When you're trying to count these:
The scenario: When we've planned a shorebird count, it's the one time when I hope not to see a falcon. But today wasn't that day.
We try to count shorebirds (sandpipers and plovers) when they're standing on the tidal flats — where we can scan across a relatively stationary flock to get a good count of the number of individuals. When a falcon flies by, all of the shorebirds take off, swerve in unison to avoid the falcon, scatter, and eventually land somewhere else, making counting very difficult!
P.S. The shorebirds in the photo above are primarily Dunlin, with a few Sanderlings, and one Black-bellied Plover.
P.P.S. I don't think I'm ever truly disappointed to see a Peregrine Falcon. Although they do complicate our surveys, it always feels special to see them.