Corvids are incredibly smart! They can undo buckles and zippers on back-packs and steal your lunch. The only closures they haven't figured out YET is Velcro. Give them time, they will!
We feed magpies in the winter (corvidae), I open the back door, clap my hands 3 times and within seconds, they swoop in. They also know who belongs where.... We had guests and the magpies started squawking as soon as they went in the yard.
Who needs a guard dog when you have these birds around?