There is no convincing evidence that magpies, even in areas where they have become abundant, cause long-term damage to songbird populations. Predation is often not nice for us delicate humans to behold but it is an entirely normal part of the natural world, and nature depends on its occurring.

http://www.rspb.org.uk/wildlife/birdguide/name/m/magpie/effect_on_songbirds.aspx

What could only be called the agricultural vandalism of the last few decades is, by a massive margin, the main enemy of songbirds.