Methods for studying stopover ecology of migrating landbirds with weather surveillance radar

Date
2017
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Publisher
University of Delaware
Abstract
Populations of numerous migratory landbird species in the eastern United States are declining and these populations may be limited during their migratory journey. Weather surveillance radar is a useful tool for monitoring large scale movements of birds during migration and particularly for mapping stopover distributions of migratory landbirds because it detects birds low to the ground as they initiate nocturnal migratory flight. This approach is sensitive to the time when flight exodus is sampled because the number of birds in the air at this time changes rapidly. Thus, in order to use radar to map densities of migrant birds on the ground, an empirical determination is needed to identify an unbiased method to sample migrant density in the air. I assessed the relationship between seasonal mean migrant bird ground densities and seasonal mean radar reflectivity, an estimate of emigrant bird density aloft, sampled at a series of sun elevation angles ranging from 1.5° to 10° below the horizon at 26 sites in Delaware, Maryland, and Virginia within 80 km of the Dover, Delaware (KDOX) and Wakefield, Virginia (KAKQ) WSR-88D stations during fall 2013 and 2014. Additionally, because the timing of flight exodus varied among nights within and among radars, I fit a logistic growth curve to the change in mean reflectivity through time during the onset of nocturnal flight to determine the sun angle at the inflection point of the curve (i.e., at the maximum growth rate) for each sampling night by radar. I computed correlations between ground bird densities and mean reflectivity among the series of radar sampling times and among a series of times relative to the inflection point of daily exodus curves. Sampling radar at the inflection point of daily exodus curves provided a consistent moderate to strong correlation and this approach is likely robust to broad spatio-temporal changes in the timing of exodus that would not be accounted for by using an absolute sun angle. Placing stopover sites for migratory landbirds into a functional framework based on intrinsic and extrinsic factors may be a key to conserving declining populations. Landbirds typically use numerous stopover sites during migration, which vary in usefulness regarding replenishment of energetic resources. To classify stopover sites across a broad spatial scale, I determined relative stopover duration at study sites mentioned above combined with data collected using similar protocols during a previous study in Alabama and Louisiana by integrating ground transect data with weather surveillance radar data. Functional types within the function framework initially included “fire escape,” “convenience store,” and “service hotel”, but clustering resulted in four distinct groups, which I redefined as “coastal fire escape,” “inland rest stop,” “convenience store,” and “full service hotel.”, a novel designation for landbirds. ☐ I incorporated hardwood forest within 5 km, distance to the coast, and insect density into the analysis as potential drivers of stopover duration. One third of our study sites were deemed as full service hotels, making the majority of our study sites coastal fire escapes, inland rest stops, or convenience stores, which typically receive less attention in conservation planning. There were regional differences, where the mid-Atlantic lacked full service hotels and the Gulf Coast lacked coastal fire escapes. Using a system of functional types facilitates the prioritization of stopover sites because I can evaluate sites within each functional type rather than across functional types. Each functional type serves a purpose and all are necessary in conservation, but all sites cannot be protected, so using a functional type system allows us to prioritize sites more easily and efficiently. Using weather surveillance radar and ground surveys allowed me to assess stopover use at a broad spatial scale, which is difficult to do with more traditional methods.
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