Effects of nocturnal celestial illumination on high-flying migrant insects

Radar networks hold great promise for monitoring population trends of
migrating insects. However, it is important to elucidate the nature of
responses to environmental cues. We use data from a mini-network of
vertical-looking entomological radars in the southern UK to investigate
changes in nightly abundance, flight altitude, and behaviour of insect
migrants, in relation to meteorological and celestial conditions. An
abundance of migrants showed positive relationships with air temperature,
indicating that this is the single most important variable influencing the
decision to initiate migration. In addition, there was a small but
significant effect of moonlight illumination, with more insects migrating
on full moon nights. While the effect of nocturnal illumination levels on
abundance was relatively minor, there was a stronger effect on the
insects’ ability to orientate close to downwind: flight headings were more
tightly clustered on nights when the moon was bright and when cloud cover
was sparse. This indicates that nocturnal illumination is important for
the navigational mechanisms used by nocturnal insect migrants. Further,
our results clearly show that environmental conditions such as air
temperature and light levels must be considered if long-term radar
datasets are to be used to assess the changing population trends of
migrants.