UC Merced

American Kestrel populations have declined since the 1960s, and the cause has yet to be identified. To further comprehend the decline of the American Kestrel, my study examined kestrel breeding productivity including occupancy, hatching success, and fledging success in nest boxes in conjunction with the identification of dietary resources. In this thesis, I monitored and documented the breeding success of the American Kestrel from nest boxes on the UC Merced’s Vernal Pools and Grassland Reserve from 2014-2016. I also analyzed food resources of the population and of individuals to quantify intraspecific and temporal dietary variation. I employed a multi-faceted approach to fully examine diet composition, breadth, and variation within a population. I used pellet and prey remain analysis as well as stable isotope analysis of carbon, nitrogen, and hydrogen in feathers and prey items. Nest box occupancy increased over the three-year study period from 60% to 80%. In 2016, the hatching and fledging success decreased drastically from 68% to 44% and 100% to 69%, respectively. Predation and nest failures occurred more frequently in 2016 compared to 2015 and 2014. Based on examination of prey and pellet remains diet composition was similar in 2015 and 2016, kestrel’s consumed mostly spiders, orthopterans, and small birds. In 2016, late nesting kestrels (May and June) consumed a greater number of orthopterans compared to kestrels that nested earlier in the season who ate spiders. Stable isotope analyses revealed that adults may feed at a higher trophic level than nestlings. Stable isotope mixing models indicated that adult diets in 2015 and 2016 were composed principally of orthopterans (2015 = 25-59%, 2016 = 18-60%) and birds (2015 = 28-58%, 2016 = 15-60%). In 2015, nestling diets were similar to adult diet with slightly different proportions of orthopterans (18-72%) and birds (17-47%). In 2016 nestlings, diets were comprised of orthopterans (15-58%) and mammals (9-56%). Overall, American Kestrels colonized the nest boxes rapidly, however, reducing predation of eggs and nestlings by mitigating predator access to nest boxes is crucial to increasing hatching and fledging success on the Merced Vernal Pools and Grassland Reserve. In terms of dietary resources, invertebrates are the largest proportion of the kestrel diet. Vertebrate consumption could also be an important factor in kestrel diet due to the higher amount of biomass in vertebrate prey. Preservation of key prey items is crucial to the conservation of the American Kestrel population. Changes in dietary resource abundance could be a predominant factor influencing the decline of the species.