Population trends of tundra-nesting birds at Cape Churchill, Manitoba, in relation to increasing goose populations

摘要

To assess potential changes in tundra-nesting bird populations relative to increased herbivory by growing Lesser Snow Goose (Chen caerulescens caerulescens) and Ross's Goose (C. rossii) populations, we estimated avian population densities in coastal tundra near Cape Churchill, Manitoba, Canada in June 1984, 1999, and 2000. We also compared bird abundance in altered and intact freshwater sedge (Carex spp.) meadows to assess bird response at a smaller spatial scale, the habitat patch. At the landscape scale, Herring Gull (Larus argentatus) densities were significantly greater in both 1999 and 2000 than 1984, and Semipalmated Sandpiper (Calidris pusilla) and Savannah Sparrow (Passerculus sandwichensis) densities were greater in 2000 than 1984. As a group, tundra-nesting passerine densities were significantly greater in 1999 than 1984. Dunlin (Calidris alpina) densities were significantly lower in 1999 than 1984, and many larger, ground-nesting birds declined in abundance and occurrence along transects in later years. At the smaller spatial scale, passerines and shorebirds were less abundant in altered than intact freshwater sedge meadows. Our results indicate that most abundant breeding birds on the study area did not exhibit lower densities in later years despite increased effects of goose herbivory on vegetation. At the habitat patch scale, the same groups of species had lower abundances in altered versus largely intact habitats. Although goose herbivory can alter tundra vegetation and lead to lower abundance of several bird species, habitat alteration may have to be widespread and severe to elicit population declines over a large area.