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Shawn H. Smith – Earlier nesting by a predatory bird is associated with human adaptations to climate change

Shawn H. Smith with falcon

Climate change impacts on phenology may create a mismatch when coinciding cycles of different species within an ecosystem no long interact at the optimal level.  Typically, mismatches have been described when prey species change their cycle, but predators remain unchanged.  In southwestern Idaho, American kestrels (Falco sparverius) have shifted their nest initiation 28 days earlier over the last 25 years, but the start of spring has not shifted.

I am studying the relationship between Normalized Difference Vegetation Index (NDVI), small mammal abundance, and American kestrel reproduction to determine if a mismatch between prey availability and the nesting period is occurring. I will use track plates to capture footprints of small mammals to index the population to determine when peak abundance occurs. I will also be estimating grasshopper abundance using a ring count technique. I will validate my prey samples using cameras to capture the diet composition of nesting kestrels. If the peak in prey availability reliably coincides with NDVI peaks, I will create hind casting models that estimate the changes in prey availability during the nesting period of American kestrels over the last 25 years. These hind casting models will indicate whether or not a mismatch occurs in our system. Results from the study will contribute to understanding how species respond to climate change and the cascading effects of changes in the annual cycle.

Thesis Title:  Earlier nesting by a predatory bird is associated with human adaptations to climate change

Shawn H. Smith with prairie mouseAbstract:  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.