Like humans, birds have excellent color vision and are very motion sensitive. Appropriate dress will also keep you in the field longer because of your increased comfort level. A scarf and stocking cap, along with good gloves, are a must.
Boots are a real boon also.
They not only keep your feet dry when treading through mud puddles but also keep those nasty grass and weed seeds out of your socks. Seeds in the socks are a prickly nuisance, especially on field trips east of the mountains. Birds should not be distracted from their normal activities for too long, especially during nesting season. Small songbirds such as sparrows, wrens, kinglets and chickadees respond best.