Resource sharing and niche overlap in some weaver birds of the Genus Ploceus and Malimbus at Ife, Nigeria

ummary Interspecific tree sharing and niche overlap was studied in six weaver bird species. The three species pairs which shared nest trees were P. cucullatus (35 colonies) and P. nigerrimus (34 colonies), M. rubricollis (71 colonies) and P. tricolor (8 colonies), M. scutatus (22 colonies) and M. nitens (12 colonies). In the first species pair high niche overlap values along all four dimensions (habitat, nest height, nest materials and diet) contradicted Pianka's (1974) niche overlap hypothesis that high overlap is the result of reduced competition. At an inter-generic level his theory appeared applicable because high overlap values along one dimension followed low values along others. May & MacArthur (1972) predicted an inverse relationship between niche overlap and intensity of competition. Low alpha (a) values of nest materials (M. rubricollis-P. tricolor) and nest height (M. scutatus-M. nitens) fitted their explanation because competition must have caused complete distinctness in the two axes. Population assessment provided evidence that under shared colony conditions, only one species numerically dominated. Hence competition risk was reduced even though overlapping of certain niche dimensions allowed for the possibility of competition. The beaks of all six species are not specialized for diet although Ploceus spp. is granivorous and Malimbus spp. and P. tricolor are insectivorous. Within the malimbes segregation along this axis was incomplete while only partial niche segregation was evident between the two groups. Ecological separation of P. tricolor nests was considered to be the result of competition from P. cucullatus and P. nigerrimus. Resume Le partage des ressources et le recouvrement des niches ecologiques de certains tisserins des genres Ploceus et Malimbusa Ife, Nigeria. On a etudie le partage des arbres et le recouvrement des niches ecologiques de six especes de tisserins. Les trois paires d'especes qui construisent leur nid dans les memes arbres sont P. cucullatus (35 colonies) et P. nigerrimus (34 colonies), M. rubricollis (71 colonies) et P. tricolor (8 colonies), ot M. scutatus (22 colonies) et M. nitens (12 colonies). Dans la premiere paire, un recouvrement important de la niche ecologique a quatre points de vue (habitat, hauteur et materiau du nid, et regime alimentaire) contredit I'hypothese de Pianka (1974) selon laquelle un recouvrement important est le resultat d'une competition restreinte. Au niveau intergenerique, sa theorie semble applicable parce que de hautes valeurs de recouvrement dans un domaine coincident avec de basses valeurs dans d'autres. May et MacArthur (1972) predisaient une relation inverse entre le recouvrement de la niche ecologique et l'intensite de la competition. Des valeurs alpha (a) basses pour le materiau du nid (M. rubricollis-P. tricolor) et pour sa hauteur (M. scutatus-M. nitens) s'accordent parfaitement a leurs explications parce que la competition doit avoir cause une distinction complete dans ces deux domaines. L'evaluation de la population montre a l'evidence que, dans les conditions d'une colonie mixte, une seule espece domine numerique-ment. D'ou le risque reduit de competition meme lorsque le recouvrement de certains aspects de la niche ecologique aurait pu creer une situation de competition. Les becs des six especes ne sont pas specialises; cependant, Ploceus spp. est grani-vore et Malimbus spp. plus P. tricolor sont insectivores. Entre les malimbes, la segregation alimentaire est incomplete, et I'on ne constate qu'une segregation partielle de la niche ecologique entre les deux groupes. La separation ecologique des nids de P. tricolor semble etre le resultat de la competition entre P. cucullurus et P. nigerrimus.