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I am looking to identify a story I read in the 1960s or 70s. It was about a scoutship that landed on Earth, prelude to an invasion force. Previous scouts had disappeared without a trace.
The crew members' names were descriptive of their function. I think the protagonist was called Pilot. They are shape-changers. One by one, the crew members defect, taking on forms of Earth creatures that fit their function and personality. Pilot tries desperately to complete the mission. One scene I remember is he assumes human form and walks into a building -- but is unable to blend in, I think because he forgets to form facial features.
He struggles to complete his crew's mission, barricading himself in a room? (perhaps to destroy the planet?) when he sees birds out the window. The people breaking in to stop him see only a large (white?) bird flying out the window, a bit awkward, but quickly gaining control of its wings and soaring away.