Habitat use by Loggerhead Shrikes in Ontario and Quebec

Abstract

Once common throughout northeastern North America, the migrant race of Loggerhead Shrike (Lanius ludovicianus migrans) has undergone a drastic decline since the middle of the last century. The subspecies was designated as endangered by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) in 1991. To gain a better understanding of the factors affecting the decline of this species in eastern Canada, habitat selection by Loggerhead Shrikes breeding in Ontario (Smith's Falls, Napanee, and Carden limestone plains) and southern Quebec was studied in 1991 and 1992. Nest trees used by breeding shrikes were compared with similar arbitrarily identified trees in suitable unoccupied habitat to determine if there was a nest-tree preference. Territories where successful nesting attempts were made (i.e., young fledged from at least one of the eggs laid) were located over a 2-year period; data were not collected for unsuccessful nests or nests used for double-brooding. Thirty-seven nests (50%) were located in hawthorn shrubs (Crataegus spp.) and 29 nests (40%) in red-cedar trees (Juniperus virginianus). White cedar (Thuja occidentalis), buckthorn (Rhamnus catharticus), and ash (Fraxinus spp.) were used infrequently (10%). Sixty-two nests (86%) were located in isolated trees or in a copse. Ten nests (14%) were located in hedgerows. Fifty-one (64%) nest sites were located in an actively grazed pasture. The other nest sites were located in idle pasture, hayfields, or old fields. Nest trees and arbitrarily chosen trees in suitable unoccupied habitat did not differ significantly in average height, width, or canopy concealment. Few differences were detected in the average height of the vegetation or the composition of ground cover within a 10-m radius of nest trees and arbitrarily chosen trees. The average numbers of shrubs per hectare did not differ between breeding sites and suitable unoccupied habitat. Nest trees in the Smith's Falls core breeding area were located significantly closer to roads than arbitrarily chosen trees in suitable unoccupied habitat. Habitat suitability was also assessed according to the density of perches (trees and shrubs), which directly affects the amount of actual utilizable habitat in a territory. Significant differences were found in the amounts of actual habitat and potential habitat. The amount of habitat around active nest sites, historic nest sites, and suitable unoccupied sites was significantly greater around active nest sites. Since few statistically significant differences were found between habitat occupied by shrikes and that which was not used, it is not possible to build a predictive model of suitable breeding habitat for shrikes in this study area.