Ardea
Official journal of the Netherlands Ornithologists' Union

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Altenburg W., Bruinenberg J., Wildschut P. & Zijlstra M. (1987) Colonization of a new area by the Marsh Harrier. ARDEA 75 (2): 213-220
This paper describes the results of a field study in the Lauwersmeer to examine the hypothesis that the Marsh Harrier colonizes new areas by summering immatures and by recruit breeders. The species colonized the Lauwersmeer in two waves, one from 1969-1977 (old site nests) and one from 1977-1982 (new site nests), both largely determined by vegetation development. Assuming fidelity to breeding place, which is very common among birds of prey, colonization by young birds would mean that 1) the whole Lauwersmeer population would be younger in the first years of colonization as compared to other populations outside the area and 2) the population of old sites would be older than that of new sites. Studies, especially those on larids, showed that older pairs arrive earlier in the breeding area, start breeding earlier, have larger clutches and raise more young in comparison with young pairs. Our data on Marsh Harriers with known age (young versus older) show the same trends (Table 3). At least until 1980, the first birds arrived c. two weeks later in the Lauwersmeer than in nearby Dutch and German breeding areas. Up to and including 1979 beginning of laying in the Lauwersmeer was later than in nearby Flevoland (Fig. SA). Up to and including 1979 beginning of laying at new sites was later than at old sites (Fig. 5B). During the whole study period clutch size at new sites was smaller than at old sites (Fig. 6). Though we did not find differences in the number of fledglings, these results strongly suggest that predominantly young birds started the colonization process. They first bred at old sites, but from 1977 onwards when numbers at the old sites did not increase any further, they also started to breed at new sites. After 1979 the initial differences had largely disappeared, probably due to replacement of older birds by younger recruit breeders. The origin of the young birds is not clear.


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