intense study, and much is still unknown about their ecology. During the breeding season, Johnny Rooks are reliant on seabird colonies, but during winter when seabirds are scarce, the caracaras must forage differently, which puts different age classes and individuals in conflict, with each other and with the Falklands’ sheep farmers. They can be quite curious and destructive!
The birds’ tolerance of human proximity and reliance on human settlements as winter foraging grounds makes them an ideal species with which to study carcass consumption over the entire period of a carcass’s availability. Juvenile caracaras have been observed foraging in groups, and exhibiting “gang” behavior, similar to rooks and ravens, in order to compete with the more aggressive adults. I hypothesize that juvenile vocalization instigates this grouping behavior, and that this behavior is an adaptive strategy for juvenile survival through their first few winters, when raptor mortality is highest. This type of study of social winter feeding dynamics is a tactic chosen by very few solitary raptor species.
My research was conducted on Saunders Island, located in the northwest Falkland Islands. Saunders is the 4th largest island in the Falklands, and though no Johnny Rooks are known to breed there, during the winter groups of 50 or more birds forage in and around the only human settlement on the east side of the island. My goal is to better understand the feeding ecology of this population, in order to reduce the conflict these near threatened birds have with farmers in West Falkland.
Gang Behavior at Carcasses in Wintering Striated Caracaras (Phalcoboenus australis) in the Falkland Islands
Avian scavengers perform vital ecosystem services by removing waste and by slowing disease. Yet few details are known about the purpose or purposes of social interactions near carcasses, and their role in the physical depletion of carcasses (Gangoso et al. 2013). The globally Near-Threatened Striated Caracara is a social scavenging falconid that relies on seabird colonies for food during the breeding season in the Falkland Islands, a principal stronghold in its range. The birds have been persecuted as livestock pests since the late 1800s. Although the population is now protected and remains stable, it does not appear to be growing. The caracaras’ reliance on human settlements as winter foraging grounds makes them an ideal species to examine age-stratified consumption over the entire period of a carcass’s availability. By providing and closely monitoring experimental carcasses, I estimated the mass of food consumed per bird over five minute intervals (mean 22.9 g/bird/5 min, ±1.2, SE), and found that this was negatively influenced by the time of arrival to the carcass, positively by the total numbers of birds feeding on the carcass, and positively by the proportion of the group that was adult birds. I also found that, as previously shown in Common Ravens, aggregation of a group to a carcass can be accelerated by vocalizations of the birds (Heinrich and Marzluff 1995). My data indicate that the so-called gang behavior in juvenile caracaras is very similar to that in ravens, and is an adaptive strategy to overwhelm adults at ephemeral resources and obtain food.