Broedduur bij Sperwers Accipiter nisus: duur van de eifase en berekening van het legbegin

The period between laying of the first egg and hatching of the first chick varies between nests (i.e. females) and within clutches. In the present study on Sparrowhawks, carried out in three regions in The Netherlands during 1990-2001, this interval is christened ‘egg-stage’. From the study sites, 156 nests were selected in which egg laying was observed. The egg stage was found to range between 32.5 and 44 days (mean 38.6 days) (Fig. 2). On average, the egg stage in first layings was of shortest duration in clutches of 2-3 eggs (38.2-38.3 days, n=6). Clutches of 5 eggs (i.e. the most common clutch size in our Sparrowhawks, with 48% of the sample of 156 nests) showed a slightly more prolonged egg stage (38.5 days), further increasing to 39.4 and 40.3 days in respectively clutches of 6 eggs (n=40) and 7 eggs (n=2). The egg stage did not differ much in 5-egg clutches that hatched completely or partly (Table 1). Clutches of 4 eggs did not fit this pattern, with an egg stage of 38.8 days. However, within this group first-year females showed a much shorter egg stage than adult females, viz. respectively 37.7 days (n=5) and 39.2 days (n=18) (Fig. 3). We had indications that these adult females suffered from inadequate food supply (laying interval >2 days, egg predation during laying, hesitant start of incubation after laying of the ultimate egg); it is also possible that senility played a role. Within each clutch size category, the egg stage shortened with progressing season. The correlation between egg stage and clutch size, as found in first layings, was absent in repeat layings (Fig. 1). Late-laying females partly compensated their late onset by commencing incubation during egg laying, thus shortening the egg stage but increasing the age-difference between chicks and with a higher incidence of runts. This strategy was most clearly found in adult females producing a repeat laying (n=7); on average, such clutches hatched 4 days earlier than similar clutches in first layings. These data indicate that the duration of the egg stage is correlated with the date at which laying is initiated, and consequently also with clutch size. To back-calculate onset of laying from the age of the oldest chick in the nest (assuming this to concur with the first-laid egg, which is not always true), we propose either a fixed egg stage of 38.5 days, or a gliding scale of 36-40 days depending on clutch size. Ideally, in clutches of 4 eggs a distinction should be made between early adult starters on the one hand, and late starting first-years and repeat layings on the other hand. The present proposal of an egg stage of 38.5 days between first egg and first chick came closest to the observed interval as compared to several other methods of back- calculating onset of laying (Table 4).