Pöysä, H. “Changes in Predator Surveillance in a Foraging Great Tit Parus Major in Response to Presence and Group Size of Yellowhammers Emberiza Citrinella”. Ornis Fennica, vol. 62, no. 3, Sept. 1985, pp. 138–140, https://ornisfennica.journal.fi/article/view/133188.