Do only large birds float?
There are many seabirds smaller than partridges. The smallest seabird (according to the National Trust of Scotland) is the European Storm Petrel, which weighs in at an average of 28g. That's not quite as small as the European Wren (6 - 10 g), but it definitely counts as 'small', nonetheless.

Being seabirds which spend their life at sea except during breeding, storm petrels float well.

Below is a European Storm Petrel weighing in at less than 35 grams in Fig. 1.