Ecological separation of small cathartid vultures in South America
During three field seasons, totalling 20 months between 1984-87, the ecological separation of four species of small Cathartid vultures was investigated at Hato Masaguaral in the flat savannas or Llanos of central Venezuela, South America. The focus of the study was an analysis of the role of migrant Turkey Vultures Cathartes aura meridionalis, in the scagenging guild of resident vultures, particularly in relation to a resident subspecies of Turkey Vulture C. a. ruficollis. Ecological separation between different taxa was achieved by differential habitat use by foraging vultures, feeding on different types of carcases and at different times. Where mixed-species flocks fed at carrion, social dominance mediated competition; heavier taxa were not necessarily dominant. During the dry season, migrant Turkey, Lesser Yellow-headed Cathartes burrovianus and Black Vultures Coragyps atratus foraged at highest densities in open savanna habitats. By contrast, resident Turkey and King Vultures Sarcor amphus papa foraged almost entirely in closed gallery forest. In the wet season, the numbers of Cathartes vultures declined by 6 fold in the absence of migrant Turkey Vultures which returned to North America. There was a distinct habitat shift in resident Turkey Vultures at the end of the dry season from gallery forest to open savanna habitats; conversely at the end of the wet season birds moved back into the gallery forest. Some residents may themselves have been migratory, moving into the ranch area at the end of the dry season, but this did not explain the decrease of Cathartes vulture density between seasons. The changes coincided with the departure of the majority of migrants in the spring and with the influx of migrants in the autumn, respectively. The core areas used by radiotragged resident Turkey Vultures shifted seasonally from gallery forest to open savanna. Most marked migrant Turkey Vultures were not resighted, suggesting that they passed through the study area on their way further south or that they had large home ranges. Compared to migrant Turkey Vultures a larger proportion of resident Turkey and Black Vultures were resighted. Comparative observations at bait sites in gallery forest and open savanna showed that carcases were detected more quickly, group sizes of migrant Turkey Vultures were greater, and agonistic encounter rates were higher in the open. Both migrant and resident Turkey Vultures occurred at similar densities over the forest but most carcases were first located by residents. Migrants won almost all agonistic encounters initiated against resident Turkey Vultures. The feeding rate of resident Turkey Vultures was significantly and negatively affected by the numbers of migrant Turkey and King Vultures, but not by other residents present at carcases. While the body condition of migrant Turkey Vultures was low after autumn migration from North America, the condition of birds trapped in subsequent months improved. However, the body condition of residents was below average throughout the dry season when migrants were present. By contrast, in the wet season when migrants were absent, the body condition of residents was above average. The wing-loading of different taxa was related to their foraging strategies; low wing-loading enabled some taxa (e.g. resident Turkey Vultures) to fly at low altitude over vegetation and forage using olfaction. Others, with higher wing-loading depended on sight for foraging (directly by observing carcases in the open or by watching the activities of other vultures) and on thermals to remain aloft (e.g. Black Vultures). Results suggested that, although there may be seasonal changes in food supply which may explain wet season changes in foraging behaviour, resident Turkey Vultures were forced to forage in gallery forest during the dry season to reduce interference competition from migrants.