Ardea
Official journal of the Netherlands Ornithologists' Union

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Visser J. (1976) An evaluation of factors affecting wing length and its variability in the Coot Fulica atra. ARDEA 64 (1-2): 1-21
This paper concerns the results of a study on the wing length of the Coot. Starting in 1967, a total of 4,797 wing measurements were performed in several places in The Netherlands, mainly the Westeinderplassen in the province of Noord-Holland (4,005 measurements). The great majority of the measurements were made in live birds. Various factors which influence wing length and which hamper comparison with data in the literature, or even make it impossible, are discussed. In general, these factors are not taken sufficiently into account in the literature. The mean wing length of male Coots is significantly longer than that of the females, and the adult birds have significantly longer wings than the juveniles. In the adult group no differences between age classes are demonstrable. In birds of known sex and age, wing measurements were used for the study of sex distinction on the basis of this parameter. From the 95% probability ranges of these measurements the following rules could be set up: Group A. Birds breeding or hatched on the Westeinderplassen juveniles with wings shorter than 200 mm = females juveniles with wings longer than 214 mm = males adults with wings shorter than 212 mm = females adults with wings longer than 217 mm = males Group B. Birds of unknown origin captured at different places in The Netherlands juveniles with wings shorter than 200 mm = females juveniles with wings longer than 212 mm = males adults with wings shorter than 204 mm = females adults with wings longer than 215 mm = males These results only hold for the measurement technique described in 2.1. It is possible that the data for Group A, pertaining to autochthonous Coots from the Westeinderplassen are not applicable to other areas in The Netherlands. The same holds for the data of Group B, pertaining to Coots from various places in and outside The Netherlands and thus representing a very wide geographical range. The mean wing length can vary appreciable from year to year. For instance, in the 1969/1970 season it was significantly smaller than in the corresponding period in the other years (Fig. 2). Little is known about the causes of these annual changes in the wing length. Comparison of the wing development during the growth period with the mean lengths in the following season (September-June) shows that in years in which the juvenile Coots show a relatively slow development of the primaries, a relatively high wing-length value is nevertheless attained. No explanation can be offered for this negative correlation. Within an inter-moult period there is a decrease in wing length. This reduction is caused by abrasion of the primaries, and is dependent on the age of the birds, being greatest in the adult Coots. Since some of the juvenile birds do not participate in the breeding cycle, it seems likely that the lower activity level of this group in the spring (territorial behaviour) and during the breeding season leads to less abrasion of the primaries. The mean daily decrease did not vary significantly from season to season. The mean wing length of the non-autochthonous Coots differs from that of the autochthonous birds, the latter being longer in both juveniles and adults (males and females). These differences are significant (Table 3). The explanation for the divergent wing length of the non-autochthonous group might be sought in the geographic origin of these birds. The ringing data show that these birds derive partially from Central European, East European, and Scandinavian populations (Table 10).


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