They do hit the odd bird, but a greater risk is a pocket of thin air given they don't have much height to correct, even taking off when fully loaded can be an issue - I've known them to clip contour banks, fences and trees sometimes with tragic consequences. When sowing seed they can fly higher as the seed is less affected by wind and air turbulence, however when spraying they need to get as low as possible to minimise those affects. We had high voltage power lines through our place and they'd sow over the top of the lines but would fly under the lines when spraying. As a kid (pre-GPS) all this work was done with flag-men on the ground for the pilot to line up, the rule was when spraying you walked BEFORE the plane got to you (you had to take a certain number of paces for each run).