Habitat shifts and parasite loads of lesser snow geese (Chen caerulescens caerulescens)1

Responding to degradation in their original coastal habitat, increasing numbers of lesser snow geese are rearing their broods farther inland. Goslings collected in this inland, fresh water habitat have substantially lower loads of two species of caecal nematodes than do goslings collected in coastal, salt marsh habitat. This likely reflects differences between the habitats in the levels of infective stages of the parasites that are ingested by goslings during their summer foraging. In the spring, several million northward migrating adult lesser snow geese use the coast of Hudson Bay for staging and feeding rather than using more inland habitat because the latter is usually still snowand icebound. The spring migrants leave behind copious amounts of feces in the coastal marshes that contain the eggs and larvae of the nematodes. By contrast, the inland habitat receives little fecal deposition until mid-summer and then only by the much smaller resident population of nesting lesser snow geese. There is some evidence that the infectious stages of these parasites survive the winter, but multi-year accumulations would only tend to amplify habitat differences in infective loads related to the spring deposition by migrants. The role of migrants in transmitting these nematodes highlights the important point that local host–parasite dynamics must be considered from a broader spatial scale.