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. / Master of Science

This thesis summarizes findings on a recognition and categorization of different sorts of stimuli at birds, it deals with predator recognition in the Red-backed Shrike (Lanius collurio) in its practical part. The aim of my study was to examine if Red-backed Shrikes follow by sign stimuli or colour of stimulus during the predator recognition. I performed my research by means of experiments with dummies, I have observed antipredatory reactions of Red-backed Shrikes on various types of dummies which differ in the presence or absence of features of raptor or in the different coloration. I concluded according to Shrikes' reactions that during the predator recognition the information on sign stimuli playes the principal role, but only this information alone is not fully sufficient. Antipredatory behaviour of the Red-backed Shrike was also influenced by sex of bird apart from type of dummy, males were more active during the nest defence than females.

In previous studies with stuffed dummies, we found interesting difference in nest defence of Red-Backed Shrike (Lanius collurio) against two similar nest predators: Jay (Garrulus glandarius) and Magpie (Pica pica). Jay was attacked very intensively, whereas Magpie almost wasn´t hited and Shrikes seemed to keep in cover. Here, I describe the antipredation behavior of the Red Backed Shrike against another stuffed corvids presented closely to Shrikes nests: Jay (Garrulus glandarius), Common Nutcracker (Nucifraga caryocatactes), Rook (Corvus frugilegus), Crow (Corvus corone) and Raven (Corvus corax). I found Jay and Nutcracker to be attacked by Shrikes very strongly, whereas Rook, Crow and Raven were attacked rarely (Shrikes only flied around them or sit at a distance, watching the dummy, sometimes accompanied with alarm-calls). Shrikes response is affected mainly by kind of predator, by age of youngs and by quality of concealment of nest. More - www.mn.ic.cz