Visser J. (1988) Seasonal changes in shield size in the Coot. ARDEA 76 (1): 56-63

The white frontal shield is a striking feature of the morphology of the European Coot and is used in the display. Its size clearly shows an annual trend, with a peak at the start of the breeding season (Figs. 1, 2). The aim of this paper is to explain this variation in size in terms of sexual maturity, sex, age, status, and condition of the individual bird. The study is based on the data of 5,588 captures over a period of twenty years, with known age, sex, status (resident or not), fresh body weight, wing length, and shield measurements. Moreover, two samples of killed birds have been used, of which weight or size of the gonads was determined. Figs. 1 and 2 show differences in mean shield size for the several age, sex and status categories. Moreover, there are differences in seasonal trends. It could be asked in how far these differences can be explained by size and condition. To check this idea shield size was corrected for size and condition. After such a correction (Fig. 3) the differences can be summarized as follows: I. the shield is larger in males than in females from March through July, both in juveniles and adults. 2. the shield of adults is larger in females than in the males in November and December, both in resident and non-resident birds, but not in the juveniles. 3. although the still developing shield of the juveniles is of course small in their first autumn, it even exceeds that of the adults in March and April, while the reverse is true after June, this 'crossing over' holds for both males and females. The first age-linked difference in March and April may indicate later territory settlement in the juveniles. The second one after June probably marks an increase in territorial behaviour at the start of a second clutch which is more common in adults than in juveniles. The relationship between shield size and fresh body weight during the greater part of the year was checked by means of a regression model. The results give a strong indication that shield size is, in addition to sexual maturity, also determined by the condition of the bird. Shield size was positively related to weight or size of the gonads (Figs. 4, 5), even if corrected for wing length and body weight. This indicates that shield size is governed by factors responsible for gonadal development, and thus can be seen as a measure of sexual maturity. Therefore it seems likely that the shield acts as a signal of rank order and sexual maturity to other Coots.