Do you call those feather markings stress bars in the USA?Image of interest to me because I can often see health issues recorded in feathers. In this case stag was doing poorly due to cocci infection at time the white and narrow portion of feather at center of yellow square was coming in. He has made a full recovery with dominant rank in his brood. Health issues any time during the season when wing feathers are coming in can have similar impacts on those feathers as they come in. For my birds that means any health issues from late April through later November can impact feather quality for the following year. Nice thing here is all feathers shown will be swapped out by Christmas, as will be those on wings and body.
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There are lots of things that can can be used to assess current and past health before you even get to point of handling.