I remember reading a few years ago that the white rump was to help evade falcons, found this in an article Bizarrely , there are rare instances of darkrumped individuals being produced naturally (several among Faeroese Rock Doves: Salomonson, 1935). It seems as though, like wild dark-rumped populations in eastern Asia, there is a latent tendency towards a dark rump. However , such individuals succumb to Peregrine attacks, leading to the conclusion that the white rump is a genetically controlled adaptation, which, through natural selection, persists in the face of a dominant gene favouring a dark rump. Research in California confirms this (Palleroni et al, 2005). Over a seven-year period, observers compared the proportions of successful attacks by Peregrines on urban flocks of domestic pigeons containing both white and plainrumped varieties. Peregrines caught far more plain-rumped birds. The researchers concluded that, during high-speed swerves, the blink of the white dorsal patch momentarily wrong-footed the assailant. Somewhat unkindly , researchers caught substantial numbers of pigeons and changed their rump colour to verify the hypothesis. http://www.birdwatchireland.ie/LinkClick.aspx?fileticket=j9yQ2XoO9Q4%3D&tabid=1413