Evaluating space use and pesticide exposure risk for burrowing owls in an agricultural environment Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/41687m392

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  • Large burrowing owl (Aihene cunicularia) populations exist in areas of intensive agriculture in California, and pesticide exposure has been identified as a potential threat to population persistence. I evaluated breeding season use of agricultural fields by adult male owls using radio telemetry, and examined egg contaminant residues to estimate population-level effects on reproduction and survival. Reproduction and survival were estimated annually, and an index of diet was inferred from pellet samples. A total of 11 adult males in 1998 and 22 in 1999 were successfully radio-tracked. Mean fixed kernel home range sizes were 172 ha (SE=68) in 1998 and 98 ha (SE=16) in 1999. Pellet analyses indicated a substantial increase in the numbers of rodents consumed in 1999, associated with an observed population explosion of California voles (Microtus calfornicus). Distance to the nest was the most important factor in differentiating between foraging and random locations, and there was no tendency to select or avoid any cover type. Owls did forage in agricultural fields, but I failed to find evidence of selection or avoidance of fields recently treated with pesticides. A total of 92 eggs were collected over 5 years. Egg contaminants were generally limited to the presence of p,p'DDE, which fluctuated by 4 orders of magnitude among years, from 0.05 ug/g to 33 ug/g fresh weight p,p'DDE. There was a general pattern of decline in egg residues over time for individual birds. The levels of p,p'DDE I documented did not appear to have any effect on either productivity or survival of adult females, nor were they clearly related to diet. I modeled the effects of various pesticide exposure impacts on demographic rates and determined that exposure rates based on field data would lead to relatively minor declines in population growth rate. An elasticity analysis of burrowing owl demographic parameters revealed a variable pattern, but generally indicated that factors influencing anyone of the demographic parameters of burrowing owls can have a substantial impact on population growth rate.
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