In a few London's older parks I see a large basket on top of a high pole that look a bit like a crows nest on a ship's mast. What are they?
The Oracle of Omigod
I will guess it is for storks just for the fun of it.
I think what you are describing is a beacon. Traditionally, beacons would have been set up in prominent locations like hilltops, and used for military signaling. A load of firewood wood be kept in the basket, and it would be set on fire when some event happened (eg when a hostile army was sighted). People at the next beacon post would see the fire and light their own fire, and then people at the third post would see the second fire and light theirs, and so on, so a chain of beacons could send a warning very quickly over a distance of tens or hundreds of miles. Beacons aren't used like that any more, but they are still used sometimes as a way of celebrating big events, such as the Queen's Jubilee or the start of the new Millennium. Modern beacons in parks and other public places were usually set up to commorate one of these events. https://www.bbc.co.uk/news/uk-england-18297618 EDIT: I've just remembered that the Lord of the Rings film features a scene where beacon fires are used as a warning to prepare for battle: https://www.youtube.com/watch?v=i6LGJ7evrAg
places for birds to nest ...........................................