The Role of Competition and Local Habitat Conditions for Determining Occupancy Patterns in Grebes

Wetland use was studied in the Pied-billed Grebe (Podilymbus podiceps) and Horned Grebe (Podiceps auritus) in southwestern Manitoba during April to August of 1997. Of the 180 wetlands surveyed, 74 had no grebe use, six were sporadically used by either grebe species, 19 were consistently used by Horned Grebe, nine switched from Horned Grebe to Pied-billed Grebe use, none switched from Pied-billed Grebe to Horned Grebe use, 68 were consistently used by Pied-billed Grebe, one was regularly used by both species but at different times, and three were concurrently used by both species throughout the breeding season. Wetlands used by grebes were larger, deeper, and had more vegetated area than wetlands not used by grebes, wetlands where occupancy changed were larger, deeper, and had more vegetated area than wetlands consistently used by one species, and wetlands that were concurrently used by both species were larger, deeper, and had more vegetated area than wetlands used by only one species at a time. Five logistic regression models were used to model the probability of observing a grebe brood as a function of the habitat characteristics of the wetland, grebe species, and their interaction. The three best models all contained a parameter describing the habitat characteristics of the wetlands. In two of the three best models, the probability of observing a grebe brood increased with increased wetland size, depth, and vegetated area. These results indicate that habitat characteristics influence breeding quality of a wetland and that grebes may be competing for high quality wetlands in this study area.