Social behavior among nocturnally migrating birds revealed by automated moonwatching

Migrating birds often fly in group formations during the daytime; whereas at night, it is generally presumed that they fly singly. However, it is difficult to quantify group behavior during nocturnal migration as there are few means of directly observing interactions among individuals. We employed an automated form of moonwatching to estimate percentages of birds that appear to migrate in groups during the night within the Central Flyway of North America. We compared percentages of birds in groups across the spring and fall and examined overnight temporal patterns of group behavior. We found groups were rare in both seasons, never exceeding 10% of birds observed, and were almost nonexistent during the fall. We also observed an overnight pattern of group behavior in the spring wherein groups were more commonly detected early in the night and again just before migration activity ceased. This finding may be related to changes in species composition of migrants throughout the night, or alternatively it suggests that group formation may be associated with flocking activity on the ground as groups are most prevalent when birds begin and end a night of migration.

Methods
We used the LunAero system (Honeycutt et al. 2020;m https://doi.org/10.1016/j.ohx.2020.e00106) to record video footage of the moon at sites in Central Oklahoma including the Kessler Atmospheric and Ecological Field Station (34.98°N, 97.52°W); private residences in Norman, Oklahoma (35.22°N, 97.44°W); and the U.S. Department of Energy Atmospheric Radiation Measurement Southern Great Plains Lab (36.61°N, 97.48°W). These sites aligned north to south and were all within the North American Central Flyway (Buhnerkempe et al. 2016). Much of the video footage we collected was unusable due to cloudy skies and/or technical issues with the hardware. Hence, we screened the footage to identify recordings where the video was suitable for further analysis. All footage used in our analyses were collected within 5 days of a full moon or when the moon was at least 66% illuminated. The steps used to extract, filter, and validate flight paths from our video footage are described in Honeycutt and Bridge (2022; https://doi.org/10.1093/icb/icac053). This workflow yielded a dataset wherein all detected bird silhouettes were incorporated into individual flight paths along with their screen coordinates (x and y) and timestamps.