IBRC 32-B-11

Idaho Bird Records Committee
Rarities Report Form

SPECIES: Mourning Warbler
NUMBER OBSERVED: 1 male

REPORTER:
Cliff Weisse
4125 Beaver Springs Rd.
Island Park, ID 83429
cliffandlisa@octobersetters.com

OTHER OBSERVERS: Lisa Weisse, Darren Clark (clarkd@byui.edu), Steve Butterworth (tntbutters@cableone.net), Bill Scheiss (rubybreleigh@yahoo.com)

DATE OF OBSERVATION: 28 May 2011
DATE REPORT PREPARED: 2 June 2011

LOCALITY OF OBSERVATION: Camas NWR, Jefferson County, specifically at the north end of a row of trees that extends north from headquarters along Camas Creek.

HABITAT: Riparian oasis, specific location was flooded Cottonwoods adjacent to Camas Creek.

CONDITIONS:

DID YOU TAKE NOTES?…
Yes, later the same day

DID YOU CONSULT A FIELD GUIDE OR OTHER REFERENCE WORK?
Yes, later the same day
The Sibley Guide to Birds (North America), Peterson Field Guide to Warblers

FIELD ENCOUNTER (written 8 hours after observation):
28 May 2011
Camas NWR, at the north end of the "Dogleg", the row of trees along Camas Creek north of headquarters.
We were trying to confirm a Common Grackle when Lisa noted what appeared to be a MacGillivray's Warbler. We attempted to get good enough looks to see the white eye arcs and confirm the ID of MacGillivray's. The bird was foraging in the grass in and around downed dead trees and branches and in brushy growth at the base of Cottonwood trees. We followed the bird around for about five minutes, during which time we both had several short looks at the eyes but could not see any white. When I first tried to get close to the bird it called once when it flushed and the chip note was striking, harder and richer than MacGillivray's, with which I am familiar. I (Cliff) finally had a good look at the bird at about 20 feet in good light and could see no white eye arcs. At this point the bird crossed to the west side of Camas Creek (which is in runoff and slightly out of its banks) so we couldn't pursue. I (Cliff) walked down to the birdge near headquarters and met Darren Clark and Steve Butterworth. Darren and I went up the west side of the creek to try to find the bird. It crossed back over to the location of the original observation so we walked around and all had several looks at the bird and Darren took photos. We were then joined by Bill Schiess who also got photos. Darren noted the long yellow undertail coverts immediately upon catching a glimpse of the bird and we all had good looks at the eyes and no one saw any white. We eventually lost track of the bird and left the area.

DESCRIPTION (written 8 hours after observation): Overall olive colored warbler with black tail, yellow underparts, and gray hood, recalling the common and familiar MacGillivray's Warbler. The bill had a flesh colored base and dark culmen but I did not note the exact pattern. The head was gray and separated from the yellow breast by a black bib, the extent of which I did not note. The lores were blackish and the entire face surrounding the eye looked dark. It struck me as darker faced than MacGillivray's, which I had not yet seen this year at the time we encountered this bird. I had several looks at the bird in good light and could see no white around the eyes. However at least one of Darren's photos shows a small light grayish area below the eye that isn't bright white. (I had several brief looks at MacGillivray's Warblers later in the morning in considerably worse lighting conditions and at greater distance and I easily saw the white eye arcs immediately. One of these was a female, making the arcs more difficult to see against the paler background color of the head) Legs were pink. Underparts were bright yellow from the hood to and including the undertail coverts. Tail was black.
ADDED JUNE 2:  All photos show partial whitish eye arcs.  They are thinner than the eye arcs of MacGillivray's and very short, slightly longer below than above the eye.  This pattern is consistent with Mourning Warbler.  Some observers eventually saw the whitish eye arcs but I was never able to get a good enough look to see them.

BEHAVIOR(written 8 hours after observation):
The bird mostly foraged in grass very low to the ground. When flushed, or when moving on its own without being approached, it consistently landed at about eye level or slightly higher, then dropped quickly into the grass below. It didn't seem to move around as much as MacGillivray's typically does. It would stay close to where it entered the grass for quite a while, often not showing itself again until we approached the location. It also seemed to want to stay in a small area, a flooded strip perhaps 40 yards long with trees and grass on the ground and several downed dead trees and branches, often circling back to that area even when pursued. The bird was mostly silent but called once during the original encounter, and several times (2 or 3) during the second encounter after it crossed back to the east side of the creek. The call note was sharp and rich, strikingly different than the flat thin call note of MacGillivray's (two pebbles clicking together). We listened to a recording of Mourning's chip note within minutes of hearing it call the second time and the call matched. We were also fortunate to hear a MacGillivray's Warbler calling a minute or two before the series of call notes made by the Mourning Warbler for comparison and the difference was obvious to all four of us (Lisa, Darren, Steve and I).

HOW AND WHEN DID YOU POSITIVELY IDENTIFY THE BIRD AND WHAT CLINCHED THE ID FOR YOU? Lisa saw the bird first and after it moved out of sight she said it was a MacGillivray's Warbler but that she hadn't seen the eye arcs. I tried to approach where it landed and saw the bird moving around in brush. I had glimpses at the face and did not detect eye arcs but I wasn't sure they were absent. The bird then flushed and made one chip note that I immediately realized was very different from the chip note of MacGillivray's which made me realize it was a Mourning Warbler. I had never heard Mourning Warbler in the field but I knew they had a distinctive chip note. We then followed the bird around about a 40 yard long strip of trees until I got a good look at the face when it held still for about five seconds at close range (20 feet+/-) and the lack of visible eye arcs confirmed my suspicion.

