Over the last year I've been working with Yaara Aharon-Rotman and Christa Beckman on a project investigating the use of torpor (a kind of short-term slow down of the metabolism to save energy, like a mini-hibernation) in passerine birds. We spent a lot of time in the woods in Armidale catching birds and equipping them with combined temperature recorders and VHF transmitters. You can read the original post here. That work has resulted in a new paper just published in Frontiers in Ecology and Environment.
The aim of our study was to examine the thermal energetics of a small passerine species, the eastern yellow robin (Eopsaltria australis) and White-throated treecreeper (Cormobates leucophaea)), during winter at a cool temperate climate site in the eastern Australian Northern Tablelands. The data on treecreepers was not great for a bunch of reasons, including the temperature loggers falling off quicker than we had hoped. We hope to gather more data on the treecreepers later but the robins furnished us with enough data for our analysis so we carried on with the work, focusing on them.
The use of torpor for energy conservation is common in a range of small mammals and in several non-passerine bird species such as hummingbirds. In contrast, the extent of the use of torpor by passerines, a group which includes over half of extant birds species, is understudied. To date, only two passerine species studied in the wild have been shown to express torpor, noisy miners (Manorina melanocephala) and fairy wrens (Malurus cyaneus). You can see in the plot below that our little robin friends dropped their skin temperature by as much as 9°C over night. For comparison, average body temperature in a human is about 37°C and if it drops by only 2 degrees we are considered to be in hypothermia and are in danger of dying. These birds have an average temperature of about 40°C and drop down as low as 27°C over night.
Our study provides evidence for torpor expression in a small ∼20 g passerine, and suggests plasticity in the use of torpor, in response to local environment. The resulting flexibility in energy requirements allows birds to optimize energy conservation during the current increasing frequency of extreme and unpredictable weather events, driven by changing climate. Indeed, despite range reduction and extreme climatic events, eastern yellow robins manage to survive in a marginal reserve and perhaps torpor allows them to overcome these energetically challenging periods. The question that remains to be answered is whether individuals that previously did not encounter extreme climatic events, or local climatic changes, are able to apply thermal strategies to adjust to new environmental conditions.
You can read the full paper here.