I'm gonna let everyone in on a lil secret.
You can absolutely fuck up a bird. Their bones are hollow and light, making them super kickable. Even a 4" human still has multiple feet of height over most species of bird, meaning you can wind up a solid kick and still probably send whatever beaked menace is after you flying. If it comes for your eyes or face, even your weakest punch will give it pause.
Now, defending yourself from a bird attack without harming the bird attacking you? Yeah that's really hard, because most of your immediate reactions like trying to grab or restrain it will likely result in hurting the bird.
Canadian Geese are how I got this experience, I used to live in a town that they regularly migrated through and got into two scraps with them in my youth. They're one of the few birds that I have zero sympathy fucking up, because both times they tried to attack me, I was just trying to quickly get through their territory with my lunch.
If you're ever attacked by one, aim for center mass and give it as much force as you can. I've never had geese swarm me after seeing one of their comrades get obliterated.