DE BAERDEMAEKER A (2017) Commensal foraging association between Little Grebes Tachybaptus ruficollis and Mallards Anas plathyrhynchos. LIMOSA 90 (2): 84-88.

In July 2017 a group of six Little Grebes was observed feeding among several duck species in a dune pond on the Dutch Wadden Sea island of Ameland. During observations on the 17th and 19th of July, several feeding associations between Little Grebes and Mallards were observed and filmed. The Little Grebes showed great fidelity to individual Mallards, which they shadowed until the duck would start to dabble. At that moment, they dove within a metre of the up-ending Mallards, with 76% of dives within half of a metre. Twelve out of 24 dives (50%) resulted in a Little Grebe surfacing with prey. Prey types were small invertebrates (75%) and small fish, probably Three-spined Stickleback Gasterosteus aculeatus. The average diving time was 9.84 seconds, with successful dives being slightly shorter than unsuccessful ones (bearing in mind that Little Grebes are able to swallow prey while still being under water). The Mallards did not seem to mind the presence of the grebes. On two occasions Mallards showed brief antagonistic reactions to the grebes: in one case by making a pecking move towards a nearby grebe, in the other by swimming away hastily after presumed contact beneath the surface.
Known feeding associations of Little Grebes are listed in Table 1, with only one prior case involving Mallards in GreatBritain in 1973. The literature review revealed that commensal feeding Little Grebes had shorter diving times, shorter recovery times between dives, and higher feeding rates than non-associated congeners. Little Grebes could profit from commensal feeding in several ways: 1) by improved foraging success per diving attempt, 2) by decreased energy expenditure per dive, 3) by better access to larger prey, and 4) by increased safety from aerial predators. It is hypothesised that feeding associations could occur more often in periods of increased food demand, like pre-migratory fattening of the Little Grebes or that they are driven by opportunities, like concentrations of moulting ducks.


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