Field sketching:
-use an angled scope
-keep both eyes open, look with the right and draw with the left!! Or vice versa
-use a smallish sketchbook at first, especially in hides where you don't want others to see. Go to A4 as soon as you feel confident. It makes you draw from the wrist not the fingers which leads to better/faster sketches. The sketchbook is for you and must not become a 'showpiece' (mine gets verrry messy at times of high action)
-use something that moves quickly over the paper. I prefer .09mm mechanical pencil with 2B leads in. I switch to a watercolour pencil lead in a clutch pencil when its wet so I can continue drawing even if my paper is damp/soaking/covered in grot.
-no rubbers, they slow you down
-if you go wrong ignore it and start a new sketch/page
-don't try to complete a full sketch of a bird at first. Draw bits and start new sketches as the bird moves,, you can come back to them later if the bird returns to the original pose. As you become familiar withthe bird's plumage/jizz your drawing will become more fluent
-Read 'Drawing On the Right Side of the Brain' by Betty Edwards
-draw sat in front of the telly whilst watching wildlife programmes etc
-buy huge quantities of cheap sketchbooks ffrom the local market (99p in mien for A4)