On Magpie marriages

As in other corvids, like Ravens and Carrion Crows, non-breeding magpies flock together. The non-breeders tend to be young birds, while territory holders, which breed, are more likely to be older. There is a hierarchy in the flock, and the dominant birds are more likely to be the next in line in obtaining a territory. Magpies form pairs in the non-breeding flock.
This morning, I witnessed my second ever 'great Magpie marriage', one or the most puzzling Magpie behaviours. A group of seven magpies (top shot) were high on a tree, calling, a short call, 'chak!' and bowing with wings half spread, displaying, pecking the branches, or chasing around the tree, but mostly watching each other.
This is a short clip of the gathering.

This behaviour, which usually takes place in the winter, in sunny mornings (although it was quite dark and cloudy today) was traditionally interpreted as the way Magpies form pairs before nest building starts. Tim Birkhead, in an article for British Birds, quotes this passage from Darwin's book 'The Descent of Man':

The common magpie (Corvus pica, Linn,), as I have been informed by the Rev. W. Darwin Fox, used to assemble from all parts of Delamere Forest, in order to celebrate the "great magpie marriage." [...] They then had the habit of assembling very early in the spring at particular spots, where they could be seen in flocks, chattering, sometimes fighting, bustling and flying about the trees. The whole affair was evidently considered by the birds as one of the highest importance. Shortly after the meeting they all separated, and were then observed by Mr. Fox and others to be paired for the season.

Birkhead's research with the Magpie population of Rivelin's Valley in Sheffield showed that instead of a pair formation congregation, this behaviour, is instead a territorial challenge by the dominant non-breeding pair. The rest of the flock attend to witness the event and, although the territory owners often keep their territory, in some cases they are ousted by the dominant non-breeders.

My first Magpie marriage, on a sunny morning a few years back (9/11/09). Individuals fluffed their white feathers in their displays.

More information

Birkhead, T. R. Studies of West Palearctic birds: Magpie 189. British Birds (1989). 82:583-600. Here.

Birkhead, T.R. 1991. The Magpies. The ecology and behaviour of Black-billed and Yellow-billed Magpies. T&AD Poyser, 270 pp.