Weight and size of eggs in the Black-headed Gull Larus ridibunuds. / Vikt och storlek hos skrattmåsens Larus ridibundus ägg.

The Black-headed Gull Larus ridibundus is a monomorphic species with little size variation over its wide distribution range. This is in agreement with the wide dispersal of young birds and change of breeding sites of adults between years, which probably means an almost panmictic dispersal of genes. In 1965 I measured and weighed 319 eggs collected at Sjolunda near Malmo, southern Sweden. When I got the eggs they were mixed so I could not distinguish variation within from variation between clutches. But this is the first report on egg size of the species in Sweden since Rosenius (1942; 205 eggs collected at different sites in southern Sweden), and it adds a new point estimate for comparison with other data sets. The measurements are given in. Most of the eggs were without embryos or with small embryos, but the weights can still not be taken as representing those of fresh eggs since eggs loose weight during incubation. The volume calculated by the formula volume=0.45923XlengthXbreadth square+1.333 (Lundberg and Vdaisanen 1979) hence gives a better estimate of the size. My sample is compared with a selection of other studies in. It can be seen that there are only small differences between the data sets that come from different sites and different years. This strengthens the property of monomorphy in the Black-headed Gull and supports the view of Lundberg and Vaisanen (1979) that any selection for larger eggs must be counteracted by other selection forces.