Rb. Srygley, Compensation for fluctuations in crosswind drift without stationary landmarks in butterflies migrating over seas, ANIM BEHAV, 61, 2001, pp. 191-203
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.