Ecologie de migrateurs tropicaux dans une zone préforestière de Côte-d’Ivoire

Abstract

Rollers (Coraciadidae) and Bee eaters (Meropidae) have been studied during three years in a guinean savanna — with galleries and patches of dense forest — in the Ivory Coast. Annual cen- suses of the breeding populations of savanna Rollers have been made by dilferent methods. For the two species a mean density of 1 individual per 4 or 5 hectares has been recorded with a production of young of 22 to 26 %, mainly in May. Three pairs of related species (Merops gularis and M. albi-collis ; Eurystomus gularis and E. glaucurus ; Coracias cyano-gaster and C. naevius) are compared from an ecological and physiological stand-point. The first-mentioned species of each pair is sedentary, the second migratory. Two of the three sedentary species are forest birds, while all the migrants are savanna birds, with the exception of M. albicollis which also lives in cleared and cultivated forest. Competition is avoided between the two populations through the occupancy of separate ecological niches. Congeneric species live in dilferent biotopes (or at least vegetation layers). Sedentaries hunt more easily in and around dense vegetation (adaptation to the conditions prevailing during the wet season), while the migrants prefer the most open spaces. The diet of the resident birds is more varied, the migratory species being more specialised in their feeding on swarms of winged ants and termites. Migratory species are much better adapted to the exploitation of this seasonal production or to the appearance of newly accessible prey (ground insects on recently burnt patches of savanna), being able to move rapidly from one place to another and to concentrate locally in large numbers. Dates of arrivals and departures are compared with the calendar of rains and it is suggested that the timing of the migratory movements in the northern part of the breeding area of these birds is more under the dépendance of the seasons than influenced by local conditions. But it is possible that the rainy season in the Ivory Coast would be too wet to allow a good survival for the populations of these species ; this could also explain the disappearance of our breeding Eurystomus glaucurus from June to September. Merops albicollis, a typical intra-african migrant, shows a premigratory fattening of about 7 %. This occurs at the end of the prenuptial moult and coincides with the beginning of the increase in the size of the gonads.