Many tropical butterfly species are well-known for their migratory behaviou
r. Although these insects can maintain a constant direction throughout the
day, the physiological mechanisms of orientation are unknown, It has been a
rgued that tropical migrant butterflies must use a time-compensated sun com
pass to accomplish their journey, but the crucial experimental manipulation
s to test this hypothesis have not been conducted. This study reports the r
esults of clock-shift experiments performed with two species of migrating b
utterflies (Pieridae: Aphrissa statira and Phoebis argante) captured during
flight across Lake Gatun, Panama. The observed constant flight bearing of
natural controls suggests that these species are capable of performing time
-compensated celestial navigation. Our clock-shift experiments suggest that
a sun compass is involved. Individuals submitted to a 4h advance shift too
k significantly different mean orientations on release compared with contro
l butterflies. The direction of this difference was consistent with the use
of a sun compass. The magnitude was approximately half the predicted value
if the vanishing bearing of released butterflies was used as the variable
to evaluate the effect of time-shifting and approximately three-quarters of
that predicted if the estimated heading was the variable used. Mean vanish
ing bearings of control and experimental butterflies did not correspond to
predicted values, This difference can be attributed largely to the combined
effects of wind and handling.