Abstract
This study provides new information on the offshore movements and flight altitudes of Common Terns (Sterna hirundo), Federally-Endangered Roseate Terns (Sterna dougallii), and Federally Threatened Piping Plovers (Charadrius melodus) within the Atlantic OCS. Common and Roseate Terns are colonial seabirds with nesting populations in the northeastern portion of the US Atlantic coast. Both species use migratory staging sites along the US Atlantic coast and migrate to South America for the non-breeding period. Piping Plovers are migratory shorebirds with nesting populations that breed along the US Atlantic coast and winter in the southeastern US and Caribbean. Species-specific information on the routes, timing, and environmental conditions associated with flights over the Atlantic OCS is needed to refine assessments of exposure to offshore WEAs and to improve estimates of collision risk with offshore wind turbines (Burger et al. 2011). Information on flight altitudes is also needed to assess exposure to the Rotor Swept Zone (RSZ), generally 25 to 250 m above sea level (asl).
In this study, adult Roseate Terns (n=150), Common Terns (n=266), and Piping Plovers (n=150) were fitted with digital VHF transmitters at select nesting areas on the US Atlantic coast from 2014 to 2017. Tagged individuals were tracked using an array of automated VHF telemetry stations within a Study Area encompassing a portion of the US Atlantic OCS, extending from Cape Cod, MA to southern VA. We developed novel movement modeling techniques to assess the frequency and extent of offshore movements over Federal waters and WEAs within the Study Area. Our specific objectives were to: (1) develop spatially-explicit, 3-dimensional models to estimate movements of Common Terns, Roseate Terns and Piping Plovers in the Atlantic OCS Study Area during breeding and post-breeding periods; (2) estimate the exposure of Common Terns, endangered Roseate Terns and threatened Piping Plovers to Federal Waters and to each BOEM Lease Area and BOEM Planning Area in the Atlantic OCS Study Area during breeding and post-breeding periods; and (3) quantify effects of meteorological conditions (e.g., wind speed, wind direction, barometric pressure, temperature, visibility, precipitation), temporal variation (time of day, date), and demographic variation (breeding population, sex, reproductive success) on occurrence of Common Terns, Roseate Terns and Piping Plovers within Federal waters and WEAs within the Atlantic OCS Study Area.
Common and Roseate terns were exposed to Federal waters and WEAs during the breeding period
through post-breeding dispersal. The highest probability of exposure occurred during post-breeding
dispersal (mid-July through late September), as terns from multiple colonies made extensive movements throughout the eastern Long Island Sound to the southeastern region of Massachusetts. Exposure to WEAs was highest among Common Terns dispersing from Great Gull Island across Rhode Island Sound to staging areas in southeastern Massachusetts. Peak exposure of Common and Roseate Terns to Federal waters primarily occurred in mid-July and August during morning hours and fair weather conditions (high atmospheric pressure). Offshore flight altitudes of terns were generally below the RSZ of offshore wind turbines (25 to 250 m asl).
Peak exposure of Piping Plovers to Federal waters occurred in early August. Piping Plovers departing from their breeding grounds in Massachusetts and Rhode Island primarily used offshore routes to stopover areas in the mid-Atlantic. Individual Piping Plovers were exposed to up to four WEAs on offshore flights across the mid-Atlantic Bight. Flights in Federal waters and WEAs were strongly associated with southwest wind conditions providing positive wind support. Offshore flight altitudes of Piping Plovers generally occurred above the RSZ.
In this study, digital VHF telemetry was an effective method for assessing broad-scale exposure of terns and plovers to Federal waters and WEAs in the Atlantic OCS. However, maximizing the detection or resolution of movement patterns in the design of automated radio telemetry station arrays involves tradeoffs, which vary by species, geographic area, and study objective. Estimating bird locations through triangulation of the detecting towers requires strategic placement of towers at relatively high densities in areas of high ecological importance. Along straight coastlines that lack islands or peninsulas, we recommend assembling towers on offshore infrastructure, such as buoys or wind turbines, where possible. The geographic coverage and scope of digital VHF telemetry can be further extended through the coordinated efforts of the Motus Wildlife Tracking System. Future studies have the potential to integrate this network with other forms of developing technology (e.g. radar, high definition imagery) for collecting detailed movements of birds in offshore environments.