Sex ratio in wintering Black-Headed Gulls in Dutch cities

In gulls there are no clear external differences between both sexes and in many gull species the measurements do have some overlap. Partly due to this in Gulls relatively little is known about the differences between both sexes in how they live. Black-Headed Gulls also show very little sexual dimorphism and both sexes can be distinguished from one another. However, within reasonable doubt it is possible from handling Black-Headed Gulls to say what the sex is, using one or more external measurements.

For the difference between the males and females a multivariate discriminant analysis by Palomares et al (1997) based on a reasonably large sample size of 583 fresh dead Black-Headed Gulls of which the sex ratio was known by means internal research. Two thirds of these were collected in The Netherlands by Berend Voslamber and one third in central Spain. From their analysis, it appears that a discriminant function with two measures gives the best estimate towards the difference between the sexes. In this article we applied their discriminant function of Z = (0.35 X head length) + (0.81 X beak height) - 34.85 for juveniles and Z = (0.33 X head length) + (1.12 X beak height) - 35.75 for adult birds. Black-Headed Gulls are being classified as males at Z > 0 and as a female at Z < 0. In applying the above mentioned discrimant function in 90.2% of the juvenile birds (89.0% of the males, 91.8% of the females) and in 94.4% of the adult birds (94.9% of the males, 94.1% of the females) the sex is predicted correctly.

Catching and measuring
The Black-Headed Gulls are caught in different ways: (1) as a group with a walk-in cage or 'clapnet' or (2) by catching them one by one with a rope on the ground. There are normally four measurements. The head length, tarsus length and beak height at the lower expansion on the lower mandible (the triangle-shaped expansion is at approximately two thirds of the lower mandible) and is measured using a calliper and the maximal wing length is measured with a stretched wing and is measured with a ruler. During the analysis juvenile (first winter plumage) and adult (after first winter) Black-Headed Gulls.

Results
Male Black-Headed Gulls are in the majority, for juveniles (76,8%; N=2368) and as well for adult birds (80.3%; N=2261). This uneven ratio for the sexes is found in all capture techniques. In addition it is clear that in case of individually caught birds the percentage of males is 10-15% higher than in Black-Headed Gulls caught as a group. The differences in sex ratio between both capture techniques are clearly significant, both in case of juvenile as in case of adult Black-Headed Gulls.
Adult Black-Headed Gulls, both males as well as females, have a significant longer wing and a significant higher beak, as opposed to juvenile Black-Headed Gulls. These age bound differences are not present with tarsus- and head length. Black-Headed Gulls in their first winter already have adult measurements for tarsus and head length, but not for wing length and beak height. It seems very likely that geographically different wintering areas, differences in biotope choice, or a combination of these are responsible for the (strong) excess number of males which we encounter in the Dutch cities.