Using Brownian bridges to assess potential interactions between bald eagles and electrical hazards within the upper Chesapeake Bay

Abstract

One of the most effective means of minimizing mortalities of large raptors from collisions with hazards is to locate hazards away from major activity centers. A reliable means of delineating bird activity centers on the landscape has been a significant impediment to progress in proactive infrastructure planning. We used Brownian bridge movement modeling to develop a population-wide, utilization probability surface for bald eagles (Haliaeetus leucocephalus) within the upper Chesapeake Bay. We used locations (n ¼ 320,304) for individuals (n ¼ 63) tracked with global positioning system (GPS) satellite transmitters between 2007 and 2011 in the analysis. We intersected the electrical network on the probability surface within Aberdeen Proving Ground, a 350-km 2 military installation to identify overlap between power lines and eagle activity centers. We also overlaid locations of eagle mortalities attributed to the lines (n ¼ 67) 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 utilization. Most electric lines were along roads and distributed away from eagle activity centers, with only 0.3% of lines located within areas with the highest estimated eagle use. Eagle mortalities were highly skewed to lines that overlapped with eagle activity centers. Eagle mortality rates (birds/100 km/yr) were 42 times higher along lines associated with the highest 10% of eagle use compared to lines associated with the lowest 10% 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. Ó 2015 The Wildlife Society.

Structure and resilience of bald eagle roost networks

“…Our study area (12,920 km 2 ) included the northern part of the Chesapeake Bay from the Bay Bridge at Annapolis, Maryland, USA, to just above the Conowingo Dam on the Susquehanna River (Watts and Mojica , Watts et al ). This area supported a significant number of eagles during the autumn and winter months and a large breeding population (Steenhof et al , Watts et al ).…”

“…The southwestern portion of the study area included urban expanses of Baltimore and Annapolis that were dominated by extensive residential and commercial development. These areas supported very little eagle activity (Buehler et al , Watts et al ). Remaining portions of the study area were primarily rural, with forest lands interspersed with agriculture.…”

“…Remaining portions of the study area were primarily rural, with forest lands interspersed with agriculture. These areas supported an extensive network of communal roosts and many significant foraging areas (Buehler et al , Watts and Mojica , Watts et al ).…”

Mapping bald eagle activity shadows around communal roosts

“…The southwestern portion of the study area includes urban expanses of Baltimore and Annapolis, Maryland, which are dominated by extensive residential and commercial development. These areas support very little eagle activity (Buehler et al , Watts et al a,b ). Remaining portions of the study area are primarily rural, with forest lands interspersed with agriculture.…”

“…Remaining portions of the study area are primarily rural, with forest lands interspersed with agriculture. These areas support an extensive network of communal roosts (Buehler et al , Watts and Mojica ) and many significant foraging areas (Watts et al ,).…”