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Waterfowl guys have a term called “blind retrieve.” It describes the situation where the dog doesn’t actually see the bird drop, yet is required to streak right to the duck and bring it back. After watching video footage from a camera I mounted on my dog, Buddy, I feel obligated to redefine this term. It looks like virtually every retrieve is blind to a dog.
It’s a matter of trigonometry. Recall your lessons on sines, cosines, and tangents? Well, a dog is low to the ground. His line of sight to something else on the ground is easily obstructed by bushes, rocks, even high spots in the terrain. The obstruction doesn’t have to be very tall—a few inches—and your bumper or dead bird is virtually invisible.
Buddy’s perspective is too darn low to see beyond even short cover. The tall stuff, I’m just glad he has a good nose. Doubt me? Try it yourself some time. Lob a bumper, kneel down and see what you can—and can’t—see.
There’s not much you can do about minimizing these obstructions, but you’ll get your dog on more “blind” retrieves if you recognize his visual limitations and appeal to his sense of smell, instead of relying so much on his sense of sight.
In the future when I send Buddy for any dead bird, I’m sending him to the downwind side of where I thought it landed, to give him every chance for a successful retrieve – blind or not.
Scott Linden Outdoors 
I always send my guys to the downwind side and let them work back to the bird. I think it was in one of my Bill Tarrant books where I read that, if you casting your dog along a side hill, send them above the bird because they will normally end up going downhill as they cross it.