Oystercatchers

Diet differences between male and female sooty oystercatchers

Male and female sooty oystercatchers (Haematopus fuliginosus fuliginosus) have an average difference in bill length of 19%. Females have pointed bills, while males have blunt bills, however they are very similar in all other respects. Is this difference a sexually selected trait, (females fancy males with short bills), or could it be related to a intersexual foraging divergence?

We studied the relationship between this sexual dimorphism and foraging ecology at coastal sites in southern New South Wales, Australia. We found striking difference between the sexes in diet, with seven prey classes eaten exclusively by one sex (male: 4, female: 3), and all shared prey classes eaten in different proportions.

We also observed divergence in the energetic rewards gained from foraging, with females gaining highest energetic benefits from eating ascidians and males from eating limpets. Within the most commonly consumed prey item, limpets, females gained higher energetic benefit from eating smaller sizes while males gained greater rewards from the largest limpet sizes. Finally, several aspects of foraging behaviour were also different between the sexes. There was a significant effect of tidal cycles upon intersexual niche partitioning in this species; the degree of diet divergence varied between tide conditions and females had a consistently more efficient dietary intake on neap tides than males.

Diet divergence in the sooty oystercatcher is greater than previously observed in any oystercatcher. We argue that competition between territorial pairs is operating to diverge male and female bill morphology.

While probably driven originally by competition, the end result is beneficial to the pair unit. By separating their foraging niche, a pair can most likely reduce the size of their territory (making it easier to defend), and foraging closer together without competition (improving vigilance).

Photo © Eleanor Sobey

A pair of sooty oystercatcher - male in foreground, female behind

Specimens of male and female sooty oystercatchers - showing average bill length and depth, measured on multiple museum specimens. Male and female bills length are significantly different sizes.