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Absolute mindblower, what a bird. Appeared suddenly with a mixed flock low in the trees by the Cottage Garden at 9:32. Spotted and called out incredulously by RZ and TH, with the former getting eyes on the rear end of the bird seconds ahead of the simultaneous earth-shattering realization of the bird’s genus level ID when it provided a clear view of its face and unequivocally delivering the first out-loud invocation of the name Phylloscopus. Quickly vanished and sent us into a panicked, single-minded scramble to secure documentation of its existence. Additional fleeting sightings at around 10:06 and 10:20 in the canopies of the London Plane trees close by, with the bird foraging like a slow kinglet, making methodical hops and occasional acrobatic sallies. After a long, stressful search by many dozens of observers, the warbler was finally relocated back near the original location at 11:48. It subsequently made its way to the shorter trees at the Urban Farm, providing great looks for the assembled horde at relatively close range and fairly low height. The bird repeatedly gave a strong, low buzzy call as it hopped about and made short repositioning flights. It wasn’t originally clear that these vocalizations were coming from the Phylloscopus, sounding superficially similar to the flight calls of a parulid like Chestnut-sided Warbler, albeit deeper and more forceful, but we did finally match the sound to the bird with confidence. Surprisingly, the bird also gave a few bouts of quiet, whispered subsong while it bounced about in the canopy: a monotone, oscillating trill not unlike that of Wilson's Warbler. Audio recordings of the distinctive call, with the subsong faintly audible in between, were fortunately locked down. Our later views of the warbler provided superior photos in addition to this audio confirmation, which allowed us to get a better look at the tertials and overall structure of the bird to rule out the original tentative supposition of Yellow-browed Warbler. Feet fairly bright yellowish with similar yellow at the base of the lower mandible. Overall plumage tones rather dusky and dingy, with thin but obvious pale wingbars. The yellow-tinged supercilium was perhaps the bird’s most striking feature, highlighted nicely by the full dark eyeline running from the base of the bill to the nape. Crown dark without any central striping, tertials and secondaries lacked obvious bright white edging, instead showing subtle yellowish fringes. Consultation with those who know the genus better than us East Coasters quickly confirmed our revised field impressions that the bird likely belonged to the Arctic/Kamchatka Leaf/Japanese Leaf Warbler subgroup. In addition to the ever-so-slightly-higher probability of Arctic showing up in eastern North America (though from which direction, one might wonder), audio recordings and live observations of a single, rich, buzzy call note strongly support this identification and counter indicate the potential confusion species. As mentioned above, this individual was preferentially favoring the canopies of London Plane trees for active foraging, but it periodically investigated the smaller, denser ornamental trees around the Cottage Garden and Urban Farm. Despite being quite active and moving dynamically, the bird was surprisingly furtive and difficult to spot when it was moving around the upper branches. Although its initial appearance occurred a mixed flock of agitated birds, it was not obviously associating directly with any other species. There were periods where the warbler was vocalizing consistently, but it also disappeared for long stretches of time without any definitive visual or audio confirmation. Original short-form field notes by TH, composed in the immediate aftermath of the bird’s discovery and initial disappearance, are included untouched below: “Compact Old World warbler flicking wings as it hopped about, tail longer and more obviously notched than superficially similar Tennessee Warbler. Drab olive-brown above and paler below, with a well-defined and striking off-white eyebrow and a full dark eyeline. Prominent, pale wingbars distinctive” Postgame write-up of salient field marks and field impressions by RZ likewise included unedited below: “Small, rather compact passerine with unmarked olive-brown upperparts and clean whitish underside, with perhaps some very light markings on the sides of the breast. Initial views were only of the back half seen from below, from which the clean white vent, dark and significantly notched tail, and incessant flicking of fairly long wings (in an agitated state) were very evident and most puzzling, presenting vaguely like an oversized kinglet-thing. A low full profile view provided the “penny-drop” moment with two bold whitish wingbars, a stout bill (relative to TEWA at least!) with orange lower mandible, and a bold, highly-contrasting pale supercilium that curved downward behind the auricular. Also contra TEWA, rather than a thin black eyeline, this bird had a thick olivey stripe extending from behind the eye down toward the nape where it joined with the mantle of the same color. Leg/foot color not noted in this initial and highly panicked look, and the bird vanished upon simultaneously trying to get AC/DA on it and trying to get pics – both failied initially.” Script of the initial find, for posterity - Bird: *appears* RZ & TH: “Uhhhh…?!” RZ: “THAT’S A [CENSORED] PHYLLOSCOPUS!!” TH: “THAT’S WHAT I’M LOOKING AT!!” AC: “What does that mean?!” *general chaos and panic* Insanity of the very best kind. An incredible and nearly unprecedented rarity discovered in the company of good friends, and successfully chased by a great many additional friends from further afield. Peak patch birding. DA’s perennially optimistic maxim that “the best Randall’s bird is yet to happen” continues to deliver the goods