The biology of canada geese Branta canadensis in relation to the management of feral populations

Feral populations of the canada goose Branta canadensis continue to grow at around 8% per year in the UK. The growing feral populations in Europe and non‐migratory populations of ‘urban’ canada geese in North America are beginning to conflict with human interests. In response to increasingly frequent calls for control of this species, we review the scientific literature concerning the biology of feral populations in an attempt to determine why such rapid population growth has occurred. We also examine the available evidence about the problems caused by canada geese and the published information on the management techniques already tested. Feral canada geese are highly fecund, producing up to six young per pair, and have high fledging success. This allows populations to continue to grow even in areas with high levels of mortality in both adult and immature birds, mostly as a result of shooting. Population growth has been most rapid in urban areas with little shooting pressure and correspondingly low adult mortality. Site faithfulness, particularly in females, has probably slowed the spread of canada geese to new habitats, many of which have been created by man. Many apparently suitable sites remain unoccupied at present, and the factors which govern the carrying capacity of existing sites are not fully understood. The upper limits to the growth of feral populations are therefore difficult to estimate, but there is little evidence that density‐dependent factors are acting to regulate population size except at long‐established breeding sites. Canada geese can cause damage to agricultural crops and amenity areas resulting in significant localised economic loss particularly in areas close to water bodies. In most countries the extent and cost of the damage caused has not been fully evaluated, and evidence in support of the need for control on a national or international level is currently weak. Work on the impact of canada geese on other waterfowl and on the possibility that they may transmit diseases to humans is continuing. In Britain, research into management has concentrated on reproductive control by treatment of eggs. Results have shown that, even if the control is highly efficient, it takes a number of years for any reduction in the population size to occur. Most researchers suggest that reproductive control needs to be combined with an increase in adult mortality if the population size at a site is to be reduced in an acceptable time. We suggest that Integrated Management Strategies (combining habitat management, behavioural modification of the birds e.g. by scaring and, where necessary, by population reduction) need to be developed. These strategies should be specific to the particular location concerned. Current research in progress in the UK is summarised and areas where further research is needed both to quantify the problem and develop effective management strategies are identified.