Flying tracks, flight altitude and directions of movement of nocturnal migrant birds, as shown by marine radar

Abstract

Many birds migrate at night, but little is known about their nocturnal ecology because it is difficult to study. We used marine radar to investigate the ecology and behaviour of nocturnal migrants at 140 sites from Hokkaido to Kyushu. Although species identification was not possible, marine radar reveals the flight tracks of nocturnal migrants, and can be used to clarify their abundance and directions. By placing the radar in a vertical position, we were able to count the number of flight tracks up to a height of two km at each site twice during both spring and autumn migrations, from sunset to three hours after sunrise. The mean number of flight tracks at a total of 560 locations was 14,415 in autumn and 4,388 in spring. We tested a GLMM model with the number of tracks as a response variable and seven explanatory variables of latitude, longitude, survey season, site altitude, terrain, percentage of radar shooting time, and cloud cover. The most important variable affecting the number of tracks was the cloud cover at the beginning of the survey. Flight altitude had a single peak at ca. 300–400 m. Flight time peaked around 80–140 minutes after sunset then gradually decreased. This study confirmed many of the migratory routes already known from previous satellite-tracking and other types of studies. Thus, it confirmed that marine radar research is a useful way to study the migration of nocturnal birds.