Hunting on dangerous ground

Distributed within tropical to temperate regions globally, the little egret (Egretta garzetta) is an aquatic bird that occupies a wide range of inland and coastal habitats (Collins Bird Guide 2011, 2nd edn; New York, NY: HarperCollins).

While monitoring avifauna in the Tanagro River (Sala Consilina, Italy), we were alerted about a little egret that was unable to fly and appeared to be stuck in the mud. After a human intervention, it became clear that the bird had one foot trapped within the closed shell of a unionid bivalve (photos courtesy of Carmen Cavallo).

To capture food in wetlands and on mudflats, E garzetta and other herons commonly rely on a technique known as “foot stirring” or “foot paddling”. Using this method, a heron will extend one leg into the substrate and then rapidly vibrate the submerged foot, thereby disturbing and forcing any hidden or benthic organisms from the subsurface to the surface; at that point, the bird will then stab its beak at any prey that attempt to escape from the disturbance (Wilson Bull 1959; biostor.org/reference/204432). It is highly possible that, while engaging this technique, the observed little egret became trapped by the bivalve.

Although similar events between shorebirds and marine bivalves have been documented (Waterbirds 1999; doi.org/10.2307/1522003), this is to the best of our knowledge the first recorded instance between an aquatic bird and a freshwater unionid. Europe hosts a large unionid biota, including the Chinese pond mussel (Sinanodonta woodiana), one of the largest (130–250 mm long) species worldwide and an eminent invader in our study area. While it is unknown how frequently such events may occur, this potential mortality source for native birds might need to be included among putative impacts of unionid bivalve species, such as the above-mentioned non-native taxon.