Ibn Majah

Benefits and Issues:

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The mamoola is a small bird whose head is large, its belly is white, and its back is green.
It preys on small birds and insects, etc. (Footnote of Ibn Majah by Muhammad Fuad Abdul Baqi, referencing Al-Munjid)
Ibn Athir said:
It is a bird with a large head and a large beak.
Its feathers are half white and half black. (Al-Nihaya, root: m-r-d)

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Our esteemed researcher has declared the mentioned narration weak in its chain of transmission, whereas other scholars have considered it authentic based on supporting evidences; therefore, the mentioned narration is valid as proof. However, what is meant by not killing is that it is prohibited to use these things as food.
And Allah knows best.
For further details, see: (Al-Irwa: 8/143)