Observational data for shift in the northern range boundary (NRB)
To analyze shift in the NRB, we used observational data on the 289 species that were selected for the study through defining data for the phenological analyses (see above; and Text S1). 2 474 498 observations on the 289 study species available through the Insect database and National Butterfly Monitoring Scheme (Saarinen et al. 2003) were sourced from the Finnish Biodiversity Information Facility (FinBIF) in December 2019 (Text S2).
To measure shift in NRB over time, we compared NRB between two five-year periods: 1992-1996 (hereafter T1) and 2013-2017 (hereafter called T2). The total number of presence cells for T2 was, however, substantially higher than for T1 due to overall increased sampling effort over time. To avoid effects caused by differences in overall observation intensity, we randomly subsampled the observations in T2 across species to equal the number of observations in T1. The temporal span of the distributional data in relation to the systematic data used to estimate phenological shift is depicted in Fig. S1.