Ardea
Official journal of the Netherlands Ornithologists' Union

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Fjeldsa J. (1977) Sex and age variation in wing-length in the Coot Fulica atra. ARDEA 65 (3-4): 115-125
Age differences and yearly variations in wing-length in the Coot have been studied by Visser (1976) without taking environmental conditions, which might account for the variation, into consideration. The present paper is based upon examination of 1,822 Coots shot in the autumns of 1972-1975. Wing-length data from these birds are compared with evaluations of feeding conditions and studies of the breeding ecology, including studies of survival of young, in eastern Denmark. Variation in wing-length was studied by plotting, on yearly scatter diagrams, flattened wing length against length of longest primary (Fig. 1). Different symbols are used for juveniles, adults and birds of uncertain age. The wing-length varied little from year to year in adult birds. Juveniles had slightly shorter and more variable wings. Age differences were insignificant after the very warm and fortunate summer of 1975, somewhat greater in the more normal summer of 1973, and greater still after the summer of 1974, which was rather cool, although still showing a high production of algae as food for the Coots. In all these years, the survival of young .Coots was high and the regression of remiges length on wing-length was similar in the two age categories. In 1972, juvenile Coots differed from adults in having fully 5 mm shorter remiges. Warm weather and high production of algae lasted shorter than in the subsequent years (Fig. 2). A high mortality of juveniles was found in close connection with some brief spells of bad weather during the periods of most rapid remiges growth. These spells may have weakened the surviving young, and arrested their remiges growth. Also the post-juvenal moult was incomplete this year. It is impossible to tell whether these showers of rain should be considered as sole responsible per se, or whether the effect of the bad weather was accentuated by poor physical condition of the young Coots in advance. Studies of museum specimens of several species of birds indicate, as a general principle, that growing birds belonging to species not strictly dependent upon flying powers for their daily maintenance tend to allocate limited energy supplies for more urgent needs, showing a delayed development of the wings and relatively short remiges.


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