Gregory, R. D., van Strien, A., Vorisek, P., Gmelig Meyling, A. W., Noble, D. G., Foppen, R. P., & Gibbons, D. W. (2005). Developing indicators for European birds. The Royal Society.

Gregory, Richard D., Arco van Strien, Petr Vorisek, Adriaan W. Gmelig Meyling, David G. Noble, Ruud P.B Foppen, and David W. Gibbons. Developing Indicators for European Birds. The Royal Society, 2005.

Gregory, Richard D., et al. Developing Indicators for European Birds. The Royal Society, 2005.