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

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. / Graduation date: 2002
Date22 April 2002
CreatorsGervais, Jennifer A.
ContributorsAnthony, Robert G.
Source SetsOregon State University
Detected LanguageEnglish

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