Fig. 2

Examples of the methodology for assessing overlap in timing of migration between age cohorts using banding data, showing examples of a species with high cohort overlap and thus concurrent migration timing of age classes (Vesper Sparrow Pooecetes gramineus, A–C) and a species with relatively low overlap and thus non-concurrent migratory timing (Sharp-shinned Hawk Accipiter striatus, D–F). After initially fitting GAMs to latitudes of banding events across the year for hatch-year and after hatch-year age cohorts of a given species across all USGS banding events 1960–2019 (A and D), we restricted the dataset to the autumn migration period, assessed as the temporal region where latitude shows a clear negative trend and refit the GAM models (southward migration; B and E). We then normalised the GAM-predicted mean latitudes for each age class to a 0–1 scale and calculated the overlap in area under the curve as an index of cohort temporal overlap during migration (C and F)