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, b ut little is known about their ability to navigate in such a fluid environm ent. Using boat navigation instruments to measure compensation for fluctuat ions in crosswind drift, I investigated the ability of butterflies (Lepidop tera: Hesperiidae, Nymphalidae and Pieridae) to orient with and without lan dmarks as they migrated naturally over the Caribbean Sea. I used the presen ce or absence of landmarks or clouds to evaluate their use by the butterfli es as guides for compensation. Forty-one per cent of the butterflies compen sated for crosswind drift, whereas only 16% did not compensate. No conclusi on could be drawn for the remainder. Without landmarks or clouds, butterfli es were significantly less likely to compensate for drift than when these l ocal cues were present. Butterflies were more likely to compensate fully in the presence of a landmark than when only clouds were present. Phoebis sen nae butterflies drifted in the morning and over-compensated for drift in th e afternoon, a pattern found both within and between individuals independen t of landmarks. Although I cannot exclude the use of clouds, this would pro bably 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 conju nction with a sun or magnetic compass. (C) 2001 The Association for the Stu dy of Animal Behaviour.