Pika, I've had great results with 40% cotton shirts, anything with less cotton can get a little hot depending on the weave. Like Tilley shirts which are 70/30. I'm looking around for something in polyester that breathes well but has some decent pockets. The fabric is called "coolmax" but any company name will do if it breathes and wicks well. Problem is that these types of shirts often are not pocketed up well like hiking shirts.
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Listen to the trees in the wind