The gemara says that the p'sul of a ba'al mum applies only to animals but not to birds. Rashi explains that the pasuk that demans - תמים זכר בבקר בכשבים ובעזים implies specifically for animals, not for birds. But, if the bird is missing a limb it is pasul. Rashi attributes the p'sul of מחוסר אבר to be a violation of הקריבהו נא לפחתך which is the same pasuk that the gemara uses on amud alef to say that you can't sacrifice the intestines without cleaning out the excrement because it is improper. Rashi ends of by saying - וכן מום מגונה. Rashi seems to understand that any significant mum that would make the animal me'guneh, would be a violation of הקריבהו נא לפחתך. Rashi 68b writes that a bird who's eye is poked out is invalid because it is missing a limb which is a violation of הקריבהו נא לפחתך. Tosafos asks on rashi that the source to exclude a bird that is missing a limb the gemara says in kiddushin is from the drasha of מן העוף, ולא כל העוף פרט לשיבש גפה, not because of הקריבהו נא לפחתך. Similarly on 35b, Rashi says the p'sul is הקריבהו נא לפחתך and Tosafos argues. It seems that there would be a machlokes between rashi and tosafos regarding types of blemishes that are more severe than דוקין שבעין and are obvious so they would be "meguneh", but aren't quite מחוסר אבר. Tosafos 35b says explicitly that anything short of missing a limb would not be pasul m'doraysa, whereas rashi would seem to hold that anything which is מגונה and looks disgusting would be pasul m'doraysa based on הקריבהו נא לפחתך.