Seaside Sparrows are larger than the other two sparrows likley to be present with it (winters), Sharp-tailed Sparrow and Le Conte's Sparrow. In the breeding season it will be the only sparrow present in the saltwater spartina grass.
The subspecies likely to be found on the UTC fisheri has buffy markings on the face and breast. Like all forms of the Seaside Sparrow it will have a yellow mark between the bill and the eye and a dark malar strip bordering a clear white throat.
Seaside Sparrow is almost a sure thing at the Bolivar Flats Bird Sanctuary. You should also find it at Searim State Park and Anahauc NWR (at the bay).
A range map is available from the Christmas Bird Count