This afternoon while mowing the lawn I noticed that a couple of baby sparrows (white-throated sparrows, I believe) were hopping about in the as-yet-unmowed portion of the grass. They had gotten out of a nest that was hidden on the ground inside the rose bush near the back of the porch. Whether it was the noise of the lawnmower that scared them out of the nest, or whether they had gone out exploring before the mower started on its rounds, I don't know. But the noisy lawnmower caused them to hop away (they could not yet fly) whenever it came close, though I kept my eye on them and would not have run the mower over them if they had foolishly gotten in the way. The thought crossed my mind to stop mowing because the noise must have been at best unpleasant for them, and possibly worse than that--so I did; and after a while I got back to mowing, thinking they must have gone back into the nest in the meantime. But not so--the mower went round and they hopped about whenever it got close. I have sometimes seen toads do the same thing, but they almost always hop away. These babies stayed on the island of tall grass that was gradually shrinking as the mower kept on around the perimeter. I was going to stop again, when one of them figured it out and, instead of staying in the taller grass, where there was presumably more protection from predators, moved onto the shorter, cut area and then hopped back over to the nest. Soon afterward, the other baby did the same.
Although it's possible that the mower shook the ground some, I'm fairly sure that it was the noise that scared the birds and agitated them. The noise disturbed the natural soundscape they were used to, and it upset them. That is another reason to use a hand mower, besides the carbon usage and exhaust in the atmosphere, but with a lawn this size--it takes an hour and half of steady mowing to complete it--that would not be practical. Better to find something other than a lawn that needed cutting and plant that instead. I know some people have deliberately done that sort of thing, and so I think I'll investigate the possibilities.