[Creagan]

About a month ago I took delivery of six ganders, aged approximately two months, and all fully wormed etc.
They've been doing fine, out on grass all day and shut in at night, with a wee feed of pellets at night time,
I noticed a week or so ago that one bird was a bit smaller than the others, and was slower to loose the last downy fluff off the back of his neck, but didn't think much of it. However in the last day or two he's gone rapidly downhill. He's not moving around much, very wobbly on his feet, and hardly eating. He looks a bit scruffy and dirty and doesn't seem to be bathing himself.

Anything obvious that this might be? Thanks!