La périodicité de la reproduction chez un Bulbul de forêt équatoriale africaine Andropadus latirostris. Ses incidences démographiques

Abstract

The pattern of reproduction of the forest Bulbul Andropadus latirostris has been studied from 1970 to 1980 at the M’Passa field station in North-East Gabon (0.4° N ; 14.0° E). On a study area of 300 ha of rain-forest, 778 individuals were marked and 248 nests were found. Most of these nests were regularly observed up to the time that fledglings leave the nest, or the destruction of the brood by predators. Andropadus latirostris is a seasonal breeder. The first clutches were always observed between 15 December and 1 January, and the last around 15 March. Sexual songs were heard from September to April, and birds in juvenile plumage were caught only between April and June. Breeding therefore takes place during the short dry season (January to March), a period characterized by a low rainfall, sunny weather, and a slight drop in fruit and insect production. It is preceded by a first peak in food production in October and November, during which period female A. latirostris put on weight before laying, and followed by a second period of food abundance in April and May, which corresponds to the emancipation of the fledglings. The building of the nest, as well as the laying and brooding of the eggs, and the feeding of the young (by the female alone), all take place during a period of the year where the climatic conditions are optimal for the thermal comfort of the eggs and young birds. The most unexpected fact noticed during the ten years’study period was a complete cessation of all breeding activities in the banded population during two years in succession (the 1971-1972, and 1972-1973 breeding seasons). Reproduction started again in January 1974, with a very high fledgling success (2.43 per nest), the predation pressure being much lower than usual (46 %). A similar situation was observed during the next breeding season (1974-1975). Later on, from 1976 to 1980, the average number of fledglings per nest decreased (1.95 per nest), and the percentage of nests destroyed by predators increased (78 %). During the last few years the reproductive success remained at a low level. The population density of Andropadus latirostris has changed considerably in the study area during the observation period. Following a good breeding season in 1970-1971, the numbers were slightly above the average, but they decreased sharply after the two years without any reproduction. When breeding was resumed, there was a large influx of young individuals, the number of juveniles caught in mist-nets in 1974-1975 being twice as great as that of the following year. Later on (1976-1980) the population again decreased, to become stabilized at a relatively low level in 1980. The possible causes of the complete cessation of breeding of A. latirostris in 1971-1972 and 1972-1973 are discussed. Food shortage is unlikely in the comparatively stable environment of a tropical rain-forest. Futhermore, no deficit in rainfall was noted in the years during which A. latirostris stopped breeding, and the 1973-1974 season, when reproduction was resumed, was not exceptionally good for fruit production either. It is also worth noting that the other sympatric species of Bulbul continued to breed normally during the years when A. latirostris did not. On the other hand, this two years pause in breeding activities might well represent an efficient way of diminishing the predation pressure in an environment where it is normally very heavy. As shown in Table VI, the annual production of fledglings from an even number of nests (100) might be considerably increased (262 against 171) by a two year pause in reproduction in this species. This could be due, at least in part, to the influence of the prolonged disappearance of a certain type of prey upon the “searching image” of adult predators, and to the delay needed by young predators to acquire a new “searching image” when breeding is resumed.