Orientation of Fall‐migrating Butterflies in North Peninsular Florida and Source Areas

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Abstract
Phoebis sennae (Pieridae), Agraulis vanillae (Heliconidae) and Urbanus proteus (Hesperiidae) migrate into peninsular Florida each fall from summer breeding areas throughout the south‐eastern United States. Fall flight directions of the three species were studied at 78 sites for clues to their means of orientation and migratory routes. Mean flight directions for a visit to a site were calculated using conventional circular statistics. The distribution of flight directions permitted using linear procedures to combine visit means. The overall mean direction (OMD) of a species at a site was defined as the mean of visit means, weighted by frequency/angular variance, with a lower limit of 0.2 imposed on angular variance. In intensive studies at two sites at Gainesville, Florida, mean flight directions during fall migration were largely independent of time of day and crosswinds. The OMDs of the three species at the two Gainesville sites were similar (142 to 156°) and not significantly different among species at a site or, for the same group of dates, between sites for a species. However, day‐to‐day variation among species and between sites was not concordant. Visits to 65 sites on a grid throughout the southeastern United States and to 11 sites along transects inland from the Gulf and Atlantic coasts yielded 10 cases of a P. sennae OMD at a grid or transect site differing significantly from its OMD at the main Gainesville site. In each of these, the OMD was more easterly at sites inland from the Gulf coast or more southerly or south‐westerly at sites inland from the Atlantic coast. By unknown means and with large, unexplained day‐to‐day variations in directions, P. sennae converge on peninsular Florida from widespread summer breeding grounds. They do this without closely following the Atlantic or Gulf coasts.