Both parents take part in the building and, as mating time approaches, their salivary glands swell greatly. Having chosen a suitable rock face, they birds fly to it, turns to dab it with saliva, tracing the outline of the base of the nest. Their saliva is sticky and thread forming. Each layer quickly dries and, as it does so, another layer is dabbed on top, gradually building up to form a white translucent cup shaped into the curve of the rock.
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