Alternative Strategies in an Avian Scavenger Guild and Their Conservation Implications

Abstract

This dissertation examines the alternative strategies used by a diverse eight species avian scavenger guild and how these enable their coexistence in Masai Mara National Reserve, Kenya. Methods include roadside transects, counts and behavioral observations of scavenger species at both natural and experimental carcasses, and a movement study using GSM-GPS telemetry. Findings suggest high association among species with similar beak morphology at natural carcasses demonstrating that resource partitioning is insufficient to explain coexistence. Instead, a series of alternative behavioral strategies occur across spatial and temporal gradients in resource availability, that occur seasonally, locally, daily, and regionally. Seasonally, social species move to areas of high food availability. As a result, higher competition caused by these social species appears to offset increases in food availability during the dry season, forcing solitary species to search throughout the day. On a local scale, trade-offs between individual dominance versus social dominance and search efficiency versus competitive ability enable socially dominant species and species with high search efficiency to use the best quality habitats, typically areas with high wildlife density. Across regional scales, variation in habitat use among Gyps vultures enables coexistence. In general, movement of competitively dominant vulture species is linked to prey mortality rather than abundance.

An understanding of alternative strategies employed by different avian scavengers has important implications for their conservation. All avian scavenger species except Bateleurs are found to be declining dramatically within and around Masai Mara National Reserve, Kenya. Poisoning, the primary threat to vultures, is expected to cause regional declines for species, such as Lappet-faced and Gyps vultures, whose ranges extend beyond protected areas. Subordinate species with low search efficiency, such as Hooded vultures and Tawny eagles, disproportionately use areas of poor quality, such as those of high human settlement density and are thus at greatest risk of poisoning. Social species, such as Gyps vultures, depend on high ungulate mortality rates and are thus most likely to be impacted by on-going wildlife declines. Management actions to prevent poisoning and continued monitoring of vulture populations in Masai Mara National Reserve will be critical steps in the conservation of avian scavengers.