Trembling movements of House Martin when nest-building

On 6th June 1975, near South Gorley, Hampshire, I watched at close range with binoculars a House Martin Delichon urbica building its nest, which was in the early stages of construction. The nest was situated below the eaves of an old thatched cottage at a height of three metres from the ground. I noticed that, when the martin placed and hel

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