Two Eurasian Blue Tits Cyanistes caeruleus roosting together in a nest-box on multiple winter nights

Birds that spend the winter in a harsh and cold climate face a suite of challenges that require optimization of energy expenditure and energy intake. Some birds roost communally, which can increase energy savings during cold winter nights. However, this behaviour is almost completely absent in chickadees, tits, and titmice (Paridae) as there are very few accounts in the literature of parids roosting together. Here I review these accounts and describe an observation where two Eurasian Blue Tits Cyanistes caeruleus were found roosting together in a nest-box on multiple winter nights in January of 2021.