Robert Cleland,

Related to your observation of Horned Grebes feeding in association with Surf Scoters, an article was published in IBIS in 1979:

Byrkjedal, I., S. Eldoy, S. Grundetjern, and M. K. Loyning. 1979. Feeding association between Red-necked Grebes Podiceps griseigena and Velvet Scoters Melanitta fusca in winter. IBIS 139: 45-50.

"Red-necked Grebes wintering off the coast of southwest Norway were found to feed either solitarily or in close association with Velvet Scoters feeding on bottom prey (mainly echinoderms) dug out from the sandy substrate. By scuba diving at spots where both species were feeding, we found that stirring the sand caused polychaete worms to emerge from the substrate. ...[P]olycheates were probably the most likely prey of the grebes."

While the situation was slightly different from what you described it shows that an other grebe species is opportunistic in its feeding associations as well.

Sincerely,
Martin Muller, Seattle
martinmuller at msn.com<mailto:martinmuller at msn.com>