The Bureau of Ocean Energy Management (BOEM) is responsible for managing renewable energy development on the Outer Continental Shelf (OCS) of the United States. The OCS extends from the boundary of each state's jurisdictional waters (generally 3 nautical miles offshore) to the outer boundary of the US Exclusive Economic Zone (approximately 200 nautical miles offshore). In the Atlantic OCS, 7,073 km² is presently under lease agreement for development of commercial-scale offshore wind energy facilities and an additional 11,235 km² is in the planning stages for potential lease (BOEM 2019). Development in the United States to date (December 2019) is limited to a 30-MW, five turbine demonstration-scale facility in state waters off the coast of Block Island, RI and a 12-MW, two turbine pilot project under construction in Federal waters off the coast of Virginia. Herein, BOEM Lease Areas and BOEM Planning Areas are collectively referred to as Wind Energy Areas (WEAs).
With large areas of the Atlantic OCS under consideration for development of offshore wind energy facilities, information on offshore movements and flight altitudes of high-priority bird species is needed for estimating exposure of birds to collision risks in WEAs and for developing strategies to manage adverse effects (BOEM 2017). The potential effects of offshore wind turbines on avian populations vary by species and include direct mortality from collisions with infrastructure and indirect effects of disturbance and habitat loss (Fox et al. 2006, Fox and Petersen 2019). Understanding these species-specific effects, including cumulative impacts from exposure to multiple commercial-scale wind energy facilities throughout their migratory ranges, will be increasingly important as offshore wind energy development advances in US waters (Goodale and Milman 2016).
This study provides new information on the movements and flight altitudes of 12 species of shorebirds: Black-bellied Plover (Pluvialis squatarola); Dunlin (Calidris alpina); Least Sandpiper (Calidris minutilla); Lesser Yellowlegs (Tringa flavipes); Pectoral Sandpiper (Calidris melanotos); Red Knot (Calidris canutus); Ruddy Turnstone (Arenaria interpres); Sanderling (Calidris alba); Semipalmated Plover (Charadrius semipalmatus); Semipalmated Sandpiper (Calidris pusilla); Whimbrel (Numenius phaeopus); and White-rumped Sandpiper (Calidris fuscicollis). These species are long-distance migratory shorebirds that breed in Subarctic and Arctic regions of North America and winter along the coast of the southern United States to southernmost South America. These species migrate over the Atlantic OCS and land to rest and refuel at a network of stopover sites along the US Atlantic coast (O’Connell et al. 2011). While broad patterns in shorebirds’ migration routes and behavior have been documented by tracking and banding studies, we still lack fine-scale information on the routes, altitudes, timing, and environmental conditions associated with flights of migratory shorebirds over the Atlantic OCS. Such fine-scale information is needed to refine assessments of exposure to offshore WEAs and to improve estimates of collision risk with offshore wind turbines (O’Connell et al. 2011).
In this study, we compiled movement data from 3,955 individuals of 17 shorebird species that were tagged with digital VHF (Very High Frequency) transmitters from 2014 to 2017 at 21 sites widely dispersed across North and South America. The movements of tagged shorebirds were tracked using a collaborative radio telemetry network, the Motus Wildlife Tracking System, which has extensive coverage from automated radio telemetry stations distributed across Eastern North America and additional coverage at key shorebird sites from Arctic Canada to South America. Our Study Area encompassed a region of the US Atlantic coast extending from Cape Cod, Massachusetts to Back Bay, Virginia, where a network of BOEM-funded automated radio telemetry stations was established for monitoring avian movements throughout adjacent waters of the Atlantic OCS (Loring et al 2018, Loring et al. 2019). These coastal stations had an effective detection radius of about 20 km, therefore the bounds of our Study Area ranged from 20 km inland to 20 km offshore. To estimate broad-scale use of our Study Area by shorebirds, while accounting for transmitter loss, we examined the migratory tracks of all shorebirds detected by automated radio telemetry stations at least 50 km from their original tagging site and within 30 km of the Atlantic coast from Mingan QC, Canada, in the north to the Texas-Mexico border in the south. Of these individuals that retained their transmitters and were detected somewhere along the Atlantic Coast of North America (n = 1,363), 65% were detected within the Study Area.
We then analyzed movements and flight altitudes of 594 individuals of 12 shorebird species with sufficient detection data by automated radio telemetry stations within our Study Area. We implemented novel movement modeling techniques to assess the frequency and extent of offshore movements over Federal waters within the Study Area, which extended approximately 20 km offshore (corresponding to effective range of automated radio telemetry stations). Our objectives were to: 1) develop spatiallyexplicit, 3-dimensional models of shorebird movements in the Atlantic OCS region; 2) estimate the presence of shorebirds over Federal waters of the Atlantic OCS region during migration; 3) assess the probability of movements into Federal waters in relation to meteorological conditions (wind speed, wind direction, barometric pressure, temperature, visibility, precipitation), temporal variation (time of day, day of year, migratory season), and sex and age class (where known).
We tracked shorebirds in Federal waters of the Atlantic OCS during both fall and spring migration. In spring, the highest probability of presence in the Atlantic OCS occurred from mid-May to early June, when winds were moderate (~10 m/s) and blowing to the north-northeast. In the fall, the probability of presence in the Atlantic OCS was highest at the beginning of July, decreased through October, and increased slightly in November. Higher probability of presence in the Atlantic OCS during fall was associated with winds blowing to the south-southeast and high atmospheric pressure. During both spring and fall, precipitation during flights in the Atlantic OCS was generally low (< 3 kg/m²).
During non-stop flights over Federal waters, model-estimated flight altitudes varied greatly (28-2,940 m) and mostly were estimated to occur above the Rotor Swept Zone (RSZ) of offshore wind turbines (25-250 m), with overall mean flight altitudes of 914 m during spring and 545 m during fall. Exposure to the RSZ was higher during fall (approximately 36% of offshore flights in RSZ), relative to spring (approximately 24% of offshore flights in RSZ).
Our array of land-based automated radio telemetry stations was effective for assessing the flight paths and behavior of shorebirds departing from the US Atlantic coast over Federal waters of the Atlantic OCS. However, using digital VHF transmitters to track movements >20 km offshore typically exceeds the limits of the technology’s current capabilities. In the future, estimates of shorebird passage rates through specific lease areas located >20 km offshore could be accomplished by placing tracking stations directly within the lease areas on offshore infrastructure such as buoys or wind turbines. The digital VHF technology used in this study has the added benefit of seamless integration with the rest of the rapidly expanding Motus Wildlife Tracking System, which uses a collaborative approach to extend the scope of tracking across the Western Hemisphere. Future studies of collision risk could benefit from the application of other forms of developing technology, especially radar and satellite transmitters, which could be used in conjunction with digital VHF telemetry to collect additional location and flight altitude data at complimentary spatial and temporal scales.