If only one could chose his/her location at the time of discovery of rarities... This time I found myself in Petoskey MI when I received the frantic call. Adam Byrne had discovered a White Wagtail almost 5 hours' drive to the south at Pointe Mouillee. And thus my detour home began; the 3 hour drive home turned into an epic 8.5 hour drive with a 4 hour stop for the bird. Fortunately, despite the miserable weather (at least 20-30 degrees colder than the shorts and tee-shirt weather of Petoskey yesterday morning), upon my arrival at Cell 3 around 4:50 PM the bird was immediately visible, though from very long range, perhaps 400m. The bird turned out to be one of the most skittish passerines I have ever encountered, flushing as a result of being approached by 'predators' over 200m away. The Killdeers weren't nearly as skittish as this bird was; at one point I had a Killdeer halfway between me and the Wagtail stay put as the Wagtail flushed. This, in combination with the wind, made for very difficult digiscoping conditions, but I was able to get these relatively decent shots as I gingerly approached the bird with the final group of birders late in the evening to probably about 150-200m at the closest.
Here are my two best video clips of the bird, taken when the wind had died down slightly.
I don't have time for a completely detailed post about the bird's identity, but after looking through "Pipits and Wagtails" by Alstrom and Mild, it is clear that this bird is either the ocularis race ('Swinhoe's' White Wagtail) or lugens race ('Black-backed Wagtail') of White Wagtail. Both forms are from e. Siberia, so it is interesting to note that this bird didn't cross the Atlantic to get here, but the Pacific and most of North America!
Had these former species (ie. White and Black-backed Wagtails) not been lumped many years ago, this bird would possibly have been an identification conundrum which could not be resolved ("Black-backed/White Wagtail"). Fortunately for us now, it is merely an issue of subspecies. Relevant traits for separating these two are complex and beyond my level of knowledge (this was a life bird for me- the first wagtail of any species I've seen). But in my quick reading of Alstrom and Mild at 2AM last night, rump and uppertail covert pattern appears to be an important trait: lugens showing a blackish lower rump and uppertail covert area and ocularis showing gray concolorous with the mantle extending all the way down to the base of the tail. My final photo in the series above seems to establish that the Mouillee bird possesses the latter condition, but there appears to be at least one caveat (as for most of the important traits!) allowing for a small percentage of lugens to show this as well. So, for the time being I am only willing to call this an ocluaris/lugens White Wagtail, and would love to have the input of birders more experienced than I.
Details of median coverts were simply not viewable due to the long distance of observation and skittish behavior, though I can confidently say there was a substantial amount of white in this tract. Greater coverts were thickly edged in white and had a substantial amount of dark on the inner webs, with perhaps some of this dark bleeding laterally into the outer web; at rest this tract appeared all or mostly white, like the white wing patch of male Bullock's Orioles in alternate plumage. We also judged the eyeline to be relatively even width in front of and behind the eye, a trait which may be suggestive of ocularis. In any event, please leave your comments on subspecific ID if you have anything to offer!