The breeding range of large-bodied waterfowl nesting in the northern boreal forest is likely influenced by breeding season length. This may be particularly true for the largest species of North American waterfowl, the trumpeter swan Cygnus buccinator, due to the extended time period necessary to raise young to fledging. This species recently recovered from near-extinction in the early 1900s to reoccupy historic breeding areas throughout the boreal forest in Alaska, although recolonization patterns may have been influenced by variation in season length over the same time period. This may have resulted in range expansion into areas that were historically unavailable due to an ice-free period insufficient for successful reproduction. We used hierarchical occupancy models to analyze trumpeter swan survey data collected over the entire breeding range in Alaska during 1968-2005. We fit models containing combinations of recolonization parameters, trend and latitude, and season length to these data to determine whether these variables explained the variation in occupancy across our survey area. Support for season length parameters would provide evidence that the recolonization process was partially related to the length of the breeding season. We expected that occupancy probability would increase range-wide due to overall population growth, while occupancy would be greatest at mid-latitudes, near the center of the species range. Because this population was recovering, we also expected that expansion would proceed outward from the range center. Our results indicated that habitat occupancy was positively related to season length, partially explaining the recently observed northward range expansion. Our results suggest that increases in annual temperatures due to climate warming would likely be associated with further range expansion in trumpeter swans and may have implications for other wetland obligates. Changes in species distributions will likely increase competition for breeding areas with potential negative effects on species not limited by season length. This may already be occurring in Alaska where the breeding distribution of trumpeter swans has begun to overlap with that of tundra swans Cygnus columbianus.