The recently published results from our experimental study describing the thermal flexibility of the rockwren (Xenicus gilviventris), combined with the unique alpine ecology of this species, led us to raise the question, as the title of our article clearly indicates, ‘Does the New Zealand rockwren (Xenicus gilviventris) hibernate?’.
The study of the rockwren is of special importance. This species combines having a small mass and largely insectivorous diet with living above the climatic timberline in the mountains of the South Island of New Zealand year-round. The response of this species to the challenging alpine conditions is of great interest. Within the Introduction of our article (McNab and Weston, 2020), we critically define the difference between a period of short-term torpor and hibernation. Although caprimulgids exhibit a range in observed torpor period lengths, there is no clear evidence that one of them, the common poorwill (Phalaenoptilus nuttalli), goes into hibernation. We introduce the characteristics of the rockwrens' alpine ecology, which provides the rationale for the hypothesis of a unique thermal behavior in this species.
As Geiser et al. (2020) state, our results are based on a limited data set of six individuals. We showed that this species enters shallow torpor at ambient temperatures well above those encountered in their natural environment. We were not able to demonstrate the length of the period of torpor, which is required to separate short-term torpor from hibernation along a temperature continuum. For that to be demonstrated, it must be done in the field. We do not claim to have answered the question within our study whether rockwrens hibernate; rather, we state that ‘evidence of an extended period of torpor is required to conclude that the rockwren hibernates…’ (p. 4, McNab and Weston, 2020). We do, however, leave the reader with the enduring question of whether it may be possible that this unique alpine passerine species hibernates. This should encourage further research on the species. Geiser et al. (2020) also agree this question ‘seems reasonable, given that these birds are small and largely sedentary, eat invertebrates and live at high altitudes, where they apparently can nest in snowbanks’.
Unfortunately, some misunderstandings of our article were present in the analysis of Geiser et al. (2020). The statement that the birds ‘did not settle in the chamber’ is incorrect: this occurred only in one individual (p. 3, McNab and Weston, 2020). The limited amount of data that we have reflects the highly endangered status of the New Zealand rockwren and the necessary restrictions around the time that individuals were held in captivity. With such a limited sample size, the application of robust statistical testing becomes irrelevant and therefore we have simply presented a summary of the raw data, always providing the variance around the data. We did not address torpor in other passerines because we were concerned with rockwrens; none of the other passerines face similar conditions. We could not expose the rockwrens to low ambient temperatures – they were neither available nor permissible for this study.
We are aware that the occurrence of hibernation in this species remains an open question, and believe if anything, that this study stands to demonstrate to students the difficulties of working with highly endangered species in remote mountainous locations. Above all, we hope that this study and the limitations that it presents motivate further research into determining the over-wintering strategy of this rare and unique alpine passerine.
