Graduate Thesis Or Dissertation
 

The role of food limitation and predation on reproductive success of burrowing owls in southern California

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  • The temporal and spatial variation in reproductive success exhibited by burrowing owls (Athene cunicularia hypugaea) facilitates the testing of mechanisms influencing this parameter. I predicted that food supplementation would increase reproductive success through one of two means: (1) directly, through increasing food availability and decreasing the effects of brood reduction, resulting in higher growth and survival of the youngest owlets, or (2) indirectly, through increasing nest attendance and decreasing predation rates of eggs and owlets, demonstrating the relationship between food availability and nest predation. Food limitation on reproductive success would also be evident if supplementation resulted in increased parental condition. I tested for the effects of food limitation and predation on the reproductive success of burrowing owls by conducting experimental food supplementation studies at the Sonny Bono Salton Sea National Wildlife Refuge from April - July, 1999 and 2000. Food supplemented nests had higher reproductive success, female condition, owlet growth, and owlet survival, demonstrating direct food limitation at the level of the individual. Male condition was not affected by food supplementation possibly indicating a lack of reproductive costs or a fixed paternal effort due to trade-offs between future survival and reproduction. Nest attendance was greater in supplemented nests although attendance was not associated with higher reproductive success, probably because predation rates on eggs and owlets were low. In nests not supplemented with food, brood size was reduced through poor growth and survival of later-hatched owlets, suggesting brood reduction is an adaptive response to fluctuating food availability. This study provides strong support for the hypothesis that burrowing owl reproduction is food-limited in the study area.
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