Carrying capacity of gravel pits of the Garonne floodplain for the Great Crested Grebe (Podiceps cristatus L.)

Human activities generate the destruction of natural wetlands, but also the creation of artificial ones e.g. rice fields, dam lakes or gravel pits. In the Midi-Pyrenees region (South West of France), the increasing number of gravel pits has allowed the colonisation and nesting of numerous species of water birds such. as the Great Crested Grebe (Podiceps cristatus L.). Howe ver, after a fast colonisation by the gravel pits, their abundance seems to be low and stable. A study of Great Crested Grebe populations was carried out during two years on two distinct sites of the Garonne alluvial plain. A nalysis of bird spatial distribution in relation with some environmental pa rameters has facilitated recognition of factors regulating grebe distributi on. During the nesting period, the presence of quite and undisturbed areas (e.g. island on the lake) and a high riparian vegetation density are the mo st important factors. In the same way, large open water area constitute sec ure habitats, particularly during the moulting period. In contrast, human d isturbance was always found prejudicial to the birds. These information wil l be useful for management purposes.