From an unused root meaning to peel, i.e. Emaciate; the gull (as thin) -- cuckoo.
H7828. shachaph
[שַׁ֫חַף] noun [masculine] probably sea-mew, gull (compare ᵑ9 ᵐ5) (from attenuated body, Thes); so PostHast. DB CUCKOW TristrNHB 210 ff; sterna fluviatilis, or tern IdFFP 135 M'Lean-ShipleyEncy. Bib. ID.; — הַשָּׁ֑חַף Deuteronomy 14:15 = Leviticus 11:16 (P), in list of unclean birds.