When the icebreaker Oden passed between 39°36’S, 57°46’W and 39°59’S, 58°11’W on 20 November 2007, an extraordinary observation was made. A feeding Pygmy Right Whale Caprea marginata, a species rarely seen, was attended by eight Grey Phalaropes Phalaropus fulicarius, two Slender-billed Prions Pachyptila belcheri, and four Wilson’s Storm Petrels Oceanites oceanicus. The birds returned to feed around the head of the whale every time it surfaced, presumably copepod plankton straining out between the baleen. The site is off Rio de la Plata estuary where nutrient rich freshwater meets cold water of the Malvinas (Falkland) Current, creating a hotspot with high levels of plankton food. The conditions were extraordinarily favourable for observation with a calm sea, no wind and only a very weak swell. Six multispecies feeding groups were seen with a total of 42 species of birds, eleven species of whales, dolphins and porpoises, and three species of seals. Dusky Dolphins Lagenorhynchus obscurus and Great Shearwaters Puf¿- nus gravis were predominant in the groups.

Greay Whales Eschristius robustus are the only large whales that are specialized bottom feeders, foraging on bottom sediments. When surfacing after a feeding dive a mud plume is formed at the surface as remaining sediment is strained out between the baleen. In the Chukchi Sea, Short-tailed ShearwatersPuffinus tenuirostris are attrackted to these mud plumes in search of food particles.