I've never seen this behaviour in a bird before, but when we were young I remember we often heard the dull THUD as a bird accidentally hit against our large sunroom windows (it was three sides glass and the birds thought they could fly through). Most of the time these birds would just be stunned, then pick themselves up and fly off with only a headache, but sometimes they would be killed. But this chaffinch was on a suicide mission - he kept coming back for more of the same punishment.
I'd never really studied a chaffinch before but they are rather beautifully marked and they have two different 'songs' - one a repetitive thrill called the 'rain call' and a loud 'pink, pink' call. At one time, definitely still by 1851 when Mayhew's London Labour and the London Poor was published, chaffinches were a fairly popular pet-bird (after linnets, bullfinches and goldfinches). They were captured in nets on the outskirts of London and in the surrounding counties, particularly Norfolk. According to Mayhew, the nets (of about 12 yards square) were spread flat upon the ground, secured by four iron pins at the edges so that the two 'wings' or 'flaps' at the sides of the net were open. On top of the net, in the middle, sat a cage containing the 'call-bird' (usually a goldfinch or linnet), which was trained to sing loudly and cheerily to attract the wild birds. Sometimes a few stuffed birds were spread around the cage if a flock of wild birds was already nesting nearby.
The bird-catcher (described as 'an intelligent man, versed in every part of the bird business') would lie still and flat on the ground about 20 or 30 yards from the edge of the net and when he thought there were enough wild birds congregated around his decoy bird he rapidly drew towards him the 'pull-line' which caused the two 'wings' of the net to collapse and fly together to encircle the cage and the wild birds. The bird-catcher would then secure the wild birds in cages or in hampers and carry them on his back to London. It was not uncommon for the bird-catcher to secure pulls of 50 - 150 birds when the young broods were all on the wing. Not surprisingly, the mortality of these birds was fairly high once they were in captivity.
Apart from the chaffinches themselves, their nests were also collector's items in Mayhew's time. He speaks to a 'young gypsy-looking lad' who is selling nests complete with the eggs. Chaffinches laid five eggs and were on sale for 3d, 'and were for cur'osity - glass cases or anything like that.'
Having seen 'my' chaffinch in action I can imagine how easy they were to capture - especially with the lure of a decoy bird - I wonder if he saw his reflection in the window and thought he'd seen a mate, or a competitor? Perhaps he saw the red balloon and fell in love with its shiny 'feathers'....I will never know.