Variation in morphological structures may indicate the existence of ecological differences between species or sexes. In birds, the bill is one of the structures most affected by selection pressures because it is directly involved with several biological functions, particularly the acquisition of food. In this study, we combined geometric morphometrics and comparisons of linear measurements to assess the presence of sexual dimorphism and differences in bill shape and size of four species of abundant zooplanktivorous seabirds that coexist during their breeding season in Southern Ocean ecosystems [blue petrel (Halobaena caerulea), Antarctic prion (Pachyptila desolata), common diving petrel (Pelecanoides urinatrix) and South Georgia diving petrel (P. georgicus)]. The results showed that bills of Antarctic prion and blue petrel differed noticeably in size and shape from those of the two diving petrels, whereas those of common diving and South Georgian diving petrels overlapped in bill shape. These differences may be due to diet segregation or factors such as diving strategy, vocalizations or olfactory development. In addition, there was sexual dimorphism in bill shape or size in all four species. However, we could not directly attribute these differences to selection pressures because of the lack of studies comparing feeding ecology between sexes in these seabirds. These results reveal relationships between the bill shape and key factors of the ecology of these species, and demonstrate the usefulness of geometric morphometrics for detecting interspecific and sexual differences in species of otherwise sexually monomorphic petrels.