In fresh plumage these smallest of the North American nuthatches are richly colored below as seen here. This color fades with time. This highly social species survives cold weather by "controlled hypothermia" in which multiple birds snuggle together in roosting cavities for warmth. Stacks of 10 birds or more sometimes roost together in formations of squares, oblongs, triangles, diamonds, wedges, and tiers. More than 150 individuals have been found roosting in a single tree
Nikon Coolpix P510.
References:
Harrap, S. (2018). Pygmy Nuthatch (Sitta pygmaea). In: del Hoyo, J., Elliott, A., Sargatal, J., Christie, D.A. & de Juana, E. (eds.). Handbook of the Birds of the World Alive. Lynx Edicions, Barcelona. (retrieved from https://www.hbw.com/node/59922 on 8 December 2018).
Kingery, H. E. and C. K. Ghalambor (2001). Pygmy Nuthatch (Sitta pygmaea), version 2.0. In The Birds of North America (A. F. Poole and F. B. Gill, Editors). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bna.567