DOF with a 500 f4 is narrow. I cant tell how far away the birds were, (100 feet or a bit more) I also do not know if you cropped the shots. But consider that you dont gain much DOF by shooting f13 over f8 (are you shooting a mannual lens? IQ is likely not noticable either, so I'd have shot it at f8 and gotton all the shutter speed I could. Now if you want to increas the DOF bump it up to at least f16 and now your talking about 10 feet as appoased to about 5. . .