Another helpful element from the original report is the bracketing of size offered by the observer. "Bigger than a junco and smaller than a robin," would generally exclude Northern Mockingbird and tells me a couple things about Wendy's observation and the way she approached her observation. First and foremost, she recognized at the time that she was looking at unfamiliar bird (very important in my opinion) and made an effort to frame its size and notice specifics about the bird as she was looking at it. She makes not mention of gray, which is the predominant color of Northern Mockingbird, describing the bird as all black and white. Too often folks see a bird in the field and don't pay much attention to it then go home and decide that it had to be something rare after the fact. The fact that she has birded many times in Arizona suggests to me that she might regularly travel and bird outside of Montana. Northern Mockingbirds are much more common across most of America than they are along the southern Oregon coast, including Arizona where they are dirt common, so I suspect that they are (like Black Phoebe) familiar to her.
The description doesn't cover every feather, but what she does describe is totally consistent with "Black-backed" White Wagtail and aside from perhaps a partially leucistic blackbird doesn't match up with anything else.
From: <obol-bounce...> <obol-bounce...> on behalf of Tim Rodenkirk <timrodenkirk...>
Sent: Sunday, October 8, 2017 6:06 PM
Subject: [obol] Fwd: Wagtail?
From: Wendy Dodson <wdodson...>
Date: October 7, 2017 at 1:24:30 PM MST
I'm a birder from Montana who recently spent 10 days at Cape Blanco State Park. As we were leaving the park on the morning of 9/26, my husband and I encountered a black and white bird flying across the road from north to south just west of the Pioneer Cemetery. I knew as soon as I saw it that it was a bird I did not know. I hit the brakes and backed up to try and relocate it for a better view. It was bigger than a junco, smaller than a robin, not a woodpecker or a shorebird. It was only black and white with a longish tail and white on the outside edges of the tail, similar to a junco. I consulted the only bird book I had with me, a Peterson's Guide to Western Birds as well as iBird Pro and Audubon Birds on my electronic device. The only bird that matched what both my husband and I saw was a Black-Backed Wagtail. We looked every day as we went by the fields and drove along the road to the horse camp at Cape Blanco hoping to see it but never did relocate it. Just thought I would let you know about the possibility of a rare bird in your area.