Parashas Vayikra · Aliyah 2 · Passuk 16
Chabad.org — English translation + Rashi
Torah Temima points to Zevachim 65a (verified; crop removed with its feathers).
English: “Its crop with its feathers” — the crop; might one dig it out with a knife [alone]? The Torah states “with its feathers” — he takes the feathers [and skin] with it.
Custom (force of the accompanying noun).
The crop is removed together with the feathers and skin covering it, not excavated under the skin alone.