We hypothesized that the ability of the semiaquatic rice rat (Oryzomys
palustris) to colonize over water is responsible for its ubiquity on
the frequently overwashed Virginia barrier islands. A mark and recaptu
re program was conducted from 20 May to 24 August 1989 on three adjace
nt islands to determine the frequency, direction and causes of the mov
ements. During this study, 11 of 166 island residents migrated for the
duration of the study to other islands. All recorded crossings were m
ade from a small island to larger islands. The demographic composition
of the overwater dispersers was a statistically random sample with re
spect to age and sex from the resident population, unlike rice rat ove
rland dispersers that were predominantly juveniles.