In addition to each party dismaying of its chances—the Conservatives, for instance, are suddenly quite keen on Michael Ignatieff—it will no doubt be said over the next two months that by-elections naturally favour the government or opposition.
Here then is Wikipedia’s list of federal by-elections. By my count—excluding the case of Bill Casey, the Conservative MP who was expelled from caucus, won as an independent and was succeeded, after retiring, by a Conservative—the government of the day has held 22 of the 31 seats contested in by-elections over the last 30 years. Opposition parties—again excluding the Casey situation—have held 26 of 38 seats.
By respective percentage, governments held 71% of the time, opposition parties held 68% of the time.
By total seats, governments went into those 69 by-elections with 30 seats and emerged with 29.