The migrant race of the Loggerhead Shrike (Lanius ludovicianus migrans) has undergone a severe decline in Canada since the mid-1950s and was designated as endangered by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) in 1991. In order to gain a better understanding of the factors that may be associated with the decline of this species in eastern Canada, we studied the breeding biology and nesting success of Loggerhead Shrikes in the three core breeding areas in Ontario, Canada, during 1991 and 1992. We located 27 breeding pairs during 1991 and 50 breeding pairs during 1992. Mean clutch size was 4.9 and 5.6, mean number of eggs hatched per nest was 4.2 and 5.4, and mean number of young fledged per nest was 3.9 and 4.2 in 1991 and 1992, respectively. Only 2.3 and 2.5 young per pair survived to independence in each year. Percent success in fledgling at least one young was 89% and 78%. We believe that predation was the cause of all but one of the nest failures in both years. One pair successfully double brooded in 1991. In 1992 one of three double-brooding attempts was successful. The probability of survival of an egg was 58.6% for nests in isolated red cedars (Juniperus virginiana), 76.2% in isolated hawthorn (Crataegus spp.) shrubs, 88.5% in hawthorn shrubs in hedgerows, and 92.9% for all other species of nest tree. Daily nest survival was 98% for nests in isolated red cedar trees and 100% for nests in isolated hawthorn shrubs, in hawthorn shrubs in hedgerows, and in all other species of nest tree. Nest depth ranged from 11.2–12.4 cm and nest height above ground ranged from 2.1–2.5 m. Most nests in red cedar and other species of trees were located adjacent to the main trunk. Nests in hawthorn shrubs were located adjacent to the main trunk, on a main branch, or in the center of the canopy.