Comparisons of thermogenic features in four coexisting songbirds: Is the northward colonized White-browed Laughingthrush different?

Thermogenic features are often invoked to illustrate animal's colonization, distribution, and response to climate change. To understand why the White-browed Laughingthrush (Pterorhinus sannio) has expanded its distribution to temperate zones in recent years, we compared its thermogenic features with three species of songbirds that co-occur in its newly colonized areas. Thermogenic parameters of these four species were measured under different ambient temperatures, ranging from 0 to 40 ​℃. The results showed that basal metabolic rate (BMR) was 44.5 ​± ​3.9 ​mL O₂/h in P. sannio, which is lower than predicted value by its body weight. This is also lower than the BMR of both its temperate congener the Plain Laughingthrush (P. davidi) and the montane, similar species Elliot's Laughingthrush (Trochalopteron elliotii). The thermal neutral zone (TNZ) in P. sannio was 15–35 ​℃, as compared to 10–27.5 ​℃ in P. davidi, 25–30 ​℃ in T. elliotii, and 7.5–32.5 ​℃ in the Green-capped Greenfinch (Chloris sinica). Thermal conductance was lowest in P. sannio, with the minimum value lower than the predicted value based on its body weight. Our results showed that the northward-colonizing P. sannio exhibited different thermogenic characteristics compared with its coexisting species in the new habitat, even its congener P. davidi, which shared similar microhabitats to P. sannio. We suggest that researchers further explore the physiological mechanisms of birds' northward expansion.