We assessed geographic variability in morphology among Yellow-headed B
lackbirds (Xanthocephalus xanthocephalus) collected from breeding popu
lations in Minnesota, North Dakota, and South Dakota in the United Sta
tes and Alberta, Manitoba, and Saskatchewan in Canada during spring. B
oth male and female Yellow-headed Blackbirds tended to be larger in th
e more northern breeding populations. Males collected in Manitoba and
southeast Saskatchewan tended to be more like populations from the Uni
ted States, whereas other populations in Saskatchewan and Alberta popu
lations were morphologically similar to each other. For females, howev
er, only the Manitoba population was similar to the U.S. populations,
and the females from the southeastern Saskatchewan were similar to oth
er Saskatchewan birds. Southeastern Saskatchewan appears to represent
a transitional region between northwestern and southeastern breeding p
opulations of Yellow-headed Blackbirds.