Friday, May 20, 2011

Philadelphia vs. Warbling Vireo

During our bike trip on 18 May down the Cannon River Trail in Goodhue Co., Minnesota, Erika and I photographed a Philadelphia Vireo (first photograph).  We also heard Warbling Vireos.  The two species can be difficult to identify.

1. They have different songs (samples courtesy of Thayer Birding Software).  The Warbling Vireo sings "a single languid warble unlike the broken phrasiology of the other vireos" (Peterson).  The Philadelphia Vireo sounds like a slow, high-pitched Red-eyed Vireo. Listen for the insertion of three-quick notes within the song.  Since the Warbling Vireo is relatively common and breeds in Rice County, I often hear them.  Unfortunately I have only heard a singing Philadelphia Vireo once. (Philadelphia Vireos breed in forests across central and northern Canada, barely reaching the northern-most United States.)

2. Notice the lores of the birds in these photos.  The lore is the dark line that connects the bird's eye to the its bill.  The top two photos are Philadelphia Vireos.  Note that the lores are relatively dark. The result is a dark line through the eye.  Now look at the third photo.  This Warbling Vireo has much paler lores, giving its face an almost whitish look.

3. The crown of a Philadelphia Vireo is darker than that of a Warbling.

4. Philadelphia Vireos have yellowish throats.  Warbling Vireos have white throats. Be careful, however, of Philadelphia Vireos with relatively pale throats. (The second photograph was taken of a Philadelphia Vireo banded and released in Northfield, Minnesota.)
5. Both Vireos vary in the amount to yellowish on their flanks.  This variation can cause  confusion among birders, who might identify a yellow-sided Warbling Vireo as a Philadelphia, while correctly identifying a paler Warbling Vireo, like the bird in my third photo (a bird banded at Sand Lake National Wildlife Refuge in South Dakota).

6. Sibley notes that Warbling Vireos have a smaller bill.  This statement is a bit confusing.  I think he is comparing western Warbling Vireos to those east of the Rockies.  (The western birds may actually be a distinct species--eastern birds like those in Minnesota sing a slightly different song than do western Warbling Vireos.)  I have always thought that Philadelphia Vireo bills are smaller and definitely less obviously hooked than those of Warbling Vireos.  This field mark is good with a bird in the hand, but may be of less use in the field.

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