Population size and differential population growth of introduced Greater Canada Geese Branta canadensis and re-established Greylag Geese Anser anser across habitats in Great Britain in the year 2000

Abstract

Capsule: Both populations are increasing at a similar rate. Aims: During 2000, an extensive survey of introduced Greater Canada Geese and re-established Greylag Geese in Great Britain was undertaken in order to update population estimates last made between 1988 and 1991. Methods: A randomized stratified sample of 1329 of the 61 510 tetrads (2 km x 2 km unit) in Great Britain was surveyed. The habitat categories, or strata, were based on the proportion of water cover, urbanization, and upland/lowland in each tetrad. Non-urbanized strata were further divided into northern and southern reporting areas. Results: In 2000 there were an estimated 88 866 full-grown Greater Canada Geese and 24 522 full-grown Greylag Geese in Great Britain. Since the 1988-91 survey, Greater Canada Geese have increased by 166% and Greylag Geese by 170%, an average per annum rate of increase of 9.3% for Greater Canada Geese and 9.4% for Greylag Geese. The increase in Greater Canada Goose numbers since the 1988-91 survey has occurred mainly in habitats which had previously held low goose population densities, particularly the 'no water' lowland habitat. Although densities were still relatively low in this habitat (<0.5 geese per km2), because of its extent it supported 56% of the total Greater Canada Goose population in 2000. The greatest increase in re-established Greylag Goose numbers has arisen from an expansion into lowland habitat with some water cover. Conclusion: There was no obvious decline in the 8.3% per annum Greater Canada Goose growth rate that caused the population to treble between 1976 and 1991. Greylag Goose numbers are increasing at a similar rate to those of the Greater Canada Goose. © 2007 British Trust for Ornithology.