At one of the local city parks I frequent I regularly see Common Mergansers scrabbling for bread as enthusiastically as any of the tame residents--Mallards, Canada Geese, pigeons, coots, feral domestics etc. The mergansers are winter visitors, arriving in the fall & leaving in the spring, & I'm at a loss to know what attracts these specialized fish eaters to such seemingly inappropriate fare. Surely they can't derive much (if any) nutriment from it? Maybe, they just get swept up in the feeding frenzy or, as you say, just like the taste?