What factor significantly contributes to the perception that the Willow Warbler might be a rare visitor?

Answer

Its subtle coloration and fast-moving habits

The bird's subtle coloration and active habits allow it to blend into the green canopy easily, leading observers who rely on fleeting glimpses to think it is rarer than it is.

What factor significantly contributes to the perception that the Willow Warbler might be a rare visitor?
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