Factors affecting loggerhead shrike mortality in Virginia

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Virginia Tech


I studied loggerhead shrikes (Lanius ludovicianus) in the Shenandoah Valley in Virginia from April 1987 to April 1988; factors affecting mortality rates wring the breeding season, fall dispersal, and winter were investigated., The high reproductive,success in this study was similar to that reported by other researchers in the United States. Reproductive success was different among varying grass heights and varying amounts of bare soil areas within 100 m of shrike nests; success was highest in habitats with numerous bare soil areas and medium grass. I radio-harnessed fledglings at a mean age of 56 days to document dispersal, fall home-range areas, and survival rates. During the postfledging dispersal period, survival rates were 100% fledglings dispersed at a minimum of 78 days of age, and fall home-ranges typically contained medium grass and numerous bare soil areas. These habitat characteristics may provide shrikes with increased invertebrate availability. Winter negatively affected the loggerhead shrike population in Virginia; raptor predation was the primary cause of subadult and adult mortality, and road kill was secondary. Shrikes moved from grassland areas to shrub-forest areas during inclement weather and preyed on small birds. Home-range sizes were larger during the winter than at other times of the year. I documented the occurrence of low levels of organochlorine and organophosphates in all birds analyzed for pesticide contamination. Also, I present statistical models to predict age and sex of loggerhead shrikes; wing chord and tail white on rectrix 5 were the most useful variables in discriminating sexes and wing chord was used in discriminating ages. More research is needed on the effects of raptors on shrike populations and on the role of foraging habitat on winter areas.