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Raptor ecology along an urban-rural gradient in Reno-Sparks, Nevada, USA
AdvisorBassett, Scott D.
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Urbanized landscapes contain some of the most complex and rapidly changing ecosystems found on Earth. The habitat characteristics of urban areas, such as noise level, building size and material, human activities, and microclimates can be more variable than those in unurbanized areas. Though urbanization has extirpated some species, and disrupted the life history and behavior of many others, research has identified patterns of species success at varying levels of urbanization. The most successful and prevalent apex predators in urban areas globally are raptors. This success is largely enabled by their high levels of mobility, intelligence, and generalism. The goal of this research is to gain insight to raptor population dynamics in the urban environment of Reno-Sparks, Nevada, USA, by examining: 1) the distribution of raptor species’ nesting sites along an urban density gradient, 2) the breeding ecology of urban Red-tailed Hawks, and 3) the human perceptions of Red-tailed Hawks in the urban area. The first study attempts to elucidate patterns in species’ nest-site selection along the urban density gradient (from the urban fringe to the urban core). Between 2014–2016 we recorded 171 individual nests belonging to nine species: Golden Eagles (Aquila chrysaetos), Osprey (Pandion haliaetus), Great Horned Owls (Bubo virginianus), Red-tailed Hawks (Buteo jamaicensis), Swainson’s Hawks (B. swainsoni), Red-shouldered Hawks (B. lineatus), Cooper’s Hawks (Accipiter cooperii), Sharp-shinned Hawks (A. striatus), and American Kestrels (Falco sparverius). The study also introduces a versatile and replicable method of modeling urban density at the land parcel-level based on the number of residents, number of employees, building height, and building footprint. Urban density was calculated at four spatial scales around each nest (nest site, macrohabitat, nearest-nest midpoint, and landscape). Based on initial plots and descriptive statistics, Accipiter species and Red-tailed Hawks nested along the widest ranges of the urban density spectrum and nested closest to the urban core, while Golden Eagles and Swainson’s Hawks nested on the urban fringe. Great-horned Owls nested in more urban areas than Red-tailed Hawks at the population level but did not extend as deeply into the urban core. Urban density was lowest at the nest-site scale, and the highest at the nearest-nest midpoint and landscape scales. Species tended to occupy a wide range of the building-footprint spectrum, indicative of their use of suburban landscapes, but not the building-height or employee spectrum. The density spectrum for each scale was binned into five equal ranges to test whether species nested in different sections of the urban gradient. A Fisher’s exact test revealed that the species showed little differentiation along the urban density gradient at the population level (P = 0.90). This study was not exhaustive of nest sites, nor of the total nesting species in the study area, but should still be a sound depiction of the distribution of nesting raptors in this urban area. The second study explores the nestling diet, parental roles, nest success, and productivity of Red-tailed Hawks relative to urban density and land cover type. Nests were observed between 6:30–8:30 h and 9:00–11:00 h during one random day in each of four date ranges after hatch (3–10 d, 11–18 d, 19–26 d, and 27–34 d) using nest cameras, and researcher and citizen scientist observations. The elements of breeding ecology were compared with urban density and land cover type at the same four spatial scales used in Chapter 1 using generalized linear or multinomial models. We calculated the diversity of prey items for each nest using Shannon’s diversity index and found that prey diversity decreased when the dominant land cover type was built-impervious or riparian and the landscape scale. The mammalian prey comprised the greatest portion of the nestling diet overall and increased in suburban densities compared to the urban core or fringe. This trend was complimented by avian prey which decreased in the suburban areas and increased in the urban core and fringe. Reptilian prey decreased with urban density. The proportion of feeding events conducted by the female decreased with urban density following a negative curve at the nearest-nest scale. The population’s nesting success rate was 84.25%. Nest success for individual pairs decreased in riparian areas, and increased in grassland and shrub-covered areas at the nearest-nest scale. The number of fledglings followed a decreasing trend with urban density at the nest-site scale and overall fledge rate per nest decreased in riparian habitat at the landscape scale. The productivity of this population was 2.13 – high relative to populations in other study areas. Many patterns were identified in this study that may result in positive or negative outcomes for the nesting attempts, but the overall nest success and productivity of this population is high relative to populations elsewhere. The results of this study reflect the high levels of dietary and habitat generalism of Red-tailed Hawks, as well as some of their habitat limitations.The third study assesses the human perceptions of raptors in the Reno-Sparks urban area as humans’ attitudes towards a species can heavily influence the species’ survival. During the fledging period of 2016 we surveyed 280 homes in the Reno-Sparks area about their perceptions of and interactions with Red-tailed Hawks. We surveyed the five homes closest to a nest, and five homes ≥740 m away from the nest and ≥100 m from each other. Survey questions addressed elements of the human-raptor relationship such as whether the residents who had domestic animals threatened by raptors were more likely to dislike them. We also observed these elements relative to age to whether residents lived near or far from nests. Among residents, 70% viewed hawks positively, 3% negatively, and 27% indifferently. Results from generalized linear mixed models indicated that whether residents had domestic animals threatened by hawks, or raised chickens, did not impact whether the resident felt positively about hawks. Age did not predict residents’ perceptions of hawks with high probability but negative perceptions were clustered between 40 and 70 years of age. A mediation analysis revealed that human experiences with hawks (e.g. observing hawks) fully mediated the relationship between resident proximity to a nest and their perceptions of hawks (P < 0.001). To clarify, residents living near a nest resulted in them viewing hawks positively but this was because residents were experiencing the hawks, not just living near them. We conducted a second mediation analysis that revealed that resident perceptions of hawks only partially mediated the relationship between resident proximity to a nest and their perceptions of hawks, and did so to a lesser extent than in the first mediation model. Sobel tests substantiated our mediation analyses (P < 0.005). These findings showed that living near a nest and experiencing hawks will lead to humans viewing them more positively while addressing the scenario in which residents observe hawks because they view them positively. While some cases of raptor persecution likely occur throughout the year in our study area, the results of our study suggest that active human persecution is unlikely to be a major influence in preventing raptors from nesting successfully.These three studies create a baseline understanding of raptor ecology along the urban-rural gradient in Reno and Sparks. Combined, they consider multiple critical facets of raptor existence by involving scientific inquiry at the community level, population level, and in the context of human attitudes. The results from these studies help portray raptor existence in this urban area, and can be used to assess ecological and cultural shifts in the future.