The objective of this study was to examine aggressive behavior of shorebirds during spring stopover on beaches and other stopover habitats in Delaware Bay, New Jersey, USA. More aggressive interactions were observed between members of the same species than between species at all study locations. The incidence of interspecific interactions was higher than previously reported for shorebirds, particularly in Sanderlings (Calidris alba), which directed almost half of their attacks toward members of other species, mainly Semipalmated Sandpipers (Calidris pusilla). Significantly more heterospecific attacks were directed toward smaller competitors (67%) than toward similarly sized (15%) or larger (18%) competitors.