I've never actually heard of a repellent for Starlings and Robins.
Perhaps a very large model of each species mounted nearby might
discourage lingering. Smith & Wesson makes a universal deterrent
but they have not (thankfully) achieved popularity in the civilized world
and are therefore best left in crazyland, where you might be able to pick
one up and toss it in your shopping basket at any community gardening supply.
How big is a Hackberry? Could you construct a frame over it and
cover it in fine-mesh netting? I guess you can tell that I'm as ignorant
about this issue as ... well, most other issues. I suppose you could
goggle mitigation of fruit crop damage from birds and perhaps glean
some insight from the pros. I'm a big proponent of fireworks but that
might not have utility for various reasons but it can be a lot of fun.