Nest site habitat selection and nestling diet of the the Texas red-shouldered hawk buteo lineatus texanus in south Texas

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2007-08

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Abstract

The following chapters of this thesis examine different aspects of the breeding ecology of the Texas subspecies of red-shouldered hawk. In the second chapter, titled “Nesting Habitat Selection of Red-shouldered Hawks In South Texas”, we create a resource selection probability function for red-shouldered hawk nesting habitat selection using a logistic regression model of characteristics measured at 15 active nest sites and 45 unused sites in 2005-2006 in south Texas. Vegetation characteristics were measured at two scales (center trees and 0.04ha sites). Models from both scales were evaluated independently with the best models from each scale then evaluated between scales. Tree height and diameter appeared to be the most important characteristics determining nest site selection at both scales, however the best model from the coarser 0.04ha site scale received a much greater weight than the best model from the finer center tree scale. The best site scale model used the parameters: canopy height, average tree diameter, and basal area to correctly predict nest sites 93.3% of the time. The resource selection probability function from this model can be applied to assess the suitably of forest stands as redshouldered hawk nesting habitat in south Texas.

In chapter 3, titled “Temporal and Spatial Variation in Prey useby Breeding Red-shouldered Hawks”, we examine daily temporal patterns in the prey use by red- shouldered hawks to determine if prey type used is influenced by time of day. Additionally, we examine fine scale and large scale spatial variation in prey use by calculating Morisita’s similarity indices across nests within our study as well as across previously published studies. Through this we find significant temporal patterns in prey type used by red-shouldered hawks breeding in south Texas. More amphibians were used during mid afternoon than during late evening, while more insects were used during late evening than earlier in the day. Breeding pairs of red-shouldered hawks within our study demonstrated differences in prey type used. Similarly, diets of red-shouldered hawks varied significantly across much of their breeding range, and coincided with latitudinal differences between study sites. These spatial patterns in prey use are likely caused by spatial differences in prey availability and demonstrate the dietary flexibility of redshouldered hawks. Furthermore, these findings indicate the potential of regional variations in prey availability to be an additional factor influencing reproductive success and survival of red-shouldered hawks in North America.

In chapter 4, titled “Nestling Diet and Adult provisioning Rates of Texas Redshouldered Hawks in South Texas” we investigate the prey provisioning rates of adult red-shouldered hawks to determine its potential effect on red-shouldered hawk productivity. Through video surveillance we identified 1320 prey items delivered to nestlings. We found, insect, mammalian and reptilian prey were used more frequently than other prey types, however insect prey contributed less to the total biomass than all other prey types except avian prey. In addition, we found that nestlings in broods of 2 and broods of 3 were provided prey in similar proportions. Adults rearing larger broods delivered more prey per hour and more grams of prey per hour, but a similar number of deliveries and grams of prey on a per nestling basis. We conclude that provisioning rates and not the size of prey used are different between adults rearing different sized broods.

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Keywords

Resource selection probability function, Buteo line

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