Hi Annabelle! I used to have this problem with my old MacBook Pro! Have you tried changing your security preferences? I found a link for you with the steps on it: http://www.tech-recipes.com/rx/45404/mac-downloaded-app-is-damaged-and-cant-be-o...
Hello! Thank you for your quick help :D It was that issue, but for some recent, Sierra/High Sierra have the "Anywhere" option hidden, so I had to use the Terminal to get it to reappear! And now it's stopped giving me that error, and it lets it open, but then... nothing shows up. The icon just sits in the dock and no birds are fed! My apologies for coming to you with more errors, and thank you again for your support!