Q: We have one birdfeeder in our backyard. For the first time in many years, a mockingbird has found us and is chasing away all our other birds. Any chance you’ve run across a mockingbird deterrent?

A: Mockingbirds are well-known backyard bullies. They are very territorial and will frantically defend their space against all invaders – cats, Cooper hawks, bluejays, etc.
My guess is that a mockingbird has declared the area around your feeder to be “his” territory. One option would be to move the feeder 20 feet away and see if the mockingbird will stop attacking birds once they are coming into an area that might not be the mockingbird’s territory. Or you could set up a couple more feeders, generously spaced apart, so the mockingbird couldn’t defend all of them at the same time.