Identification of Offshore Nocturnal Avian Migration Patterns Using Weather Radar

Despite evidence of passerine and shorebird migration over the northwestern Atlantic, oceanic flyways and migrant use of these offshore regions are poorly known. WSR-88D (NEXRAD) weather radar units can detect migratory... [ view full abstract ]

Despite evidence of passerine and shorebird migration over the northwestern Atlantic, oceanic flyways and migrant use of these offshore regions are poorly known. WSR-88D (NEXRAD) weather radar units can detect migratory animals in the atmosphere and are used to document migratory activity over large areas, but the limitations of NEXRAD data collection offshore have prevented their widespread use for monitoring migrations in the marine environment. Specific issues have included an inability to detect animals during precipitation events; limitations in radars’ geographic detection range offshore; and limitations related to the altitudes at which offshore migration can be detected. New analytical approaches developed for the mid-Atlantic U.S. address these limitations by 1) Filtering out precipitation events that interfere with detection of migratory activity, so that we could monitor offshore migration in a wider range of environmental conditions, and 2) Utilizing a modeling framework that allows for less biased predictions of migratory activity levels at varying altitudes and distances from shore. We assessed 144 sampling sites over land and water along the Atlantic coast of the U.S., from New York to North Carolina, during spring and fall migration in 2010-2013. Analysis focused on comparing nocturnal migratory activity levels between sites and determining how weather influenced overwater migrations. Sites were located up to 80 km offshore and NEXRAD units could detect birds as low as 82 m above sea level at most sites. During spring migration, most migratory activity occurred onshore, with the exception of areas offshore of the Carolinas. During fall migration, offshore migratory activity was statistically indistinguishable from onshore migration levels, with offshore activity highest near Long Island and the Carolinas. Autumn offshore activity in the northern part of the radar coverage area was largely dependent on westerly winds, while offshore activity in the south was high regardless of wind direction. This study provides evidence that birds are regularly migrating overwater up to 80 km out on the mid-Atlantic Outer Continental Shelf, and that nocturnal migratory activity is higher at certain locations and times of year, as well as under particular weather conditions. Given the levels of migratory activity predicted in offshore locations, regulators for offshore wind energy development may want to consider potential impacts to migrants in development scenarios, particularly in locations with consistently higher levels of migratory activity, such as the New York Bight and areas offshore of North Carolina.