HOW DID YOU ELIMINATE SIMILAR SPECIES AND WHAT WERE THEY? Mourning is very similar to MacGillivray's but not easily confused with any other species. The skulking behavior, olive upperparts, yellow underparts and gray hood are not like any other species except perhaps Connecticut, which has a shorter tail, complete white eye ring, and walks rather than hops and this bird did not walk (I saw it hopping along fallen tree trunks and through thick brushy growth several times). MacGillivray's Warbler is easy to eliminate by the absence of distinct wide white eye arcs and the distinctive chip note of Mourning Warbler. MacGillivray's has a longer tail/shorter undertail coverts and less extensive black bib. Hybrids between Mourning Warbler and MacGillivray's Warbler are a problem though.

MOURNING X MACGILLIVRAY'S WARBLER HYBRIDS
Recent studies have confirmed fairly extensive hybridization between these species in a narrow overlap zone in northeast British Columbia. Plumage, structure and song in this overlap zone are a mix of traits typical of Mourning and MacGillivray's Warblers to the extent that they did not predict DNA findings. IE, hybrids could look like pure members of either species or display a combination of traits shown by the parent species. No evidence that the influence of hybridization extends beyond the contact zone was found. Call note was not described but songs are blended in the overpal zone. So the bottom line is that you can't say with 100% certainty that any Mourning Warbler, or MacGillivray's Warbler, is not a hybrid. Lacking the ability to assess DNA I feel we have no choice but to examine appearance and vocalizations and assign individuals to species based on the evidence available, much as we do for large gull species that are known to hybridize. When all field marks are well within range for a "pure" species and nothing suggests hybridization we accept the identification despite the remote possibility that hybridization may have occurred at some point in the bird's background, and in the case of gulls like GWGU hybridization probably occurred at some point in the background of all individuals.
EYE ARCS: Presence or absence of bold white eye arcs are the easiest field mark with which to separate these species. MacGillivray's rarely if ever lacks eye arcs and Mourning usually lacks eye arcs. However partial eye arcs are not uncommon in many populations of Mourning Warbler throughout their range. A sample of Mourning Warblers from Saskatchewan included 35% with eye arcs and as far east as NY up to 19% showed eye arcs. The presence of eye arcs in such a high percentage of individuals, and so far removed from the contact zone, suggests that eye arcs are typical for Mourning Warbler and NOT the result of hybridization, especially since no evidence of hybrid traits was found outside the contact zone.  The eye arcs on the 28 May individual at Camas NWR are very short and narrow and are well within range for Mourning Warbler.
Here's a link to a photo of a Mourning Warbler in Ontario with far more extensive eye arcs (this individual also shows dark lores):
http://www.ofo.ca/photoalbums/checklist/Warblers/Mourning%20Warbler/slides/MourningWarblerke.html
DARK LORES: Dark lores are typical for MacGillivray's but Mourning is said to have gray lores rather than dark. This too is quite variable and dark lores were found to be much more common than eye arcs in Mourning Warblers range wide, up to 54 percent in some samples and many with 1/3 or more displaying dark lores. As a result I would suggest that dark lores are typical for Mourning Warbler.

In summary I feel the combination of field marks shown by this individual are typical for Mourning Warbler and eliminate MacGillivray's as well as hybrids between these species. Not even the eye arcs represent an extreme of variation. While it's never possible to completely rule out hybridization, especially after the first generation, nothing about this bird suggests it's a hybrid. The shorter tail, longer undertail coverts, dark face, extensive black bib, lack of bold white eye arcs, limited narrow whitish "eye arcs", and chip note identical to that of Mourning and not at all like MacGillivray's are all consistent with Mourning Warbler so I feel the bird should be endorsed as such despite the slight possibility of hybridization. In addition I would suggest that there are substantially more Mourning Warblers displaying this suite of field marks than there are hybrids in the narrow overlap zone which to me makes it far more likely that it's a "pure" Mourning than a hybrid that shows field marks only consistent with Mourning Warbler.  
References - I used these articles to prepare the hybrid section of the report:
 http://www.zoology.ubc.ca/~irwin/PDFs/Irwinetal2009%20MacMourning.pdf - Extensive hybridization in a contact zone between MacGillivray’s warblers Oporornis tolmiei and mourning warblers O. philadelphia detected using molecular and morphological analyses
Darren E. Irwin, Alan Brelsford, David P. L. Toews, Christie MacDonald and Mark Phinney, J. Avian Biol. 40: 539!552, 2009
http://elibrary.unm.edu/sora/Condor/files/issues/v094n01/p0198-p0209.pdf - PLUMAGE AND SIZE VARIATION IN THE MOURNING WARBLER’
JAY PITOCCHELLI / The Condor 94: 198-209

EXPERIENCE WITH THIS SPECIES: I've only seen one Mourning Warbler previously but I see numerous MacGillivray's annually and am quite familiar with its chip note and appearance.

GENERAL BIRDING EXPERIENCE: 18 years

WERE PHOTOS, VIDEO, AND/OR AUDIO OBTAINED BY YOU? No, but I have included a series of four photos taken by Bill Scheiss during the time I observed this bird.

These photo taken by Bill Scheiss on 28 May 2011 at Camas NWR.