Ardea
Official journal of the Netherlands Ornithologists' Union

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Visser J. (1974) The post-embryonic development of the Coot Fulica atra. ARDEA 62 (3-4): 172-189
The growth (body weight and wing length) of young Coots Fulica atra was studied in the field between 1967 and 1972. The curves for wing length and body weight show different patterns (Fig. 1). Compared with the weight, the wing length increases slowly during the first weeks. At the age of 5-6 weeks the weight has reached 50 % of the final value as compared with 30 % for the wing length. At the age of 10-12 weeks this lag disappears. The wing has almost reached its final length at the age of 12-13 weeks, the mean value being 208 mm. A mean value of 804 g was found for the body weight at this age. The individual differences in weight and wing length within age groups are extremely large (Tables 1 and 2). The possible causes of these differences are discussed. The widest variation occurs in the first half of the developmental period (0-8 weeks), and the variation in weight and wing length within a single brood shows the same pattern as the total variation. Variation is greatest at the age of 1-3 weeks (body weight) and 4-10 weeks (wing length), after which there is a gradual decrease in the variation within the brood. There are indications that the lightest young in a brood has the greatest chance of dying at an early age, as a result of which the weight variation within the brood decreases with increasing mortality, hence with the age of the survivors (Tables 3A and 4A). The relationship between age and variation in body weight and wing length between broods generally corresponds with the trends in the total variation and in the variation within broods (Tables 3B and 4B). There are differences in growth pattern between males and females. At the age of 2-4 weeks the males are significantly heavier than the females. The wing length of both sexes does not show a significant difference until the age of 12-14 weeks (Fig. 2). The growth rate can vary widely from year to year. In the present study conditions were most favourable in 1969, the endpoint of growth having been reached about two weeks earlier than in the other years (Fig. 3). Sub-optimal conditions resulted in a shift of the time at which growth terminated, but did not influence the final weight or wing length. Differences in growth rate in relation to the hatching period occur mainly at the age of 0-4 weeks (Fig. 6). In most years the young which hatched during May (the main period of hatching) showed higher weights during the first few weeks than did those hatched earlier or later in the season. Of the many external factors that can influence the growth of young Coots, only one was investigated, i.e. the air temperature. In early springs (1971 and 1972), when the young were exposed to low temperatures during April, this was reflected in low body weights and high mortality of the young chicks. For the rest, the data show no distinct relationship between temperature and growth rate. The growth rate is also influenced by the brood size. In the first four weeks the weight of the young from small broods (1-4 young) was significantly higher than that of young from large broods (5-8 young). This difference gradually disappeared.


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