Publication Date


Date of Final Oral Examination (Defense)


Type of Culminating Activity


Degree Title

Master of Science in Raptor Biology



Major Advisor

Julie Heath, Ph.D.


James Belthoff, Ph.D.


Christopher J. McClure, Ph.D.


Advancing growing seasons and prey abundance drive earlier breeding in dietary specialists because, ultimately, consumers benefit by timing their reproduction to coincide with peak prey abundance. The selective pressure to breed earlier may be lower for species that forage on diverse prey items that vary in abundance both spatially and temporally. The selective pressure may be reduced further if predators have access to a mosaic of habitats, each of which having different shifts in growing seasons. We studied whether earlier breeding of a predatory generalist, the American kestrel (Falco sparverius) nesting in a mosaic of habitat types was associated with changes in local growing seasons and prey abundance. The study area was predominately mixed sagebrush steppe/invasive grass cover types and irrigation-dependent cover types, which included crops, pastures, and lawns. Both cover types could typically be found within an American kestrel’s territory. From 1992-2015, we examined the potential relationship between prey abundance (small mammals) and Normalized Difference Vegetation Index (NDVI), recorded seasonal changes in NDVI to estimate the start of the growing season (SoGS) in irrigated and non-irrigated land covers, and used annual SoGS estimates to predict the timing of kestrel nesting. Finally, we related changes in the timing of SoGS in irrigated land cover to planting of crops and weather. The positive relationship between maximum NDVI values and small mammal abundance indicated that as maximum NDVI values increased so did small mammal abundance. This suggested that NDVI was a useful proxy for estimating shifts in the timing of prey abundance over time. NDVI-estimated SoGS advanced significantly in irrigated land cover (β = -1.09 ± 0.30 SE) but not in non-irrigated land cover (β = -0.57 ± 0.53). Date of kestrel nest initiation was positively associated with the SoGS in irrigated land cover and the date of nest initiation advanced 15 days in the last 24 years. Irrigated SoGS advance was associated with earlier planting of crops following warmer winters, which is a commonly reported human adaptation to climate change. Within their territories, most kestrels had access to both irrigated (shifting SoGS) and non-irrigated (no change in SoGS) land covers, suggesting that kestrels may preferentially track prey in irrigated land cover compared to prey from non-irrigated land cover. Kestrels may track irrigated SoGS because irrigated land cover provided higher quality prey, or earlier prey abundance may enable kestrel response to other selective pressures on nesting phenology, such as seasonal declines in fecundity or competition for high-quality mates. Future studies of climate change and wildlife in human-dominated environments should consider synergies between climate and human adaptations. Finally, studies of climate change effects should consider utilizing direct measures of growing seasons, such as NDVI, that may be more reliable indicators of environmental change than temperature alone.