DIJKSEN AJ & OUWENEEL GL (2005) Commensalism between Black-necked Grebes Podiceps nigricollis and Mute Swans Cygnus olor?. LIMOSA 78 (4): 155-156.

On the morning of November 7th, 2005, the authors observed a scattered flock of foraging Mute Swans at Lacul Saraturi, a saline lake near the village of Murighiol, just south of the delta of the Danube, Romania. Several swans were being escorted by Black-necked Grebes. Each time the swans put their head and neck under water to feed, the grebes dived. As soon as the swan raised its head from the water the grebes also surfaced. Judging from small waves caused by the feeding grebes, and from the positions where the grebes surfaced, it was apparent that the grebes remained in the direct vicinity of the up-ending swans. It seemed that the grebes stayed in the neighbourhood of the swans to take prey that was disturbed by the swan's feeding activities. Given that grebes normally feed on fish and invertebrates, and swans on waterplants, and that the swans did not seem to be hindered by the feeding grebes, we can classify this coexistence as commensalism. Although several examples of commensalisms between grebes (Podicipedidae) and herbivorous waterbirds have been documented, this is the first record of Blacknecked Grebes in combination with Mute Swans. Black-necked Grebe Podiceps nigricollis Mute Swan Cygnus olor

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