Tuesday, September 14, 2010

Mourning Warbler (VCFW)

If you observed this VCFW (VERY Confusing Fall Warbler) in the field, you might be tempted to identify it as a Connecticut Warbler.  I think that there are two reasons why this mystery bird is not a Connecticut Warbler.  The narrow eye ring is not complete.  In the next photograph, note the wide, complete eye ring of a Connecticut Warbler I banded near Dundas last fall. The measurements are wrong for a Connecticut.  According to Pyle's Identification Guide to North American Birds: Connecticut Warbler wing length minus their tail length = 19-27 mm.  The formula for the bird above gives you 7mm.
What about a MacGillivray's Warbler? The wing minus tail formula for MacGillivray's works out to 2-12 mm, within the range of the mystery bird. First, only one documented record exists for this western warbler in Minnesota, a bird that was found in the far western part of the state.  Second, as you can see in the next photo of a MacGillivray's Warbler banded in Aberdeen, South Dakota, even in the fall this warbler's eye ring is broad and clearly broken into upper and lower halves. 
The only remaining candidate for the mystery bird is the Mourning Warbler.  The wing minus tail measurement should be 9-18 mm.  The first bird's 7 mm is a tad low, falling into the MacGillivray's range!  A few other little problems nag me.  In my photos, the MacGillivray's throat is slightly yellowish and the mystery Mourning Warbler's throat is whitish--exactly opposite of field marks cited by Sibley in his Guide to Birds.  Sibley does caution that those field marks are only "usually" present.  The loral area--between the bill and the eye, is blackish on the MacGillivray's Warbler but whitish on the Mourning.  I once thought this difference was significant, but apparently Mourning Warblers can sport dark (as well as gray) lores.  The area above the lores, the supralorals, should be white in a MacGillivary's and yellowish in a Mourning.  The the color of the supralorals are hard to see in both of my photographs--but curiously they are both yellowish (trust me).  Finally, note that hybirds between these warblers have been reported.

The bottom line?  I believe the relatively narrow, broken, white eye ring of the mystery bird indicates that it is a Mourning Warbler and that the wing and tail measurements indicate that it is definitely not a Connecticut.  The eye ring is the wrong shape, and the supralorals the wrong color, for a MacGillivray's Warbler. Finally, I hope I have convinced you that these three species certainly are confusing!


  1. I feel your pain. Some of these fall birds seem to be even harder in the hand! The only other characteristic I would think to look at is the length of the legs. I don't know if this is valid but Connecticuts seem "long-legged" to me. Don't have any measurements to back it up though.

    Good catch!

  2. Based on range, eye-ring, breast - I would have to agree! Confusing - but good call here.