Estimating the social burden of bald eagle management within the lower Chesapeake Bay

OBJECTIVES:

Because the management of bald eagles includes activity restrictions and occurs primarily on privately owned land there is an inherent conflict between management objectives and property rights. The objectives of this project were to 1) estimate the amount of privately held land that was subject to bald eagle management guidelines and 2) to estimate the value of lands under restrictions as a surrogate indicator of social burden.


PROJECT DESCRIPTION

As the infrastructure associated with modern society expands across the globe, man-made hazards represent a growing source of mortality for many bird species. Fishing gear, roadways, buildings, oil spills, wind turbines and many other structures represent passive hazards within the landscape that contribute to annual mortality rates. For a growing number of species, mortality attributed to man-made hazards has been shown to be unsustainable and is contributing to documented declines. How to mitigate the demographic impacts of such hazards has become an important theme in conservation biology. Overhead electrical lines represent a hazard to flying birds and have been the topic of numerous symposia and management plans focused on understanding and mitigating their impacts. Two general strategies have been pursued to mitigate the impact of electrical lines on birds including 1) making poles and lines “safer” for birds and 2) reducing the overlap between birds and the hazard by placing lines away from the primary activity centers of vulnerable species. “Avian-safe” standards have been developed for line engineering that includes specifications for wire spacing to reduce mortality for species with long wingspans and guidelines for grounding and insulating transformers to prevent pole electrocutions. Although arguably the preferred strategy, much less progress has been made in reducing the interaction between birds and lines by avoiding avian activity centers when installing new lines or the removal of existing lines within such locations. Identification and delineation of major activity centers for vulnerable species has been one of the major impediments to progress. In this project, we used satellite telemetry and Brownian bridge movement modeling to develop a population-wide, utilization probability surface for bald eagle 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 overlaid the electrical network on the probability surface within Aberdeen Proving Ground, a 350-km2 military installation in order to identify intersections between power lines and eagle activity centers. We overlaid line-attributed mortalities (n = 60) documented on the installation to assess the relationship between mortality rates and utilization probabilities. Areas of high bald eagle use were relatively rare on the landscape with only 0.1% and 5% of the area accounting for 10% and 30% of estimated activity respectively. Most electric lines were distributed away from eagle activity centers with only 0.3% of lines located within areas with the highest estimated use. Eagle mortalities were highly skewed to lines that overlapped with eagle activity centers. Eagle mortality rates were 42 times higher along lines associated with the highest eagle use compared to lines associated with the lowest use suggesting that estimated utilization may be an effective proxy for mortality risk associated with electric line hazards. The majority (71.9%) of high-use bald eagle areas delineated within the study area have no existing electric lines. Utilization probabilities may be a potential tool for site-specific infrastructure planning.


Years: 2007-2011

Status: Complete

Project Partners: Virginia Department of Game and Inland Fisheries, United States Fish and Wildlife Service, United States Geological Survey, United States Park Service, Army Corps of Engineers, Virginia Society of Ornithology

CCB Staff: Bryan Watts

Project Contact: Bryan Watts bdwatt@wm.edu (757) 221-2247