Compensation for fluctuations in crosswind drift without stationary landmarks in butterflies migrating over seas

Migrating insects may fly over large bodies of water that lack landmarks, but little is known about their ability to navigate in such a fluid environment. Using boat navigation instruments to measure compensation for fluctuations in crosswind drift, I investigated the ability of butterflies (Lepidoptera: Hesperiidae, Nymphalidae and Pieridae) to orient with and without landmarks as they migrated naturally over the Caribbean Sea. I used the presence or absence of landmarks or clouds to evaluate their use by the butterflies as guides for compensation. Forty-one per cent of the butterflies compensated for crosswind drift, whereas only 16% did not compensate. No conclusion could be drawn for the remainder. Without landmarks or clouds, butterflies were significantly less likely to compensate for drift than when these local cues were present. Butterflies were more likely to compensate fully in the presence of a landmark than when only clouds were present. Phoebis sennae butterflies drifted in the morning and over-compensated for drift in the afternoon, a pattern found both within and between individuals independent of landmarks. Although I cannot exclude the use of clouds, this would probably result in undercompensation. Hence, a ground reference in conjunction with a sun or magnetic compass is the most likely orientation cue. In the absence of clouds, one butterfly compensated, at least in part, indicating that it was using ripples on the sea surface as a ground reference in conjunction with a sun or magnetic compass. (C) 2001 The Association for the Study of Animal Behaviour.