The applicability of age ratio and brood size counts in population dynamic studies of the brent goose Branta b. bernicla

In wintering Brent Geese, annual assessments of the proportion of juveniles and the mean brood size have been made since the 1950s, allowing population dynamic analyses. Field procedures have never been quantitatively explored and were studied in four areas along the Dutch Wadden Sea in 1975/76. Juveniles were shown to be overrepresented in the front and adults in the centre ofgrazing flocks. A separation between families and other birds is accomplished already at landing. As a consequence of the flock structure, the composition of temporary subflocks can differ largely. The proportion of juveniles went down from 50% in autumn to 25% in midwinter, and increased to 40% in spring. With normally 15-20% of the Brent wintering in the entire Wadden Sea, age counts from the southern areas will overestimate the population average. In late winter and spring systematic differences in age ratio were found between different feeding habitats. Brood size ranged from 1 to 7 juveniles. The average declined from 4 in autumn to 3.2 in spring. The mean brood size in a flock was positively related with its proportion of juveniles. Changes in family behaviour with season are discussed. Field counts can best be run in conjuction with the annual midwinter census, but require a tightly organised scheme. Sampling techniques are discussed. Results will probably apply to other goose species, but consequences of shooting warrant more study. Zoological Laboratory, Rijksuniversiteit Groningen/Delta Institute for Hydrobiological Research. Address: Vierstraat 28, 4401 EA Yerseke, The Netherlands.