Ecology of a marginal Carrion Crow population. I. — Distribution and abundance

Résumé En

A low density population of the Carrion Crow Corvus c. corone was censused in the Camargue delta region, southern France. Three methods were used : (A) a line transect along 45 km roadside, (B) mapping of the crows on a 16 km² study plot by car, and (C) a complete survey revealing all resident crows during the breeding season on the same plot. Accuracy of the two estimates (A) and (B) was checked with those of method (C). This revealed that 7 censues gave accurate results for both method (B) and for the main belt census sector for method (A). Fortnightly counts using methods (A) and (B) served to monitor number, group size and habitat use of the crows during one whole annual cycle. Size and composition of the population of the study plot, which lays entirely in marshland did not show significant differences between seasons, while a significant increase in number of crows in winter and larger group size and a preference of open cultivated land during the moult/dispersal period, was found along the roadside circuit. Extrapolation of crow distribution and density for the whole Camargue region (750 km²) revealed to be difficult because neither methods (A) and (B) gave results about the population structure and percentage of non-breeders. It is concluded that only repeated surveys (C) in different habitat could provide accurate results.