Birds :-) They seem to know at least a day before you do that the cherries are ripe. And then tell all their friends. Birds are kind of a big deal with ripe cherries......more so than with most other fruit trees.

When I was growing up we had a full sized Montmorency, a Rainier and two Bings in our garden and my dad used to hang old tin pie plates from the trees, my Grannie's old foxfur scarf (looked like two flattened foxes biting each others' tails) and sat out on the patio with his sling shot beaning every bird that he could see. We still only got maybe a third of the crop.

I figured since they were sour the birds wouldn't like them

Birds don't care if they're sour or not :-) They eat unripe fruit all the time, as well as the berries from hollies and cotoneasters and other plants that are toxic to humans or taste pretty foul to us.

Cedar Waxwing eating my green sour cherries.

I had to cover my Honey berries yesterday, used row cover, Cedar Waxwings already eat them green... none are showing any blue!

Here a video on Starlings eating cherries.

I had the same thing happen to a montmorency tree 20 years ago. It died or I cut it down after a while because I saw that I would never see a ripe cherry.