Etter, M.A. 1995. 1995 Minnesota loggerhead shrike survey. Report submitted to the Nongame Wildlife Program, Minnesota Department of Natural Resources. 19+ pp.
The Loggerhead Shrike (Lanius ludovicianus) historically bred throughout the United States, southern Canada, and northern Mexico. Declines in Loggerhead populations have been recorded in all areas of the birds breeding range (Peterjohn and Sauer 1994, Yosef 1994). The most severe declines have occurred in the Midwest and in New England, where the species has been virtually extirpated. In Minnesota, it is considered a threatened species (Coffin, B. and Pfanmuller, L. 1988). It breeds in southern and western Minnesota between April and September but is not present during the winter. Anecdotal evidence suggests that densities of Loggerhead Shrikes were considerably higher early in the century. (Table 1, Roberts 1937).
Several hypotheses have been proposed for the decline of Loggerhead Shrikes. For this report I will simply list them and elaborate only on those hypotheses which I will later relate to results of this study. Those presented here are adapted from recent papers by Yosef (1994) and by Yosef and Lohrer (1995).
- Low reproductive success (but see Brooks 1988)
- Loss of breeding habitat, including removal of fencelines which serve as observation perches and nest sites.
- Reduced survival on winter range
- Pesticide ingestion
- Inclement weather during nesting cycle
- Interspecific competition
- Collisions with cars.
The most comprehensive study of Loggerhead Shrikes in Minnesota was done by Bonnie Brooks for her Master's project "The breeding distribution, population dynamics, and habitat availability and suitability of an upper midwest Loggerhead Shrike population." between 1986 1987. She studied three fundamental aspects of the biology of Loggerhead Shrikes in Minnesota. First, she carried out a survey of breeding shrikes in 12 counties. In 1986 she found 32 nests and classified an additional 5 cases as likely nesting, though the nest was never found. In 1987 she found 27 nests. In both years nests were monitored and clutch size, hatching success and fledging success were recorded. Her results showed high overall fledging and hatching success (72% and 76% respectively) (Brooks 1988).
The second aspect of Bonnie Brooks' study was to build a population projection model parameterized by the results of nest monitoring from 1986 and 1987 and using male return rate (47%) as an estimator of adult survival. The results of this model suggested that the Minnesota population was not stable, and predicted a 20% annual decline in population size (Brooks 1988).
Finally, Ms. Brooks used quantitative methods to measure habitat availability for Loggerhead Shrikes in Minnesota. Her results showed that, while shrike habitat may be limited, there is more habitat available in Minnesota than there are breeding Loggerhead Shrikes to occupy it (Brooks 1988). For a more complete discussion of these results, please refer to the original documents.
Ms. Brooks' study was expanded into an annual monitoring program for the MN DNR. Her methods were reproduced in subsequent years to monitor the breeding population of Loggerheads in Minnesota. That monitoring program has continued, with some modifications of methods, until the present.