Egg-Focused Aspects of the Reproductive Ecology of Black-Winged Stilts Himantopus himantopus and Pied Avocets Recurvirostra avosetta Coexisting in an Unstable Wetland in the Sahara (Algeria)

Despite its hyperarid climate, the Sahara Desert encompasses some large oases with extensive but rather fragile wetlands that are inhabited by Charadriforms. In this study we analyse the reproductive ecology of two species of Recurvirostridae, Black-winged Stilts Himantopus himantopus (more numerous) and Pied Avocets Recurvirostra avosetta (less numerous), nesting in an unstable habitat of the Saharan wetland of Chott Ain El Beida, Algeria, North Africa. The onset of egg laying was more stable and similar between years in Black-winged Stilts (15–19 April, on average, N = 315) than in Pied Avocets (13–21 April, on average, N = 73). Black-winged Stilts and Pied Avocets showed opposite yearly trends in egg laying dates. Clutches composed of 4 eggs dominated in both these species. They did not differ in hatching success of complete clutches (on average 73% and 77%, respectively). Average weights of eggs decreased from year to year during 2005–2007, suggesting that the individual quality of breeding birds decreased over the study years, probably corresponding with the progressive deterioration of the habitat. In the Black-winged Stilt, but not in the Pied Avocet, egg sizes decreased with the progress of the laying period. Another difference between egg-related aspects of the breeding ecology of these species was that only in Black-winged Stilts egg shape tended to affect hatching success of clutches, so that clutches of more elongated eggs had higher hatchability. To conclude, some aspects of breeding performance of waders may have a different form even in closely related species, depending on changing ecological conditions.