Body Condition Varies with Migration and Competition in Migrant and Resident South American Vultures

We investigated seasonal changes in the body condition of migrant and resident Turkey Vultures (Cathartes aura meridionalis and C. a. ruficollis), as well as Black Vultures (Coragyps atratus brasiliensis), in the Llanos of central Venezuela. Although the condition of adult migrants was below average in October and November following migration from the breeding grounds, it increased through the dry season to reach a peak before northward migration in March and April. Juvenile (FY; first-year) migrant Turkey Vultures followed a similar trend, but their condition improved more slowly than adults, perhaps due to intraspecific competition. The condition and mass of Black Vultures also increased through the dry season. This suggested that abundant carrion was available for vultures in the Llanos during the dry season. However, the condition of resident Turkey Vultures was below average when sympatric with migrants, whereas they were in above-average condition when migrants were absent. Unlike the linear relationship between condition and date in migrants, this relationship for residents was quadratic (second-order polynomial). Strikingly, condition of resident Turkey Vultures was negatively correlated with the density of Cathartes vultures (almost entirely migrants) during the study period. Migrant Turkey Vultures were significantly larger than resident Turkey Vultures in the majority of body measurements, explaining why they were dominant to residents in agonistic interactions at carcasses. These results may reflect the detrimental consequences of competition between migrant and resident Turkey Vultures on the condition of residents, and the physiological effects of migration on the condition of migrant Turkey Vultures.