Acta Zoologica Cracoviensia, 43(1-2) 2000

Issue:

VOL 43(1-2) 2000

Keywords:

Rook, Corvids, wintering assemblage, zone of daily dispersion, feeding grounds, foraging strategy, energy saving, communal roost, city

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The wintering strategy of Rooks Corvus frugilegus Linnaeus, 1758, in Poznań, west Poland

• Aleksander WINIECKI

Abstract:

The functioning of the wintering assemblages of the Corvids in Poznań is based on the system of daily flights to feeding grounds, which is stable in time and space. The elements of the system are: the basic roost, routes of flights to feeding grounds and sites of temporary stay (post-feeding concentration points, pre-roosting concentration points), and feeding grounds. According to the author, the main reason for establishment of the system, including the basic roosting place, is the fact that it facilitates the exchange of information about food resources. The energetic and antipredatory functions are of secondary importance. The wintering Rooks roost in a communal roosting place and during the day they disperse over an area distanced up to 25 km from this place. At the beginning and end of the winter season, they are foraging in agrocenoses, using the strategy of active search for food. In the peak of winter they come to urbanized environments and use the expectation of food strategy. The effectiveness of this strategy depends on the experience of individuals, therefore young birds remain outside the urbanized areas. The energy costs of the Jackdaws’ stay in the least urbanized areas, e.g. parks and in agrocenoses, are similar. The Rooks’ penetration of the city is caused by the need to save energy (less energy is used for flights). The most attractive source of food for Rooks are waste dumping grounds. Strong fluctuations in the number of Rooks foraging there are the result of increased food competition. The change in the type of foraging is accompanied by shortening of the routes of daily flights between the roosting place and feeding grounds. The changes are induced by: a decrease in the availability of food and its irreproducibility in agrocenoses, the need to save energy for flights, and a shortening of daylight equivalent to shortening of foraging time.