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Case Study: Migration
Case Study: Migration

Context

We focus on the phenomenon of migration because it is a critical stage in the life cycle of many animals. The consequences of interactions between migratory wildlife and offshore wind facilities are unclear. Some species may have increased collision risk. Others may have increased energetic expenditures from avoidance during migratory movements, although these effects will depend on the scale and number of offshore wind facilities along a migration route.

Download Mid-Atlantic Wildlife Studies: Distribution and Abundance of Wildlife along the Eastern Seaboard 2012-2014. This 32-page summary publication explores aspects of the mid-Atlantic ecosystem; describes our survey and analytical approaches; and presents a range of results, featuring several case studies on specific species or phenomena.

The Executive Summary for the technical report is also available here.

 

Take Home Messages

Our research suggests that a wide variety of animals migrate through areas that have been proposed for offshore wind energy development in the mid-Atlantic region. Additional research on migrant populations may be warranted for sites proposed for development or other offshore activities.

Background

Background

Migration is a difficult phenomenon to study, particularly in offshore areas, but a wide range of taxa move over or through open water habitats during migration. If we are to understand the potential effects of offshore activities on wildlife populations, we must determine when and where this phenomenon occurs.

We employed several methods to document the timing and routes of animal migration through the mid-Atlantic region, including analysis of weather radar data, the use of avian passive acoustic recorders, satellite telemetry, and boat and aerial surveys.

Rays

Rays

The Cownose Ray is a species of eagle ray that primarily eats mollusks and shellfish. Large groups of rays migrate north and into inland bays, such as Chesapeake Bay, to breed during summer. While their breeding habits are well known, the migratory period is poorly understood.

Digital video aerial surveys recorded immense migratory schools near the water’s surface in the mid-Atlantic as many as 75 km from shore. We observed almost 48,000 rays in the summer and fall.

Bats

Bats

Bats are not commonly thought of as offshore migrants, though anecdotal observations of migrating bats over the Atlantic Ocean, particularly in fall, have been reported since at least the 1890s. In September of 2012 and 2013, two bats were sighted during boat-based surveys (one per year), and 15 were documented in high resolution video surveys. All were sighted between 16 and 70 km from shore.

Most of these were identified as Eastern Red Bats, a tree-roosting species that migrates long distances and sometimes collides with land-based wind turbines. Despite their nocturnal habits, these bats were observed flying during the day. Most were estimated to be flying several hundred meters above sea level. Weather conditions were generally good at the time of these observations, suggesting the bats were deliberately migrating offshore rather than having been driven offshore by wind or weather.

 

Songbirds

<p>Songbirds regularly flew over open water, particularly in the fall, when migratory activity was often higher than over land. For many songbirds, expansive areas of open water on the Outer Continental Shelf may not be the barrier to movement that we previously thought.</p>
<p>The movements of individual songbirds can be difficult to track because of their small body size. Like bats, they generally migrate at night, making the study of their migrations particularly difficult. Weather radar can detect migratory activity in the atmosphere, and this allowed us to document broad-scale geographic and temporal patterns of nocturnal migrants (including songbirds) offshore..</p>
<p>Predicted average levels of nocturnal migratory activity during fall migration at 144 study sites along the eastern seaboard, correcting for nuisance variables like distance from radar unit and elevation. Values are model predictions based on data from six NEXRAD units located between New York and North Carolina (Sept.-Oct., 2010-2012). Red and orange colors show areas of highest migratory activity.</p>

Songbirds regularly flew over open water, particularly in the fall, when migratory activity was often higher than over land. For many songbirds, expansive areas of open water on the Outer Continental Shelf may not be the barrier to movement that we previously thought.

The movements of individual songbirds can be difficult to track because of their small body size. Like bats, they generally migrate at night, making the study of their migrations particularly difficult. Weather radar can detect migratory activity in the atmosphere, and this allowed us to document broad-scale geographic and temporal patterns of nocturnal migrants (including songbirds) offshore..

