Sure, Magpies do raid nests of other birds, but I'm not entirely in agreement that they alone are responsible for the decline of song birds. I happen to live in an area where song birds are still flourishing despite a healthy population of Magpies.
There are other birds such as Jackdaws, Jays, Hooded/Carrion Crows, Starlings and in the UK, Great Spotted Woodpeckers, to name but a few, that raid nests.Don't forget also mammalian predators such as Stoats/Weasels, Rats, Squirrels and feral/ pet cats do just as much damage. It's the latter that are most the problematical in my area.
It's unfortunate that the Magpie is always made a scapegoat on this topic when other factors are often ignored.
I,m sure Mr Heseltine was bowing to negative press and mythology when he brought up this case. It's also very unlikely he made any proper studies on the subject to justify his stance. Typical politician really.