Those are chunks of ice frozen on to his feathers reflecting the extreme cold weather we had in February 2015.
After resurfacing from diving, droplets of water will sit on top of their feathers like drops on a waxed car.
But if he does not dive again soon the very cold temperatures cause the droplets to freeze to ice.
They will not melt underwater since the water is already at freezing temperature.
Birds that dove more frequently did not have these ice chunks.