Does a passerine hibernate?

The thermal physiology of the highly endangered Rock Wren (Xenicus gilviventris) from New Zealand is examined. It is a member of the Acanthisittidae, a family unique to New Zealand. This family, derived from Gondwana, is thought to be the sister taxon to all other passerines. Rock Wrens permanently reside above the climatic timberline at altitudes from 1,000 to 2,900 meters in the mountains of South Island. They feed on invertebrates and in winter face ambient temperatures well below freezing and deep deposits of snow. Their body temperature and rate of metabolism are highly variable. Rock Wrens regulate body temperature at ca. 36C, which in one individual decreased to 33.1C at an ambient temperature of 9.4C, which returned to 36C at 30.1C; its rate of metabolism decreased by 30%. The rate of metabolism in a second individual twice decreased by 35%, nearly to the basal rate expected from mass. The Rock Wren food habits, entrance into torpor, and continuous residence in a thermally demanding environment suggest that it may hibernate. For that conclusion to be accepted, evidence of its use of torpor for extended periods is required. Those data are not presently available. Acanthisittids are distinguished from other passerines by the combination of their temperate distribution, thermal flexibility, and a propensity to evolve a flightless condition. These characteristics may reflect their phylogenetic status, but they are so different from those found in other passerines that it is more likely that they reflect the geographical isolation of acanthisittids in a temperate environment for 85 million years in the absence of mammalian predators.