Predicted average levels of nocturnal migratory activity during fall migration at 144 study sites along the eastern seaboard, correcting for nuisance variables like distance from radar unit and elevation. Values are model predictions based on data from six NEXRAD units located between New York and North Carolina (Sept.-Oct., 2010-2012). Red and orange colors show areas of highest migratory activity.

 
<p>Interpolated movement patterns of Peregrine Falcons with satellite transmitters along the Atlantic U.S. coast during fall migration, 2010–2014 (n=16) indicating extensive use of the offshore environment.</p>
<p>Peregrine Falcons are the world’s fastest animal, and their aerial dexterity allows them to catch birds on the wing. This foraging prowess, among other attributes, allows them to migrate over large expanses of the Atlantic Ocean. They are able to fly for several consecutive days over open water, soar and forage at night, and often roost on offshore structures and vessels. Satellite telemetry data indicated that though peregrines often migrated relatively close to shore, individuals were capable of flying hundreds of kilometers offshore and staying in those areas for weeks.</p>

<p>During migration, Peregrine Falcons primarily prey on other migrating birds, like songbirds and shorebirds. It is possible that falcon migratory routes in offshore areas are dictated by the migratory paths of their prey.</p>

Interpolated movement patterns of Peregrine Falcons with satellite transmitters along the Atlantic U.S. coast during fall migration, 2010–2014 (n=16) indicating extensive use of the offshore environment.

Peregrine Falcons are the world’s fastest animal, and their aerial dexterity allows them to catch birds on the wing. This foraging prowess, among other attributes, allows them to migrate over large expanses of the Atlantic Ocean. They are able to fly for several consecutive days over open water, soar and forage at night, and often roost on offshore structures and vessels. Satellite telemetry data indicated that though peregrines often migrated relatively close to shore, individuals were capable of flying hundreds of kilometers offshore and staying in those areas for weeks.

During migration, Peregrine Falcons primarily prey on other migrating birds, like songbirds and shorebirds. It is possible that falcon migratory routes in offshore areas are dictated by the migratory paths of their prey.

 

For More Information

For more information, see Chapters 5, 17, 25, 26, and 27 in the technical report.
 

References

  • Cochran WW (1975) Following a migrating peregrine from Wisconsin to Mexico. Hawk Chalk 14:28–37 
  • Desorbo CR, Wright KG, Gray R (2012) Bird migration stopover sites: ecology of nocturnal and diurnal raptors at Monhegan Island. Report BRI 2012-08 submitted to the Maine Outdoor Heritage Fund, Pittston, Maine, and the Davis Conservation Foundation, Yarmouth, Maine. Biodiversity Research Institute, Gorham, Maine. 43 pp plus appendices. 
  • Goodman MA, Conn PB, Fitzpatrick E (2011) Seasonal occurrence of Cownose Rays (Rhinoptera bonasus) in North Carolina’s estuarine and coastal waters. Estuaries and Coasts 34:640–652 
  • Hatch SK, Connelly EE, Divoll TJ, Stenhouse IJ, Williams KA (2013) Offshore observations of eastern red bats (Lasiurus borealis) in the mid-Atlantic United States using multiple survey methods. PLoS One 8(12): e83803. doi:10.1371/journal.pone.0083803 
  • Johnson JA, Storrer J, Fahy K, Reitherman B (2011) Determining the potential effects of artificial lighting from Pacific Outer Continental Shelf (POCS) region oil and gas facilities on migrating birds. Prepared by Applied Marine Sciences, Inc. and Storrer Environmental Services. US Department of the Interior Bureau of Ocean Energy Management, Regulations and Enforcement. Camarillo, CA OCS Study BOEMRE 2011-047 29 pp. 
  • Voous KH (1961) Records of the Peregrine Falcon on the Atlantic Ocean. Ardea 49:176–177
 
Photo Credits: Header Image © Soar.ou.edu; Boat survey © -Emily Connelly; Cownose Ray and Eastern Red Bat images © HiDef Aerial Surveying, Ltd.; Common Yellowthroat © BRI-Ian Johnson; Falcon photo © BRI
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