Age-assortative pairing in birds has rarely been documented in passerines, most examples coming from relatively long-lived non-passerines with stable pair bonds. In a sample of 17 pairs of breeding McKay's Buntings (Plectrophenax hyperboreus) from St. Matthew Island, Alaska, documented through archived specimens, 14 were paired assortatively by age class (SY or ASY). This is one of the highest levels yet observed in a passerine. The mechanism(s) responsible for this are unknown in this little-studied taxon, but evidence to date suggests that active mate choice is involved. This observation adds to our limited knowledge of the breeding biology of one of North America's least studied birds. It also highlights one of the many uses of specimen series in our understanding of complex natural history phenomena that can be difficult to assess though field observation.