Interspecific variation in extraction of buried seeds within an assemblageof sparrows

Most North American sparrows forage almost exclusively on herbaceous seeds during the winter months. Limited availability of surface seeds fords some birds to scratch for seeds buried beneath soil, snow, or litter. Artificial seed trays were used to test the ability of five different sparrow species to extract seeds buried at different depths in soil. The results suggest t hree functional groups based on relative scratching ability. Strong scratch ers, which included eastern towhee and song and white-throated sparrows. me l or exceeded their energetic requirements when foraging on seeds buried at ail depths (down to a maximum depth of 1.50-2.25 cm). A weak scratching sp ecies, Savannah sparrow, scratched with the same frequency as the strong sc ratchers, but experienced negative energy budgets when forced to forage on sub-surface seeds. Finally, a non-scratching species, field sparrow, failed to extract any buried seeds. Level of scratching ability may influence foraging efficiency at low resour ce densities. As a result, interspecific differences in scratching ability may contribute to habitat selection. Strong scratchers may be adapted to fo raging near woody vegetation where intense resource competition and abundan t litter limit the availability of surface seeds. Weak scratchers, on the o ther hand, may be forced to feed away from areas where surface seeds are li mited. Since woody vegetation serves as a primary source of protective cove r in early successional habitats, a tradeoff between foraging efficiency an d the risk of predation may promote the local coexistence of species that d iffer in relative scratching ability and adaptations to evading predators.