The Snow Bunting was much bigger than I expected when seen close up. Size matters. Volume contains cells that generate thermal energy. The bigger the volume, the more warmth for polar species. Thermal energy is lost through the surface. Consider a sphere. Volume = 4/3 * pi * r^3. Surface area = 4*pi*r^2. Hence if I double the radius, volume is up by a factor of 8 whilst surface area is up by a factor of 4. 8x the thermal energy generated but only 4x the rate of heat loss. So it pays to be big to stay warm.
