Geslachtsbepaling bij Torenvalken Falco tinnunculus op basis van biometrische gegevens

During a study of Kestrels in the central Netherlands in June 2001 through May 2011, a total of 568 full-grown birds were captured. Of these birds, the following measurements were taken (after Bijlsma 1997): body mass, wing length (maximum), tail (longest), tarsus, tarsus width (minimum and maximum), bill length, head + bill, eight primary (distally) and forearm. Ages were categorized as first calenderyear, second calender-year and older. Removing birds with incomplete or dubious measurements from the dataset, a total of 213 females (41 first cy, 43 second cy and 129 older) and 154 males (respectively 30, 31 and 93) remained for analysis. Except for tarsus length (all age classes) and rear claw (in first calender-year birds), all measurements showed significant sex-specific differences. Using the most distinctive features to separate males from females (bill length and wing lenght in first cy, body mass and forearm length in second cy and older), 13-17% of the birds would have been wrongly sexed if only morphometries would have been used. However, features of the plumage can be used as well, especially those of the upper tail coverts (as exemplified by photographs).