Browsing by Author "Strobel, Bradley N."
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Item Nest site habitat selection and nestling diet of the the Texas red-shouldered hawk buteo lineatus texanus in south Texas(2007-08) Strobel, Bradley N.; Boal, Clint W.; Wallce, Mark; Perry, GadThe 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.Item REPRODUCTIVE SUCCESS AND ANNUAL HABITAT PREFERENCE BY RED-SHOULDERED HAWKS IN SOUTHERN TEXAS(2011-05) Strobel, Bradley N.; Boal, Clint W.; Andersen, David E.; Haukos, David A.; Griffis-Kyle, Kerry; Blankenship, TerryIdentifying the vegetation characteristics resulting in habitat use by nesting raptors provides information important to species conservation and management. I examined nesting habitat preference of red-shouldered hawks (Buteo lineatus) using conditional logistic regression on characteristics measured at 27 occupied nest sites and 68 randomly selected unused sites in 2005–2009 in southern Texas. I measured vegetation characteristics of individual trees (nest trees and unused trees) and corresponding surrounding 0.04 ha plots. I evaluated the importance of tree and plot characteristics to nesting-habitat selection by comparing a priori tree-specific and plot-specific models using Akaike‘s Information Criterion corrected for small sample size (AICc). Models with only plot variables carried 14% more weight than models with only center-tree variables. The model-averaged odds ratios indicated red-shouldered hawks selected to nest in taller trees and in areas with higher average diameter at breast height (dbh) than randomly available within the forest stand. Relative to randomly selected areas, each 1-m increase in nest-tree height and 1-cm increase in the plot-average dbh increased the probability of selection by 85% and 10%, respectively. My results indicate characteristics of individual trees and the structure of the 0.04 ha area surrounding the tree, were associated with red-shouldered hawk nesting habitat selection. My results show red-shouldered hawks preferred taller nest trees (average 16.4 m) surrounded by larger diameter trees (average 25.4 cm) relative to the surrounding forest in southern Texas. Preference for taller nest trees by raptors is well documented and suggests an association with increased fitness. Further study is required to understand the mechanism responsible for this preference. Widespread alteration of forest communities combined with the well-documented preference for mature forests by breeding red-shouldered hawks (Buteo lineatus) has caused concern over the conservation of the species. Although habitat preferences of breeding red-shouldered hawks have been well documented, few data describe non-breeding-period habitat selection. I studied the seasonal habitat preferences of 12 adult red-shouldered hawks in southern Texas. I used radio-telemetry to collect >1,800 locations during 3 discrete phenological periods (winter, breeding-spring, and non-breeding-spring). I constructed phenological stage-specific discrete-choice models to compare the proportion of a 0.5-ha circle around each location (used and available) comprised of mature forests, young forests, brush, grass, wetland, and open-water cover types. I used general linear models to assess whether the amount of mature forest covertypes within used areas was dependent upon the sex of the individual, its breeding status, or the time of year the data were collected. During the breeding-spring-period, red-shouldered hawks preferred areas with greater amounts of mature forest cover types (selection ratio = 1.40, 95% CI = 1.16 - 1.69) and higher cover-type richness (selection ratio = 1.10, 95% CI = 1.04 - 1.21). Characteristics associated with habitat use during the breeding-spring-period were different than those associated with winter-period or non-breeding-spring-period habitat selection. Habitat use during the winter was associated with greener vegetation measured using the Normalized Difference Vegetation Index (NDVI), and avoidance of grasslands, brushlands, and wetlands. My data indicate the amount of forested cover types preferred by individual red-shouldered hawks depends upon the sex of the individual as well as its breeding stage (i.e., breeding, non-breeding). Habitat quality is often evaluated based on relative resource preferences, usually by examining disproportionate use of available resources. Alternatively, the Ideal Despotic Distribution hypothesis predicts that given a suite of territories to select from, an individual will select the highest quality territory in regards to its perceived fitness benefit. Presumably, the long-term occupancy rates of specific territories will be positively correlated with their quality. However, the predictions of the ideal despotic distribution hypothesis hinge upon the individual‘s ability to accurately assess territory quality (perfect knowledge), a condition that is difficult to meet under temporally and spatially variant environments. Using a 5-year data set on a resident population of red-shouldered hawks in southern Texas, I examined the predictions of the ideal despotic distribution hypothesis. My results were consistent with the prediction of the ideal despotic distribution hypothesis that the proportion of years a territory supported a nesting attempt was negatively related to the relative nest initiation dates. However, in contrast to the predictions of the ideal despotic distribution hypothesis, my data show that proportion of years a territory supported a nesting attempt was unrelated to the average number of eggs laid and negatively related to the average number of young fledged annually. Whereas the proportion of years a territory supported a nesting attempt varied across territories, the annual reproductive success of breeding red-shouldered hawks varied markedly from year to year and was positively associated with the number of breeding pairs observed. Overall, my results suggest that red-shouldered hawk reproductive success is influenced heavily by stochastic characteristics that violate the ideal despotic distribution hypothesis‘ assumption of perfect knowledge by individuals.