Relatively little is know about the Cape Sable seaside sparrow and its foraging behavior. However, some studies have been conducted on the diet of nestlings. Sparrow appear to prefer insects in the dragonfly family. Additionally, the adult sparrow will feed their young grasshoppers, butterflies and caterpillars later in the season as their numbers in the field increase.
Source Lockwood et al., 1997. Life History of the Endangered Cape Sable Seaside Sparrow
The Wilson Bulletin. 109: 720-731.