Fertilizing isn't the big problem. There are organic fertilizers out there that won't harm the birds. Kelp meal, for instance, is something that some people give to there birds free choice for the minerals. If my girls nibble on that while free ranging I'm not concerned. The seed is more of an issue, in that it will never have a chance to take off and grow of you let them at it. They will range it all away as seed snacks. What I have done is over seed smaller areas at a time and put up temporary fencing to keep them out. Once it's established move on to another area. I know, what a pain in the bum, but what else can you do.