**Supplementary data** ---------------------- ---------- **Highly pathogenic avian influenza causes mass mortality in Sandwich tern (*Thalasseus sandvicensis*) breeding colonies across northwestern Europe** ---------- Ulrich Knief, Thomas Bregnballe, Ibrahim Alfarwi, Mónika Ballmann, Allix Brenninkmeijer, Szymon Bzoma, Antoine Chabrolle, Jannis Dimmlich, Elias Engel, Ruben Fijn, Kim Fischer, Bernd Hälterlein, Matthias Haupt, Veit Hennig, Christof Herrmann, Ronald in ‘t Veld, Elisabeth Kirchhoff, Mikael Kristersson, Susanne Kühn, Kjell Larsson, Rolf Larsson, Neil Lawton, Mardik Leopold, Sander Lilipaly, Leigh Lock, Régis Marty, Hans Matheve, Włodzimierz Meissner, Paul Morrison, Stephen Newton, Patrik Olofsson, Florian Packmor, Kjeld T. Pedersen, Chris Redfern, Francesco Scarton, Fred Schenk, Olivier Scher, Lorenzo Serra, Alexandre Sibille, Julian Smith, Wez Smith, Jacob Sterup, Eric Stienen, Viola Strassner, Roberto G. Valle, Rob S. A. van Bemmelen, Jan Veen, Muriel Vervaeke, Ewan Weston, Monika Wojcieszek, Wouter Courtens ---------- We provide five **data files** containing: 1. Breeding and HPAI data from all colonies in northwestern Europe and the westernmost Mediterranean. 2. Environmental variables collected in HPAIV-affected colonies. 3. Longitudinal data on the cumulative numbers of dead Sandwich terns in eleven colonies. 4. Counts of first-year and adult birds at major European roosts in Denmark from 2020–2022. 5. Sero-surveillance in adult Sandwich terns caught in July in Belgium. A detailed description of all files is in the README. ---------- We furthermore make our **analyses and plotting scripts** available. All analyses have been conducted in R (v4.2.0). For analyzing spatial data, we make use of the Global Self-consistent, Hierarchical, High-resolution Geography Database (GSHHG) that can be downloaded in its latest version (v2.3.7) from the [University of Hawaiʻi at Mānoa webpage][1]. [1]: https://www.soest.hawaii.edu/pwessel/gshhg/hhg/