Egg Size Variation and Breeding Characteristics of the Black-Winged StiltHimantopus himantopusin a Saharan Oasis

Different aspects of the breeding biology of the Black-winged Stilt were studied in a natural oasis wetland area in the Sahara Desert, the Chott of Aïn El Beïda, near Ouargla, E Algeria in 2004–2007. In this wetland the water level is independent of rainfall and Stilts start egg laying when average daily temperature increases to ca. 15°C, mostly in April-May, with no significant differences between years. Complete clutches usually consist of 4 eggs but 3 egg clutches happen with low frequency (2.9–5.6%). Egg size traits (mass, length, breadth and volume) tended to decrease from 2004 to 2007, probably due to deteriorating hydrological conditions of the wetland. Within-clutch variability showed both significant repeatability and laying-sequence-dependent differences among eggs, with the final egg being smaller than the clutch mean value. We found that all egg traits studied showed a negative correlation with the date of laying, which seems to be a phenomenon analogous to the seasonal decrease in clutch size, typical of birds with more variable clutch sizes. Hatching tended to be asynchronous, with average clutch hatching time being 1.84 days. The small final egg and hatching asynchrony are typical components of the adaptive brood reduction, the phenomenon not being recorded so far and remaining for future studies on the Black-winged Stilt. These are the first published data on breeding characteristics for any N African population of the Black-winged stilt.