Yesterday's by-election in Aird and Loch Ness saw the SNP and Lib Dems finish more or less level-pegging on first preferences (just 29 votes separated them). But in contrast to the East Ayr by-election a few weeks ago where the SNP were fractionally behind on first preferences and were still able to win, it was inevitable that the Lib Dems were going to take this one on transfers, because there were a substantial number of unionist votes to be redistributed after the elimination of the third-placed Tory.
Aird and Loch Ness by-election result (first preferences) :
Liberal Democrats 33.5% (+21.2)
SNP 32.5% (+4.6)
Conservatives 15.2% (+8.3)
Independent - Fraser 9.5% (n/a)
Greens 9.3% (n/a)
The by-election was caused by the resignation of an SNP councillor (Drew Hendry), so the SNP were "defending the seat", but only in the most technical of senses - they finished in second place in the ward last time, with even the combined vote for their two candidates still placing them more than 9% behind the winning independent candidate. So the headlines will scream "Shock Lib Dem Gain From SNP", but that's utterly meaningless - the SNP vote is up 4.6% and they remain in second place. What appears to have happened is that the Liberal Democrats have claimed the lion's share of the independent vote from last time, allowing them to leapfrog into first place. Without being aware of local factors, it's impossible to know how they pulled off that feat - it could be, for example, that their candidate is well-known and popular.
The other point that should be noted is that the increase in the SNP's vote is only slightly lower than the average increase they enjoyed in the seven by-elections last week (of which they won six).