Wednesday, June 04, 2008

Dichotomous Key

OK, folks, even duller entry than usual today, so if you're not into a somewhat scientific look at some bird photos:
Immature male Orchard Oriole
Female Orchard Oriole

The species of the top bird was called into question by one of Minnesota's better birders. This led me to also question the ID of the second bird shown. I confirmed my field judgment with the help of two books:

"A Manual for the Identification of the Birds of Minnesota and Neighboring States" by T.S. Roberts
"Identification Guide to North American Birds, Volume I" by Peter Pyle

The first book contains a dichotomous key for all the various species. This is a fancy way of saying that you choose either answer A or answer B to a series of questions until you get to the final answer. Pretty neat if you can follow along! To go though the full set of questions for the first photo (yes you can tell all the answers from the picture):

1. Whole head, throat and upper breast bright yellow (YES) or not (NO)?

The NO eliminates adult male Yellow-headed Blackbird
2. Body below dark chestnut (YES) or not (NO)?
The NO eliminates adult male Orchard Oriole
3. Underparts, including throat and breast, wholly black, brown or slate (YES) or not (NO)?
The NO eliminates a whole host of blackbirds
4. Throat bright yellow, pale yellow or buffy (YES) or not (NO)?
The NO eliminates Meadowlarks and a number of female blackbirds and orioles (we will revisit this item shortly)
5. Entire head and throat pure black (YES) or not (NO)?
The NO eliminates the adult male Baltimore Oriole
6. Throat pure black, underparts greenish-yellow (YES) or not (NO)?
The YES arrives at immature male Orchard Oriole!

For the second photo, questions 1, 2 and 3 are answered the same. However, question 4 is answered with a YES which takes us to:
5. Upperparts striped (YES) or not (NO)?
The NO eliminates Meadowlarks
6. Bird uniform grayish-brown (YES) or not (NO)?
The NO eliminates female Brown-headed Cowbird
7. Head buff-colored (YES) or not (NO)?
This eliminates young Yellow-headed Blackbird
8. Entire underparts pale yellow or yellow-orange (YES) or not (NO)?
The YES eliminates female and young male Yellow-Headed Blackbird
9. Entire underparts greenish-yellow (YES) or orange-yellow (NO)?
This is the most difficult to judge, but the YES leads to female Orchard Oriole and the NO leads to female Baltimore Oriole. The choice of the former was confirmed in hand by measurements of wing and tail as detailed in Pyle's book, the Orchard Oriole being a smaller species than Baltimore Oriole.

The question had arisen if the first photo was of a Hooded Oriole. This was a very good possibility, but the some blackish feathers in the bird's crown (as barely seen in the photo) and some chestnut feathers above the bird's tail (not seen in the photo but seen in the field) were present. The plumage clues were once again confirmed by the smaller measurements of wing and tail detailed in Pyle.

Isn't science fun? For even more fun check out the hybrid warblers at Powdermill's web site.

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