Welling, P., M. Orell, K. Koivula, and K. Lahti. “The Frequency and Timing of Copulations in the Willow Tit Parus Montanus”. Ornis Fennica, vol. 74, no. 1, Mar. 1997, pp. 51–58, https://ornisfennica.journal.fi/article/view/133457.