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. LAY SUMMARY Birds that migrate in groups during the day are easily observed, but most bird species migrate at night when it is much harder to discern how individuals interact while flying. We automated an old observational method called “moonwatching” to monitor nocturnally migrating birds as they flew across the Moon, and this approach allowed us to determine how frequently they fly in groups. We found that group migration was generally rare at night (never exceeding 10% of birds observed), but it was especially rare during fall as opposed to spring migration. We observed a strong overnight pattern during spring migration wherein group migration was most frequent early in the night and then again as migration activity declined in the hours before sunrise. Our results suggest that group formation during nocturnal migration may be due to different species flying at different times of night or also to social activity on the ground before and after migratory flights as groups were more prevalent just after sunset and before sunrise. Las aves migratorias a menudo vuelan en formaciones grupales durante el día, mientras que, por la noche, generalmente se presume que vuelan individualmente. Sin embargo, es difícil cuantificar el comportamiento grupal durante la migración nocturna, ya que hay pocos medios para