Barton J. Paxton

Ospreys of the Chesapeake Bay: Population Recovery, Ecological Requirements, and Current Threats

Abstract The Chesapeake Bay supports the largest Osprey (Pandion haliaetus) breeding population in the world. The population experienced a dramatic reduction due to biocide-induced reproductive suppression in the post World War II era and reached an estimated low of 1,450 pairs by the early 1970s. By the mid 1990s, the population recovered to an estimated 3,500 pairs and breeding was documented on 427 of 878 named tributaries of the tidal Bay. Recovery has been exponential but spatially variable with average doubling times for defined geographic areas varying by more than an order of magnitude. Rates of population growth have been negatively related to salinity with the highest rates occurring within tidal fresh reaches suggesting that recovery has progressed from the main stem of the Bay toward the fall line. Virtually nothing is known about the breeding ecology of Ospreys in the lower saline waters of the Bay. The increase and diversification of man-made structures used for nesting has made a fundamental contribution to recovery and current distribution. A synthesis of information from several field sites throughout the Bay shows a collective increase in reproductive rate (young/active pair) from less than 0.8 in the 1960s to more than 1.2 by the mid-1980s followed by a reduction to below 1.0 in the late 1980s. Threats to the population continue to be the release of new classes of contaminants into the estuary and anthropogenic activities that have the potential to suppress reproductive rates and juvenile/adult survivorship.

Seasonal Variation in Space Use by Nonbreeding Bald Eagles Within the Upper Chesapeake Bay

Abstract Access to food resources is essential to self-maintenance and reproduction and, for species of conservation concern, foraging areas are considered critical habitat. Human disturbance is an important factor restricting access to prey resources for Bald Eagles (Haliaeetus leucocephalus) and guidelines in the Chesapeake Bay have been developed to mitigate its impact. However, our ability to implement such guidelines has been limited by a lack of information on important foraging areas. We used Brownian bridge movement modeling to develop a population-wide utilization probability surface for Bald Eagles along shorelines within the upper Chesapeake Bay. We used locations (n  =  320 304) for individuals (n  =  63) tracked with GPS satellite transmitters between 2007 and 2011 in the analysis. We examined seasonal variation by developing utilization surfaces for summer and winter. Although shoreline use was widespread, segments receiving high levels of activity were relatively rare. Shoreline classified as having the highest category of use and accounting for 10% of the total utilization made up 0.41% and 0.55% of the total shoreline for winter and summer, respectively. From a management perspective, there is a clear pattern of diminishing returns in conservation value for including sequentially lower-use shorelines in land-use management plans. Shoreline use shifted dramatically in both location and extent between seasons. During the summer months, use was highly concentrated on shorelines along the main stem of the Chesapeake Bay or along major (>1 km wide) tributaries. During the winter months, use shifted away from the main stem of the bay and was more focused on minor (<100 m wide) tributaries and inland ponds. Seasonal shifts in shoreline use suggest the need for season-based management objectives.

Status and Habitat Use of the Wayne's Black-throated Green Warbler in the Northern Portion of the South Atlantic Coastal Plain

Abstract Dendroica virens waynei (Waynes Warbler) is a unique, disjunct subspecies of Dendroica viren virens (Black-throated Green Warbler) that is restricted to the South Atlantic Coastal Plain from southeastern Virginia to South Carolina. We surveyed a network of 265 fixed-radius plots to examine seasonal occurrence, spatial distribution, and patterns of habitat use by Black-throated Green Warblers. Survey plots were chosen to represent the full gradient of forest types within the region. Plots were surveyed 7 times between early April and mid-June, 2001. Detections of Black-throated Green Warblers began in early April, increased to a peak in late April, and then declined throughout May and early June. Birds were detected during 251 (13.5%) of 1862 point counts conducted. Detections were widespread and included 114 of 266 (52.6%) survey plots. Forest composition had a significant influence on the distribution of breeding sites. The frequency of plots classified as breeding sites was higher than expected for plots containing Pinus taeda (Loblolly Pine), Chamaecyparis thyoides (Atlantic White Cedar), and Taxodium distichum (Bald Cypress). The density of these tree species within survey plots was significantly higher for plots classified as breeding sites compared to plots classified as unoccupied. This response was particularly significant when all three tree species were combined.