Brooks, B.L. 1988. The breeding distribution, population dynamics, and habitat availability and suitability of an Upper Midwest Loggerhead Shrike population. M.S. Thesis, University of Wisconsin-Madison. 58 pp.
Chapter 1: The breeding distribution of the loggerhead shrike in Minnesota: a preliminary report
Chapter 2: Dynamics of an endangered loggerhead shrike population in the upper Midwest
Reproductive, and demographic data were collected on 48 breeding pairs of Loggerhead Shrikes (Lanius ludovicianus) in Minnesota during 1986 and 1987 in an attempt to identify possible causes of a noted decline in midwestern shrike populations. These data, combined with data taken from other passerine studies, were used to construct a stochastic model of the population dynamics of a hypothetical Loggerhead Shrike population based on the following variables: productivity per pair, annual adult survival rate, and annual juvenile survival rate. Our model predicted a 20% mean annual rate of decline for the population, which closely resembles and observed 29% decline in breeding pairs from 1986 to 1987. Reproductive success was high; therefore, we conclude that a decline in the upper midwestern shrike population is probably due to factors on their non-breeding range.
Chapter 3: Habitat availability and suitability for loggerhead shrikes in the upper Midwest
A decline of the Loggerhead Shrike (Lanius ludovicianus) in the eastern United States has evoked much concern over the status of the species and the need for conservation activities. Several explanations for the decline have been proposed, including: poor reproduction, shortage of suitable breeding habitat, and high overwinter mortality. This paper describes a system for assessing the suitability and availability of Loggerhead Shrike breeding habitat in the upper Midwest. Using information form previous studies to supplement our own data, we have constructed a habitat evaluation model using procedures analogous to the U.S. Fish and Wildlife Service's Habitat Suitability Index (HSI) models. Our model is based on habitat features, such as the percent grassland cover, availability of perch sites, and availability of nest sites. It also incorporates measures of the fitness of shrikes occupying various habitats. We were unable to find convincing evidence that the abundance of Loggerhead Shrikes is limited primarily by availability of breeding habitat.