Die Massenvermehrung des StarsSturnus vulgaris in fortpflanzungs-physiologischer Sicht

Zusammenfassung

In der vorliegenden Arbeit wird dargestellt, daß die zunehmende Überwinterung des Stars im Brutgebiet bzw. in dessen Nähe sowie die frühere Rückkehr ins Brutgebiet offenbar sowohl die Zahl brütender Einjahresstare als auch den Prozentsatz von Zweitbruten erhöht, woraus sich eine allmähliche Massenvermehrung der Starpopulationen in Europa, besonders aber in Nord-Amerika, ergeben haben dürfte.

Summary

In the present paper, an attempt is made to explain the enormous increase in the number of European Starlings since the middle of the past century. Up to now, the following reasons for the increase are given: The climatic changes since the middle of the past century (milder springs, autumns and winters in north and middle Europe), the nestboxes offered everywhere, the extension of meadows, pastures and fruit-cultures (as new feeding grounds). Recent experiments showed that Starlings, which do not breed before their second year in their natural population, breed already in their first year if autumn and spring migration fail to take place. Adult (more than two years old) Starlings show a significantly premature gonadal cycle, if autumn and spring migration fail to take place. There is a direct correlation between non-migration resp. early return to the breeding area, and the number of first year Starlings breeding as well as the course of the gonadal cycle of the population. Furthermore, it can be observed that European Starling populations have continously antedated their arrival at their breeding grounds, and that increasing numbers are wintering at their breeding grounds, or between the breeding grounds and the typical winter areas. This means, that the extent of migration has diminished. Consequently, the following hypothesis can be suggested: The number of first year Starlings already breeding has generally increased as well as the number of second broods (because of earlier first broods). Since Starling populations without regularly breeding first year birds and without regular second broods are already accumulating, the increase of the number of breeding first year birds and the increase of second broods must have resulted in an abnormally high reproductive rate and finally in an increase of the population density and the area of the distribution. The reasons for the enormous increase in numbers of American Starlings are discussed, too: Compared with Europe, the high rate of second broods and breeding first year birds as a possible consequence of the highly resident behaviour may have been responsible.