by Bryce W. Robinson
I recently illustrated the above Baltimore Oriole for someone. I’ve mentioned earlier that I’d like to supplement the exercise of my illustrations with reading and reporting on an academic article relating to my subject. I chose an article that is a bit dated (Rohwer and Johnson 1992), but it peaked my interest as it discussed differences in timing of molt between the two subspecies of the then Northern Oriole. Now we understand that the Northern Oriole is comprised of two species, the Baltimore Oriole (pictured) in the east, and the Bullock’s Oriole of the west.
I learned from reading the paper that the two species differ in the timing of their pre-basic molts. Bullock’s Orioles begin their pre-basic molt after fall migration whereas Baltimore Orioles begin their pre-basic molt before and molt during migration south. The paper then suggests that the difference is under genetic control. Rohwer and Manning (1990) found a female “hybrid” (in 1990 it would be considered an intergrade) in fall undergoing a second pre-basic molt. This suggested that the bird had molted before its migration south and was undergoing a second molt at that time. This was thought to be a result of the confused genetic control of molt timing for both species, as it molted on its breeding grounds as Baltimore Orioles do, and was again molting after migration as Bullock’s do.
The interesting aspect of this article is that it discusses the differences in molt timing and attributes this difference to a genetically controlled mechanism. This supports the genetic difference between the two birds, and is likely an article used for support when the decision came to split the Northern Oriole into what we now understand as two species today, the Baltimore and Bullock’s Oriole.
I really enjoy looking through the vast scientific literature of the ornithological world. I quickly find something that interests me, and always come away with some degree of enlightenment following each read. Pairing illustration with reading is absolutely a worthy exercise that I will continue.
Rohwer S. and J. Manning. 1990. Differences in timing and number of molts for Baltimore and Bullock’s Orioles: Implications to hybrid fitness and theories of delayed plumage maturation. Condor 92: 125-140
Rohwer S. and M. S. Johnson. 1992. Scheduling differences of molt and migration for Baltimore and Bullock’s Orioles persist in a Common Environment. Condor 94: 992-994