Author contributions
A.A.P., M.R.K. and J.C.d.R. conceived of the study; V.T. carried out population genomic analyses; A.A.P. and S.N. carried out sequencing; K.L.A. performed gene expression studies; T.J.B.d.M. assisted with bioinformatics; F.X.V. led sampling of western monarchs; A.A.P. and J.C.d.R. carried out and analyzed flight performance experiments; A.A.P., V.T., M.R.K. and J.C.d.R. wrote the manuscript and all other authors edited the manuscript.