Use of Torpor in Migratory Hummingbirds
Rufous Hummingbird image courtesy of James Ownby
|There have been studies performed by Carpenter et al. (1988 and 1993) that reveal another use of torpor in wild migrant hummingbirds. It appears that at least one species (the rufous hummingbird) uses torpor to conserve energy needed for migration. This was initially observed and recorded in the 1988 paper, where Carpenter showed documentation of a rufous hummingbird with a body mass that was %60 above lean body mass. This observation was very unusual at the time because up until this point, torpor in hummingbirds was associated with low energy reserves and not with birds containing large amounts of body fat.|
The later study revealed that migrating rufous hummingbirds will enter torpor before and during their migration to minimize energy loss. This allows for more energy for the actual process of migration. Additionally, even when food sources were present and abundant, the migrating hummingbirds would enter torpor rather than burn off excess fat (Carpenter 1993). It was hypothesized that this behavior comes from the fact that rufous hummingbirds lack access to large amounts of amino acids. Thus, they conserve fat so that they do not use all of its energy and have to shift to breaking down muscle. In the few cases where hummingbirds were observed without large fat stores, they were unable to quickly regain mass on migration stop-overs, probably due to the above theory.
It has become obvious that torpor is not simply a tool that hummingbirds can use in energetic emergencies but instead can be used to regulate energy use in a much broader way. Hummingbirds are able to use torpor to manage energy use over long periods, including migration and the periods leading up to migration (see section on seasonal variations in torpor).
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