Mean, standard deviation (sd) and range (min/max) of study variables in 26 juniper woodlands in Spain in winters 2009–2010 and 2010–2011.

Relative abundance of tits refers to the mean number of birds counted in four censuses without detection distance limit (two censuses per winter). Values for temperatures are averages for 77 days (1 December to 15 February) in two consecutive winters. Tree height and abundances of fruits and arthropods are estimated in each juniper woodland as the average of several samples (see Methods for more details). R2: square of the correlation between day length at winter solstice and each variable (**: pp