Data Associated with \u27Earlier Nesting by Predatory Bird is Associated with Human Responses to Climate Change\u27

Summary: 1. Warming temperatures cause changes in growing seasons and prey that drive earlier breeding by birds, especially dietary specialists within homogenous habitat. Less is known about how generalists respond to climate-associated shifts in growing seasons or prey, which may be occurring at different rates across land cover types used by predators. 2. We studied whether breeding phenology of a generalist predator, the American kestrel (Falco sparverius), in a mosaic of non-irrigated shrub/grasslands and irrigated crops and pastures, was associated with changes in growing seasons and, presumably, prey abundance. We examined the potential relationship between prey abundance (small mammals) and Normalized Difference Vegetation Index (NDVI) within a breeding season, recorded changes in NDVI to estimate the start of the growing season (SoGS) in irrigated and non-irrigated lands, and used both estimates of annual SoGS to predict the timing of kestrel nesting from 1992-2015. Finally, we related changes in the timing of SoGS in irrigated lands to planting of crops and weather. 3. A positive relationship between NDVI and small mammal abundance 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) since 1992. Average date of nesting was positively associated with the SoGS in irrigated lands and the average date of nesting advanced 15 days in the last 24 years. Irrigated SoGS advance was associated with earlier planting of crops after warmer winters, which have become more common. 4. Results show that despite different patterns of change in land cover types, kestrels responded to shifts in prey. Kestrels may preferentially track prey in irrigated areas compared to prey from non-irrigated areas because irrigated areas provided higher quality prey, or earlier prey abundance may release former constraints on other selective pressures to breed early, such as seasonal declines in fecundity or competition for high-quality mates. 5. This is one of the first examples of an association between human adaptations to climate change and shifts in breeding phenology of wildlife