Morphological variation and status of the orioles, Icterus galbula, I. bullockii, and I. abeillei, in the northern Great Plains and in Durango, Mexico

The phenetic affinities of 56 specimens of the orioles Icterus galbula and I. bullockii, and specimens intermediate between them, from the northern Great Plains, are discussed. Phenetically intermediate individuals appear there in lower proportion than they do at most localities in the southern plains, suggesting that there is selection against such individuals in the north. The range of I. galbula seems to be extending southward m the northern plains at the expense of I. bullockii; this is possibly correlated with changes in the climatic environment. I. bullockii and I. abeillei both are uncommon in north-central Durango, Mexico, the only place where they are known to be sympatric.