Hybrid zone movement
For eight of the transect-years, the best fit of was the model with free minimum and maximum frequencies and no tails, for six transect-years it was the model with fixed frequencies and no tails, and for one transect-year it was the model with free frequencies with tails (Table 3; Supplementary Figures S1 and S2). For the Massachusetts transect, the analyses for 2009, 2011, and 2012 resulted in cline shapes with minimum or maximum frequencies that did not approach 0 and 1 (Supplemental Figure S2). This result was likely due to the fact that for those years, we did not sample far enough into the Bruce spanworm (2009) or winter moth (2011 and 2012) dominated areas. Consequently, these years were left out of Figure 7 as the centers could not be accurately estimated (Table 3). Cline analyses of the hybrid zones along the Massachusetts and Connecticut transects indicated that the center of the hybrid zones moved westward during our study period (Figures 7 and 8). Along the Massachusetts transect, in 2007 the center of the hybrid zone was located approximately 131 km from trap T00, and by 2018 the center of the hybrid zone was located approximately 103 km from trap T00 (Table 3 and Figure 7). During our survey period, the location of the center of the hybrid zone moved 3.1 km from 2007 to 2008, then moved more rapidly from 2008 to 2010 (6.2 km/yr), then moved steadily westward from 2014-2017 (2.4 km/yr), before retreating eastward 7.7 km in 2018. Along the Connecticut transect, in 2016 the center of the hybrid zone was located approximately 109 km from trap CT01, in 2017 it moved approximately 8.9 km west, and in 2018 the center of the hybrid zone retreated approximately 3.1 km east (Table 3). It is therefore possible that while we did not sample this transect prior to 2016, that a similar westward shift occurred along the Connecticut transect as we found along Massachusetts. For both transects, the mean population density of moths increases dramatically just east of the hybrid zone center in the region where winter moth is dominant (Figures 7 and 8). For the Massachusetts transect, the cline width varied from 9.62 to 69.42 km across years with a mean of 39.03 ± 7.01 km, and for the Connecticut transect, the cline width varied from 26.82 to 50.3 km across years with a mean of 40.18 ± 6.97 km (Table 3).