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Season length influences breeding range dynamics of trumpeter swans Cygnus buccinator

TitleSeason length influences breeding range dynamics of trumpeter swans Cygnus buccinator
Publication TypeJournal Article
Year of Publication2011
AuthorsSchmidt, JH, Lindberg, MS, Johnson, DS, Verbyla, DL
JournalWildlife Biology

The breeding range of large-bodied waterfowl nesting in the northern boreal forest is likely influenced by breedingseason length. This may be particularly true for the largest species of North American waterfowl, the trumpeter swanCygnus buccinator, due to the extended time period necessary to raise young to fledging. This species recently recoveredfrom 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 periodinsufficient for successful reproduction.Weused hierarchical occupancy models to analyze trumpeter swan survey datacollected over the entire breeding range in Alaska during 1968-2005. We fit models containing combinations ofrecolonization parameters, trend and latitude, and season length to these data to determine whether these variablesexplained the variation in occupancy across our survey area. Support for season length parameters would provideevidence that the recolonization process was partially related to the length of the breeding season. We expected thatoccupancy probability would increase range-wide due to overall population growth, while occupancy would be greatestat mid-latitudes, near the center of the species range. Because this population was recovering, we also expected thatexpansion would proceed outward from the range center. Our results indicated that habitat occupancy was positivelyrelated to season length, partially explaining the recently observed northward range expansion. Our results suggest thatincreases in annual temperatures due to climate warming would likely be associated with further range expansion intrumpeter swans and may have implications for other wetland obligates. Changes in species distributions will likelyincrease competition for breeding areas with potential negative effects on species not limited by season length. This mayalready be occurring in Alaska where the breeding distribution of trumpeter swans has begun to overlap with that oftundra swans Cygnus columbianus.

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