Loki borrows Freya's falcon cloak in the story of the Theft of Idunna's Apples. Loki is a shape shifter. In the Eddaic stories, he appears in various forms, including a fly, a seal, etc. In Continental European folklore, he appears as a red fox. It's very likely that he could have turned into a bird without Freya's divine object. For what purpose did he borrow it? It would make sense in the story to borrow it to have a greater flight time, but that would only be the answer if his shape shifting power was limited, and there does not appear to be any evidence for that. So, I think the answer is that he borrowed it to mask as Freya. So that when he flew over Jotunheim, the giants below would not know it was him.
Freya must have known that Loki intended to mask as her when she lent him the cloak. Freya also once helped Thor mask as her, in the story in which Thor puts on a wedding gown to retrieve his hammer.
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