As others have stated, it’s a hard-wired trait. Boxers actually come from an ancestor that was bred for hunting (Brabanter Bullenbeisser). Does she only exhibit this behaviour in your yard, or on walks as well? What commands does your dog know? Do you have a friend who has a pet bird?
An easy method, which does not involve trying to change or correct your dog, is putting a bell on her collar. This alerts the birds to her presence. I’ve seen this work… as annoying as the bell can be to us, it does alert the birds.
There are also some things you can try to help redirect/stop the behaviour.
First off, I don’t believe in punishing a dog for doing what comes naturally to them. This usually results in making the dog anxious.
You can take your dog to a park with a large number of wild birds (like pigeons, seagulls, etc). She must be on a short leash. Every time she shows interest in the birds, distract her (by saying her name or some other command she might know like “leave it”) and immediately lead her in the other direction. Once the bird is no longer visible or she shows no interest, pet/praise her and give her a treat. This should be repeated daily, whether in a park or on walks, until she has little or no interest in the birds.
You can tie a long rope/line to your dog’s collar. This can be done at a park or in your backyard. This will allow her to walk near the birds and if she gets too close, you can grab the rope. Don’t pull her though. Call her name or other command you use to get her to come to you, and then walk away from the bird (with the rope in hand). If she doesn’t not respond to your voice command, you can pull her gently. The rope is there as a safety measure. Again, repetition and quantity are the keys.
You can also teach your dog chase games like Fetch. It will let her do what dogs love so much… chasing and catching (which is what she does to the birds). It’s great exercise for her and can really help with unwanted behaviour caused by boredom. Make it part of her routine (a play time). You can use frisbees, balls, sticks, flirt poles… etc. Make sure to put these “chase” toys away after play time though. She will be excited every time you take them out. Also, always quit playing the game when she is at the height of her excitement (especially if she’s not used to playing). It seems mean, I know. But you want her to keep that level of excitement for those specific toys/game. Make sure you get excited as well. You may look a little silly but it’ll help in her excitement.
If your dog is under-trained. I’d suggest teaching her the basics (come, sit, stay, off, down). It is so much easier to help a dog in this situation when they are trained to certain commands. If nothing you try works. Get her into obedience classes with a professional trainer (preferably one who works with dogs who exhibit this type of behaviour at unwanted times/objects).
I know there are loads more techniques you can try but those are the ones I can think of off the top of my head. If you have any questions, please feel free to PM me.