In the winter of 2011 Ryanair grounded 80 of its fleet in the US State of Arizona. With fuel accounting for 40-50 % of costs, more airlines are flying significantly less in winter.
According to research carried out by anna.aero in the article entitled "Grounding planes in winter improves profitability: The new network planning reality explored among Europe’s airlines" in July 2012:
Airlines generally earn their profits in summer when demand, yields (and capacity constraints) are highest. In the past, when fuel was much cheaper, airline economics were dominated by fixed costs – aircraft ownership, salaries, and the perceived wisdom that aircraft should be kept flying even in winter, even if loads and yields were significantly lower, as this helped to reduce unit cost per Available Seat Kilometre (at least on paper).
Although flying less pushes up the unit cost of the flying that does take place, the idea is that unit revenues will go up by a higher percentage as there will be fewer empty seats during the off-peak period as reduced capacity should lead to higher average fares and better load factors.
It would appear that other airlines are following suit eg Easyjet, German Wings.
The full article including analysis: