Environmental factors associated to group formation in a communal roosting bird, the Andean condor

LAMBERTUCCI, S. A.; RUGGIERO, A

San Carlos de Bariloche, Argentina

Congreso; VI Southern Connection Congress; 2010

Universidad Nacional del Comahue

Group size in animals is a dynamic variable that may respond to environmental factors and internal group forces. We used a communal roosting bird, the Andean condor (Vultur gryphus), as model species to test how group size in communal roosts may respond to environmental factors representative of four hypotheses: room availability, climatic refuge, insolation and size of food eaten. We estimated the number of condors that roost daily (=group size) at each of seven communal roosting places in north-western Patagonia, Argentina. A total of 2082 groups were observed over 1042 days of survey. Partial regression analysis was used to partial out different components of group size variation. Critical, mean and median group size varied from 7-28, 4-28, 2-24 individuals respectively, and increased in larger roosts with rainy and colder climatic conditions, low seasonality in precipitation and temperature and moderate levels of insolation. The shared effects of room availability and climate explained a greater proportion of group size variation that either of them considered separately. The size of food items locally consumed had little influence on group size variation. Temperature was the main factor affecting group size along the year. In general, group size increased with unfavourable conditions, but some seasonal variations exist. The Andean condor supported the climatic refuge hypothesis as suitable explanation to account for aggregation patterns in communal roosting birds in temperate regions. Our analysis emphasizes the importance of exogenous climatic factors to account for group formation in social birds.