Displaying White Storks in Madrid

A few weeks ago when I passed through Madrid I observed White Storks nesting colonially in Casa de Campo. Pairs were displaying on the nest and in fact both sexes appeared to perform the same display. This involves some very strange looking postures in which the bird begins to squat and rapidly (though fairly gracefully) throw its head backwards with the neck bending right back until the head is resting on the back. Then, more slowly, the head is brought back forward, continuing until the whole bird was leaning forward, tailed raised, wings half-open, until the bill touches the nest. Some single birds appeared more aggressive in their display, particularly as other storks flew in and landed on neighbouring trees and it probably performs different functions in different situations. The display is often accompanied by loud bill clattering which was difficult to hear as some nearby construction involving pneumatic drills made this a little difficult.