Has Scotland suddenly become one of Labour's weakest areas?

It's been fascinating since the Holyrood election to see how Labour's support in Scotland seems to have slumped to a level closer to that you'd expect in the south of England, rather than one of their heartland areas. As an illustration, here are the party's regional voting intention shares from the last four GB-wide YouGov polls -

18th May :

North of England 52%
Midlands/Wales 43%
London 41%
South (excluding London) 31%
Scotland 28%

19th-20th May :

North of England 55%
Midlands/Wales 44%
London 44%
Scotland 35%
South (excluding London) 33%

22nd-23rd May :

North of England 55%
Midlands/Wales 46%
Scotland 39%
London 38%
South (excluding London) 31%

23rd-24th May :

North of England 56%
Midlands/Wales 47%
London 40%
Scotland 38%
South (excluding London) 31%

Of course, this may well be nothing more than a transitory side-effect of the SNP's honeymoon period. All the same, it's a timely reminder that taking your core vote for granted is all very well - up to the point that your core vote finds somewhere else to go.