BIJLSMA RG (1999) Sex determination of nestling Common Buzzards Buteo buteo. LIMOSA 72 (1): 1-10.

In 1988 and in 1992-98 a total of 48 nestlings of Common Buzzards on 19 nests were weighed and measured (almost) daily from hatching till fledging or death in the province of Drenthe (NE-Netherlands). Each of the 610 nest visits (mostly in the evening) lasted 5-20 min (including climbing), during which 11 measurements, among which maximum wing chord, lateral tarsuswidth, hind claw, body mass and crop size, were recorded. None of the broods was disturbed as a result of the frequent nest visits. Hatching date and hatching order were known for each nestling. Of 48 nestlings, 18 males (8x position A in nest, 7x B and 3x C) and 20 females (l2x A, 4x B, 3x C and Ix D) survived through fledging and this group was used to produce sex-specific growth curves of body mass, lateral tarsus-width and length of hind claw (Appendix 1-4). Nestlings were sexed using the criteria in Bijlsma (1997), based on a combination of wing length, lateral tarsus-width, length of hind claw and claw span after day 30 (hatching day =0). The reliability of this method was confirmed with DNA-analyses from blood samples of 5 males and 6 females (analysis by C. Dijkstra, University of Groningen). The impact of crop size on body mass was studied in 1998 at 3 nests; nestlings were fed ad libitum with known quantities of mice and other vertebrates, and resulting crop sizes estimated. Crop content of well-fed nestlings varied between 35.1 and 115.5 g, depending on age but not on sex (Tab. 1).
      Overlap in body mass between the sexes continued to exist throughout the nestling stage, but mean body mass already differed by 50-100 g during day 15-24, and by >100 g after day 25. Halffull and ful1/bulging crops in the second part of the nestling stage may on average add respectively 50-75 and 75-115 g. Prolonged starving, and birds with bulging crops resulted respectively in females showing male weights and males attaining female weights. Such birds could nevertheless be sexed from day 26 onwards (wing length >192 mm), using lateral tarsus-width (maximal, i.e. no pressure on callipers as soon as skin of tarsus is touched: <9.7 mm = male, >10.0 mm = female) and length of hind claw (<19.7 mm = male, >20.0 mm = female). Nestlings should preferably be ringed and sexed from day 26 onwards (wing length >192 mm), recording body mass, crop size, wing length, lateral tarsus-width and length of hind claw (Tab. 2). Differences of >20-25 mm in wing length between successive nestlings are indicative of retarded growth (also visible in excessive presence of fault bars on coverts and flight feathers), and may jeopardize reliable sex determination. Within-nest positions of young and retarded growth should therefore be established by means of wing length (combined male-female growth curve can be used up to 192 mm, and sex-specific curves thereafter) and body mass. Within-nest positions truthfully depict laying and hatching order.

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