Role of exotic tree stands on current distribution and social behaviour of Swainson's hawk Buteo swainsoni in the Argentine pampas

SARASOLA, J.H.; NEGRO, J.J.

JOURNAL OF BIOGEOGRAPHY

WILEY-BLACKWELL PUBLISHING, INC

Año: 2006 vol. 33 p. 1096 - 1101

0305-0270

Aim: The Argentine pampas was extensively and abruptly altered by European colonization. Between 1880 and 1885 the indigenous human inhabitants were completely displaced and native grasslands were replaced by exotic pasture plants and crops. One of the most important ecological changes in the pampas landscape, the introduction of tall exotic tree species, has received little attention and its effects on wildlife has never been assessed. We have made an intensive survey of habitat use of Swainson´s hawk Buteo swainsoni in their most important non-breeding quarters, the pampas of Argentina, aiming to characterize the sites used by hawks for communal roosting. Location: Pampas grasslands, Argentina. Methods: We surveyed 30,000 km of roads by car during the austral summer from 2001 to 2004, covering the main non-breeding area occupied by Swainson´s hawks. Their roost sites were located by direct observation of birds roosting in tree stands close to the roads, by surveying potential roost sites around places where groups of pre- and post-roosting hawks were recorded and by gathering information from local farmers. Results: Swainson´s hawks exclusively used stands of exotic tree species for roosting. Eucalyptus viminalis (Labill. 1806) was present in all 34 roosts surveyed and in 59% of them it was the only species present. The remaining exotic tree species were the Siberian elm Ulmus pumila (Linneaus 1753), pines (Pinus spp.) and cypress (Cupressus spp.). Flock sizes at roost sites were unusually high for a raptor, with an average of 658 individuals (range 8-5000 hawks, n = 27 flocks). Main conclusions: The introduction of exotic trees may have resulted in the expansion of the suitable habitat for Swainson´s hawks, permitting a recent colonization of the Argentine pampas. Tree stands may have also changed communal roosting behaviour of this raptor, mediated by these new structural elements in a region that almost completely lacked trees prior to European occupation.