Zoological Studies

Vol. 49 No. 4, 2010

Flocking Behavior and Sexual Segregation in Black Grouse Tetrao tetrix during the Non-Breeding Period

Michał Ciach*, Dominik Wikar, Małgorzata Bylicka, and Marta Bylicka

Department of Nature-Cultural Heritage, Animal Ecology and Wildlife Management, Faculty of Forestry, University of Agriculture, al. 29 Listopada 46, 31-425 Kraków, Poland

Michał Ciach, Dominik Wikar, Małgorzata Bylicka, and Marta Bylicka (2010) Black Grouse tend to form flocks outside the breeding period and to segregate sexes during flocking.  However, the mechanisms of sex-specific and mixed flock formation remain unclear.  In 2002-2005, we studied flocking behavior of Black Grouse during the non-breeding period in open habitats of the Kotlina Orawsko-Nowotarska valley (southern Poland).  The results were grouped into 4 seasons: autumn, early winter, winter, and early spring.  Relative abundances of Black Grouse did not differ among seasons.  Birds were observed in unisex, containing only males or females, and mixed flocks.  Male flocks dominated, and differences in their size among seasons were close to being significant.  There was a tendency of male flock size to increase in winter, when the extended snow cover appeared.  Female flocks were recorded less often, and differences in their size were not significant among seasons.  Mixed flock size did not differ among the defined seasons.  However, a tendency of the mixed flock size to decrease as the seasons progressed was found.  The sex structure in mixed flocks significantly differed among seasons.  In autumn and early winter, males dominated the mixed flocks.  This rapidly decreased in winter and early spring.  Males gradually left the mixed flocks, probably joining unisex flocks during the winter, and hence mixed flock size decreased together with sex-structure changes.

Key words: Flocking behavior, Sex ratio, Non-breeding period, Black Grouse, Tetrao tetrix.

*Correspondence: E-mail:mciach@ar.krakow.pl