BIJLSMA RG (1996) Incubation time and hatching order in the Goshawk Accipiter gentilis. LIMOSA 69 (2): 67-71.

Northern Goshawk Accipiter gentilis In the sexually dimorphic Goshawk, where secondary sex ratio is biased towards males and laying date is positively correlated with sex ratio, it should be tested whether egg hatching sequence mirrors egg laying sequence, particularly because incubation time varies relative to clutch size. During 1991-95, egg laying and hatching sequences were determined in 16 clutches (Appendix 1). Incubation time (defined as time between laying and hatching) showed within-clutch variation, with mean incubation time for 1st eggs being 41.3 days (SD=1.1, N=14), for 2nd eggs 40.1 days (SD=1.3, N=14), for 3rd eggs 38.7 days (SD=1.0, N=lO) and for 4th eggs 38.0 days (SD=O.O, N=3). Clutch size also influenced incubation time, as can be demonstrated for first-laid eggs: 42.4 days (SD=O.4, N=4) in C/4, 41.1 days (SD=0.9, N=9) in C/3 and 39.0 days in C/2 (N=l). Incubation times for completed clutches varied between 46-47 days in C/4 (mean 46.5 days, SD=0.5, N=2), 42.545 days in C/3 (mean 43.4 days, SD=0.9, N=5) and 39.5 days in a single C/2. Infertile clutches may be incubated up to 52 days (N=l). Hatching intervals between successive eggs were smaller than laying intervals. However, hatching rank equalled laying rank in all cases studied (Appendix 1). In six out of 16 nests two eggs hatched on the same day (5-14 hours apart), but hatching order still equalled laying order and this hatching order could still be quantified by wing length differences at the age of ringing. In a much larger sample of nests on which all nestlings were ringed, sexed and measured (N=246), 58 nests had two nestlings with an age-difference of less than one day. Based on wing length, 42 of these couples differed in age by 0.5-1 day, still sufficient to distinguish hatching order. In the remaining 16 nests with age-differences of less than 0.5 day between two nestlings, 12 couples were of the same sex and only four of different sex. It can be concluded that in Goshawks hatching order reflects laying order, and that nestling rank at the time of ringing can be used to establish hatching order

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