Growth and biometry of Mute Swan Cygnus olor cygnets in Groningen, the Netherlands

Growth curves of 10 Mute Swan cygnets, weighed and measured from hatching until fledging, are presented. Biometrical data of 340 cygnets were collected during July, August and September in differentyears. The growth of cygnets is almost complete at that time, except for the flight feathers. The onset of primary growth was related to a sex specific level of weight development: females reached that stage at a lower weight than males did. Furthermore, an analysis of variance in the growth of primary feathers was made. There was no relation between the timing of primary growth and sex or colour morph. The number of cygnets in a brood proved to be the most importantfactor explaining deviations in the onset of primary growth. The effect of brood size on primary growth differed between years and is suggested to depend onfood conditions, which might influence the rate of development of the cygnets.