Dwergeieren bij Sperwer Accipiter nisus

On 9 June 2000, a nest of a Sparrowhawk contained a single cold egg (32.1x22.0 mm) with little pigmentation; on 12 June it was still cold and collected for further examination (no yolk visible). The female was present during both visits and proved to be the same as the year before (as comparison of moulted feathers showed), when she nested but did not lay eggs. In 1999, she was already at least in her third calender-year. The territory, with the same female, was again occupied in 2001, with her nest containing a single egg without yolk (31.7x21.9 mm) on 27 May (Photo 1). Both eggs (in 2000 and 2001) can be defined as runt eggs when compared to the average egg size of 39.7x32.1 mm (Table 1). Energetic constraints, one of the possible causes of the production of runt eggs, are not relevant in this case, as the female was paired to an adult male in 1999 and 2001 and occupied the same territory for at least three consecutive years. A female in poor condition is probably not able to hold a territory for some years in a row, and attract a mate.