Sexing Black‐legged Kittiwakes by measurement

The effectiveness of using biometric data to sex Black‐legged Kittiwakes Rissa tridactyla in northeast England has been examined. No single measurement or group of measurements was successful in sexing all individuals. The head and bill length was the best measure and correctly sexed 94% of individuals. Mass was an unsatisfactory value to identify sex, as it varied with the time since feeding, throughout the season and probably between years. There was also a significant trend for older individuals to be heavier. Wing length showed considerable overlap between the sexes and both wear of the tip of the longest primary and age adversely influenced the reliability of this measure. There was no obvious or appreciable advantage in using a combination of measurements, such as wing length and head and bill length to produce a combined discriminant value, and the improvement in sexing was marginal and of dubious significance. The Kittiwake shows geographical variation in size and it is suggested that, in other areas, plus and minus 2.6 mm of the average head and bill length of the sample taken from that area will adequately estimate the mean head and bill lengths of male and female Kittiwakes respectively, while the overall mean approximates to the best separation value.