REPRODUCTIVE SUCCESS AND ANNUAL HABITAT PREFERENCE BY RED-SHOULDERED HAWKS IN SOUTHERN TEXAS

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2011-05

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Identifying 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.

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