Environmental change and the cost of philopatry: an example in the lesser snow goose.

The consequences of philopatric and dispersal behaviours under changing environmental conditions were examined using data from the colony of Lesser Snow Geese (Anser caerulescens caerulescens) breeding at La Pérouse Bay, Manitoba, Canada. In response to increased population size and decreased food abundance over time, increasing numbers of family groups have been dispersing from the traditional feeding areas. Goslings from dispersed broods were significantly heavier (7.3%), and had longer culmens (3.1%), head lengths (2.6%) and marginally longer tarsi (1.9%) on average than goslings that remained within La Pérouse Bay itself. These differences were consistent in each of 5 years. There was no evidence that the larger size of dispersed goslings was due to either a tendency for larger adults to disperse to alternative sites, or increased mortality of smaller goslings among dispersed broods. The most likely cause for the larger size of goslings from dispersed broods was the significantly greater per capita availability of the preferred salt-marsh forage species at non-traditional brood-rearing areas. The larger goslings in non-traditional feeding areas showed significantly higher firstyear survival, suggesting that the use of deteriorating traditional feeding areas may currently be maladaptive in this population.