Le cycle de reproduction de la Glaréole Glareola nuchalis : ses déterminants écologiques et comportementaux
Authors: Brosset André 
Published: November 2022

The reproduction of Glareola nuchalis has been studied during a 9 years’ period at M’Passa, near Makokou, Gabon (Lat. 0.4° N ; Long. 12° E). The observations deal with an isolated population, numbering between 93 and 200 individuals, and localized along about 3.5 km of the bed of the Ivindo river. Regular census was made of the number : (1) of individuals in the whole population ; (2) of “ bachelors” or non-breeding birds ; (3) of territorially-breeding pairs ; (4) of clutches ; (5) of fledglings in juvenile plumage.

The colony roosts on two types of traditional sites : Site No. 1 : on the branches of an isolated tree, growing in the bed of the river, during the periods when the water level is high (October-December, and March-June). Site No. 2 : emerging rocks, when the water level is low (January-March and June-September). The non-breeding birds are gregarious, and are usually encountered in a single, compact group. The breeding birds are territorial, and strongly defend a large area surrounding the rock where the nest is located. During the breeding season, the recorded percentages of the breeding birds in the population were always low, ranging between 20 % of the total population in 1978 and only 4 % in 1977.

There are two breeding seasons each year. The clutches are laid immediately after the emergence of the rocks, at the beginning of the two dry seasons. The date of the emergence of the rocks varies according to the year, differences of nearly two months having been noticed. The dates of the emergence of the rocks and the laying of the eggs are remarkably synchronized.

The stimulus triggering the timing of the breeding season is discussed. Food accessibility is probably the ultimate factor. In effect, Glareola nuchalis is mostly a crepuscular bird, hunting in the early morning and late in the evening, apparently because its insect prey does not fly during sunshine. Nevertheless, during the two dry seasons, these prey fly even during the day when the sky is cloudy and the light poor ; this is a common situation, especially between June and September. Thus, the birds may hunt and feed the young quite regularly during the day in the course of the dry seasons — an impossible task during most of the rainy seasons, when hunting is restricted to the periods around sunrise and sunset.

The proximate factor triggering the onset of gonad development may be a visual stimulus, namely the sight of the rocks emerging from the river. This type of stimulus would explain the remarkable synchronism between the emergence of the rocks, and the start of reproduction. The existence of a social stimulus for gonad development may be ruled out, since only a few days separate pair formation in a defended territory from egg laying.

Population turnover seems low. Each clutch contains one or two eggs ; 30 clutches had one egg, 41 clutches had two eggs. Most of the eggs, laid on barren rock, are drowned or destroyed by predators. On the other hand, the young are protected by efficient morphological and behavioural adaptations. Census of the fledglings, after each breeding season, showed great annual variations. Thus, there were 38 fledglings in April 1978, while only 11 fledglings were recorded for the whole year 1977. Over a 9-year period, the average number was 25 fledglings annually for an average population of 150 individuals.