Tjernberg, M., K. Johnsson, and S. G. Nilsson. “Density Variation and Breeding Success of the Black Woodpecker Dryocopus Martius in Relation to Forest Fragmentation”. Ornis Fennica, vol. 70, no. 3, Sept. 1993, pp. 155–162, https://ornisfennica.journal.fi/article/view/133377.