Conséquences de la protection du héron cendré sur sa dynamique de population et sur ses stratégies d’occupation de l’espace en France

Abstract

Before being afforded legal protection (1975), the French Grey Heron population (ca = 3 400 pairs) was limited by hunting, and the nesting colonies (N = 110) were located on the safest available sites ; the «floating population» of the species was also apparently small. Protection led to an increase in the number of breeding pairs (from 9 500 in 1981 to about 13 000 in 1985). This population increase can only in part be ascribed to better survival rates (which increased from 0.55 to 0.63 for adults, and from 0.35 to 0.49 for immatures, between 1971-75 and 1977-82). Newly recruited breeders first settled on old breeding sites, which quickly became «saturated», and then started establishing new colonies. The settlers of the new sites were provided both by birds from the largest «old» colonies (presently declining in size), and by the «floating population». The present Grey Heron population (240 colonies in 1985) is now more or less food regulated, and each colony seems strongly limited by the number of individual feeding territories available in the foraging area of the colony. Radiotracking shows that most breeding Grey Herons have only one individual feeding territory, located at 2 to 38 km from the colony ; the birds which have feeding territories closer to the colony breed earlier than those which have to travel farther to feed.