The following information is also given in the main manuscript. The individual contributors should be contacted for more information or permissions to work with their respective data sets. 276 of the plots come from the British Trust for Ornithology's (BTO) Common Bird Census (CBC) and the BTO/Joint Nature Conservation Committee/Royal Society for the Protection of Birds, Breeding Bird Survey (BBS) (Marchant, 1990; Freeman et al., 2007). Two additional plots from the UK were obtained from Williamson (1975) and Gaston and Blackburn (2008). 78 plots from Germany were provided by Bowler and Schwarz (pers. comm.), see Schwarz and Flade (1989) and Kamp et al. (2021). 2 plots from Estonia were provided by Leivits (pers. comm.). 7 plots from Poland were obtained from Tomiałojć and Wesołowski (1996) and Wesołowski et al. (2002). 6 plots from Sweden were obtained from Enemar et al. (2004), Svensson (2006 & 2009). 2 plots from Finland were obtained from Palmgren (1987) and Lehikoinen et al. (2016) 3 plots from Norway were included (own data, as well as Moksnes (1978) and Hogstad (1993)). References: Enemar, A., Sjöstrand, B., Andersson, G. & Von Proschwitz, T. (2004) The 37-year dynamics of a subalpine passerine bird community, with special emphasis on the influence of environmental temperature and Epirrita autumnata cycles. Ornis Svecica, 14, 63-106. Freeman, S.N., Noble, D.G., Newson, S.E. & Baillie, S.R. (2007) Modelling population changes using data from different surveys: the Common Birds Census and the Breeding Bird Survey. Bird Study, 54, 61-72. Gaston, K. & Blackburn, T. (2008) Pattern and process in macroecology. Hoboken. NJ, John Wiley & Sons. Hogstad, O. (1993) Structure and dynamics of a passerine bird community in a spruce-dominated boreal forest. A 12-year study. Annales Zoologici Fennici, pp. 43-54. JSTOR. Kamp, J., Frank, C., Trautmann, S., Busch, M., Dröschmeister, R., Flade, M., Gerlach, B., Karthäuser, J., Kunz, F. & Mitschke, A. (2021) Population trends of common breeding birds in Germany 1990–2018. Journal of Ornithology, 162, 1-15. Lehikoinen, A., Fraixedas, S., Burgas Riera, D., Eriksson, H., Henttonen, H., Laakkonen, H., Lehikoinen, P., Lehtomäki, J., Leppänen, J. & Mäkeläinen, S. (2016) The impact of weather and the phase of the rodent cycle on breeding populations of waterbirds in Finnish Lapland. Ornis Fennica, 93. Marchant, J.H. (1990) Population trends in British breeding birds. British Trust for Ornithology. Moksnes, A. (1978) Vurdering av ornitologiske verneverdier og skadevirkninger i forbindelse med planene om tilleggsreguleringer i Neavassdraget, Tydal kommune. Palmgren, P. (1987) On the constancy of annually repeated bird censuses. Ornis Fennica, 64, 85-89. Schwarz, J. & Flade, M. (1989) Ergebnisse des DDA-Monitoringprogramms. Teil I: Bestandsänderungen von Vogelarten der Siedlungen seit, 87-106. Svensson, S. (2006) Species composition and population fluctuations of alpine bird communities during 38 years in the Scandinavian mountain range. Ornis Svecica, 16, 183-210. Svensson, S. (2009) A stable bird community during 27 years (1980—2006) in the nemoral broadleaf wood Dalby Söderskog National Park. Ornis Svecica, 19, 237-243. Tomiałojć, L. & Wesołowski, T. (1996) Structure of primaeval forest bird community during 1970s and 1990s [Bialowieza National Park, Poland]. Acta Ornithologica, 31, 133-154. Wesołowski, T., Tomiałojć, L., Mitrus, C. & Rowiński, P. (2002) The breeding bird community of a primaeval temperate forest (Białowieża National Park, Poland) at the end of the 20th century. Acta Ornithologica, 37, 27-45. Williamson, K. (1975) The breeding bird community of chalk grassland scrub in the Chiltern Hills. Bird Study, 22, 59-70.

Understanding patterns of species diversity is crucial for ecological research and conservation, and this understanding may be improved by studying patterns in the two components of species diversity, species richness and evenness of abundance of species. Variation in species richness and evenness has previously been linked to variation in total abundance of communities as well as productivity gradients. Exploring both components of species diversity is essential because these components could be unrelated or driven by different mechanisms. The aim of this study was to investigate the relationship between species richness and evenness in European bird communities along an extensive latitudinal gradient. We examined their relationships with latitude and Net Primary Productivity, which determines energy and matter availability for heterotrophs, as well as their responses to territory densities (i.e., the number of territories per area) and community biomass (i.e., the bird biomass per area). We applied a multivariate Poisson log-normal distribution to unique long-term, high-quality time-series data, allowing us to estimate species richness of the community as well as the variance of this distribution, which acts as an inverse measure of evenness. Evenness in the distribution of abundance of species in the community was independent of species richness. Species richness increased with increasing community biomass, as well as with increasing density. Since both measures of abundance were explained by NPP, species richness was partially explained by energy-diversity theory (i.e., the more energy, the more species sustained by the ecosystem). However, species richness did not increase linearly with NPP but rather showed a unimodal relationship. Evenness was not explained either by productivity nor by any of the aspects of community abundance. This study highlights the importance of considering both richness and evenness to gain a better understanding of variation in species diversity. We encourage the study of both components of species diversity in future studies, as well as use of simulation studies to verify observed patterns between richness and evenness.

Funding provided by: The Research Council of NorwayCrossref Funder Registry ID: https://ror.org/00epmv149Award Number: SFF-III 223257