Fish - or at least their eggs - probably do occasionally get transported on birds' feet or feathers. Such rare, chance events may well be biologically important, given that freshwater species have few other dispersal options available to them.
Some freshwater invertebrates are thought to actively seek out birds' feet. One study found 18 waterfleas, rotifers and shrimps clinging onto a gull that had been paddling in a pool.
In addition, snails and beetle eggs are known to survive the journey between ponds inside a duck's stomach, and cockles can hitch rides on birds' feet by snapping shut on their toes.