Les particularités démographiques et socioécologiques du Coliou strié dans le Nord-Est du Gabon I. - Données d'observation

Abstract

A population of speckled mousebirds Colius striatus was colour-marked in Makokou, North-East Gabon, and studied from 1973 to 1977. Two different types of social groupings were found : (1) sedentary family groups, generally consisting of 1 to 3 adult males, 1 to 3 adult females, 1 or 2 young bachelor males, and sometimes one «satellite » adult female, and (2) nomadic groups of 2 to 7 birds composed either of adult (mostly male) birds, or of young of the year (again mostly male). Such nomadic groups are most frequently seen in between the two annual breeding seasons. The annual home range of a sedentary group varies from 10 to 15 ha. It contains a core area of about one hectare where the group roosts, breeds and takes its «dust baths», and which is defended against conspecific intruders. During the most favorable environmental conditions of the rainy season, sedentary mousebirds reach an average breeding density of 1.3 bird/ha, subsequently increasing their numbers to 3 birds/ha later during the dry season. During the dry «lean period » 55 to 62 % of the birds wander through the whole area and/or move away to colonize marginal habitats or patches of secondary growth scattered in the surrounding rain-forest area. Males predominate among young and subadult speckled mousebirds ; up to 75 % males being found among subadults. However, the sex-ratio is more balanced among adults (57.5 % males), and is quite evenly divided among breeding individuals. The modal clutch-size is 2 eggs, but a female may lay clutches 4 to 8 times successively during a single year. The mortality rate during incubation and nestling stages is high (76.5 %), but much lower among sedentary adults which live more than 5 years in the study area. Whereas all young females of the year leave their nesting grounds and scatter beyond the study area, one of the young males will often stay within the parental family group for a further 1 to 3 years. A reverse situation is found among adults, those moving away being more frequently males. Adult females are less nomadic ; they often settle temporarily in an area rich in fruit, before becoming «satellites » to an already established family group, or mating (as beta-females) with a sedentary male. The rate of population turnover is high amongst adult speckled mousebirds ; 38 % of the sedentary adults moved out from the study area between July 1976 and July 1977. Both the young bachelor males staying with their parents (thereby delaying their sexual maturity for 6 to 14 months), and the «satellite » adult females, can be considered as a «potential breeder reserve » for the local mousebird population. The area surrounding the nest(s) and roost(s) of a sedentary group is defended by all group members against conspecific intruders. Food sources on the group’s home range are also defended, at least to a certain extent. A dominant-subordinate hierarchy can be established between females of a given family group, and between adjacent sedentary groups with partially overlapping home-ranges. Two females belonging to a same family group may occasionally lay their eggs in the same, well protected, nest. «Helpers at the nest » are of common occurence among speckled mousebirds. In most cases they are subadult males from the preceding generation helping their own parents. However, helpers can also be unrelated adults of both sexes. «Bachelor » male offspring can provide as much food to their sibling nestlings as their parents do. Pair bonds among sedentary breeders may be long-lasting. Out of 12 colour-marked pairs, 7 bred again the next year, 5 two years later and one three years later. One case of polyandry is reported. The size of the foraging flocks increases when food resources become scarce. When threatened by a predator, flocks split up in smaller groups using a different foraging technique. During the dry season, which is also the coldest in the Makokou area, mousebirds «huddle » in tight clusters at night, but separate during the day.