The characteristics of a kosher or non-kosher bird are very debatable since the Torah only lists names of non-kosher birds and not definitive signs like by animals and fish. Nevertheless Acheirim say ( Chulin 65a) that we can determine the kashrus of a bird by seeing which other birds it keeps its company. "HaShachen Im Temei'im Tomei Im Tehorim Tahor"; a species of bird who dwells with non kosher birds must be non kosher and vice versa. "Lo Lechinam Holach HaZarzir im Ha"oreiv"; not for naught does the Zarzir (starling) hang out with the Oreiv (raven) says Rebbi Shimon Ben Elazar, it is therefore not kosher as well.
The pasuk says the same about people "Yagid Olov Rei'oi"; look at his friends and you will know who he is. Even if you start out better than your friends the fact that you associate with a certain element reveals a certain root beneath your surface that will be exposed outwardly over time.
The Zarzir Sings (Yeshaya 61:9): V'Nodah BaGoyim Zar'am...Ki Heim Zera Beirach Hashem"; It is clear to all the other nations that Bnei Yisroel who lives alone without any friends among the goyim, that we must be the children of Hashem since we live only with the Torah of Hashem.