Red-backed Hawks supply food to scavenging Chimango Caracaras

Abstract

Positive interactions between species of terrestrial vertebrates occur frequently and it is common among birds. However, such interactions have generally been overlooked in raptor field studies. We analysed interactions between Red-backed Hawks (Buteo polyosoma) and Chimango Caracaras (Milvago chimango) in order to determine the characteristics of the association between these two South American raptors. We observed 171 bouts of activity, 40 when Red-backed Hawks were feeding and 131 when the Hawks were undertaking other activities. Chimango Caracaras were more often present when Red-backed Hawks were feeding (81%) than during other activities of Red-backed Hawks (69%), but they attacked Red-backed Hawks more frequently during other activities of the Hawks (66%) than when the Hawks were feeding (43%). In addition, the rate of attacks by Chimango Caracaras was higher during other activities of the Hawks (mean = 2.21 attacks per Caracara) than when the Hawks were feeding (mean = 0.68 attacks per Caracara). Duration of feeding activities was not affected significantly by the presence of Chimango Caracaras or by attacks by the Caracaras. During 16 of 40 feedingbouts of Hawks, an average of 1.81 ± 0.23 (s.e.) Caracaras fed on prey discarded by the Hawks. Thus, Red-backed Hawks indirectly supplied food to Chimango Caracaras, which, in turn, reduced their agonistic behaviour towards Hawks. Our findings represent one of few cases reported about facilitative associations among raptors, but it seems likely that it represents a common strategy among these birds.