Sympatric nesting Eastern Prairie Population Canada geese and lesser snow geese on the Hudson Bay lowlands: Nest survival and spatial distribution.

Abstract

The distribution and abundance of breeding lesser snow geese ( Chen caerulescens caerulescens ) in northern Manitoba, Canada have expanded considerably during the last 30 years, and snow geese now use areas where they did not previously occur and were traditionally used solely by breeding Eastern Prairie Population (EPP) Canada geese ( Branta canadensis interior ). Breeding and migrating snow and Ross's geese ( Chen rossii ) have dramatically altered sub-arctic vegetation along Hudson Bay, but the direct or indirect effects of these growing populations on other species, particularly sympatric nesting EPP Canada geese, is not well documented. To better understand relationships between snow geese and EPP Canada geese, I evaluated factors influencing survival and spatial distribution of Canada goose nests across a range of spatial and temporal scales. I employed nest distribution data, 2001-2007, and spatial point-pattern analyses to assess (1) whether nesting Canada geese exhibited territoriality, (2) whether current spatial patterns of Canada goose nest distribution were evolutionarily stable, and (3) whether spatial patterns of Canada goose nest fate were associated with the density of nesting snow geese. In addition, I employed logistic-exposure models to evaluate factors influencing within- and among-year variation in the probability of nest survival for Canada goose nests from 2005-2007 in areas with different histories and densities of snow goose nesting. Specifically, I tested the apparent-competition hypothesis and the nesting-association hypothesis as mechanisms to describe interactions between nesting Canada geese and snow geese. I examined factors influencing the spatial distribution of nesting EPP Canada geese in a coastal tundra landscape between 1976 and 2007, and assessed whether associations with these factors have changed in light of changes to the coastal salt marsh vegetation from snow goose herbivory. Lastly, I modeled Canada goose nest count data collected during range-wide aerial breeding population surveys conducted annually for EPP Canada geese in northern Manitoba, Canada, 1987-2008. I employed mixed-effects Poisson regression to (1) quantify spatial relationships between nesting lesser snow geese and Canada geese; (2) assess factors influencing the distribution of nesting Canada geese at a range-wide spatial scale (101,500 km 2 ); and (3) evaluate how well local (i.e., nest or study area) spatial-scale associations predict range-wide distribution patterns.

Point-pattern analyses indicated territoriality among nesting Canada geese and that changes in snow goose nest proximity and density may influence Canada goose nest fate. However, logistic-exposure models did not support the apparent-competition or nesting-association hypotheses as mechanisms governing associations among Canada goose nest survival and nesting snow geese; particularly relative to the strong effects of collared lemming ( Dicrostonyx richardsoni ) and arctic fox ( Alopex lagopus ) population dynamics. Canada goose nest density was negatively associated with increasing distance to coastal salt marsh brood-rearing habitat; however, from 1976 to 2007, this relationship appeared to be changing to where in more recent years increased distance to salt marsh was positively associated with density of nesting Canada geese. Three models of Canada goose nest counts from range-wide aerial surveys--all containing factors related to previous Canada goose nest density (representing recruitment), distance to Hudson Bay, general habitat type, and lesser snow goose nest density--had the lowest mean-squared error among all models considered and were unbiased. The relationship between Canada goose nests and snow goose nest density suggested that the abundance of nesting Canada geese and snow geese varied similarly across habitats throughout the entire EPP breeding range (e.g., both abundant near the coast), but that very high snow goose abundance may have negative impacts on Canada goose nest density.

My analyses of data from multiple spatial and temporal scales suggested that associations between the distribution of Canada goose nests and nesting snow geese was likely related to alteration of coastal salt marshes from snow goose foraging, and subsequent changes in the distribution of Canada goose brood-rearing habitat use; rather than influences related to Canada goose nest survival. My analyses highlight that a continuing shift in the spatial distribution of nesting snow geese along western Hudson Bay will have implications for the spatial distribution of sympatric nesting Canada geese.