Spatial variation in parasitism is commonly observed in intermediate host p
opulations. However, the factors that determine the causes of this variatio
n remain unclear. Increasing evidence has suggested that spatial heterogene
ity in parasitism among intermediate hosts may result from variation in rec
ruitment processes initiated by definitive hosts. I studied the perching an
d habitat use patterns of wading birds, the definitive hosts in this system
, and its consequences for the recruitment of parasites in snail intermedia
te hosts. Populations of the mangrove snail, Cerithidea scalariformis, coll
ected from mangrove swamps on the east coast of central Florida are parasit
ized by a diverse community of trematode parasites. These parasites are tra
nsmitted from wading birds, which frequently perch on dead mangrove trees.
I tested the hypothesis that mangrove perches act as transmission foci for
trematode infections of C. scalariformis and that the spatial variation of
parasitism frequently observed in this system is likely to emanate from the
distribution of wading birds. On this fine spatial scale, definitive host
behaviors, responding to a habitat variable, influenced the distribution, a
bundance and species composition of parasite recruitment to snails. This ca
usal chain of events is supported by regressions between perch density, bir
d abundance, bird dropping density and ultimately parasite prevalence in sn
ails. Variation between prevalence of parasites in free-ranging snails vers
us caged snails shows that while avian definitive hosts initiate spatial pa
tterns of parasitism in snails through their perching behaviors, these patt
erns may be modified by the movement of snail hosts. Snail movement could d
isperse their associated parasite populations within the marsh, which may p
otentially homogenize or further increase parasite patchiness initiated by
definitive hosts.