August 2004 Field Journal 8.10 Louisiana Waterthrush Seiurus motacilla
This afternoon, Calvin Lou and I drove down to Oak Grove Park in Los Gatos. Upon arriving there, we found a number of birders including Dave DeSante, Keith Hanson, Jim White, Dave Weber, Mike Feighner and others. After searching for about 30 minutes or so, I managed to refind the LOUISIANA WATERTHRUSH on the far bank of the stream, more or less directly opposite the parking lot and not far from the Blossom Hill Road overpass. The following description is a composite based on a sequence of views over roughly 1/2 hour, none of which exceeded more than 10-15 seconds.Description:
A chunky, warbler-like bird that frankly gave me the impression of a Spotted Sandpiper. The bird was almost exclusively confined to the immediate banks of the stream, and moved its tail non-stop while appearing to forage at the edge of the stream. The motion of the tail seemed to mostly to entirely up and down vertically. The bird definitely preferred the cover of the underbrush; when it was in the open and perceived the horde of twitchers trying to view it, it often moved up or downstream, or sought cover in denser underbrush. At one point, it was motionless in a thicket of bamboo at water's edge. Lighting in the riparian varied from local spots of bright illumination to the more typical shade of the riparian with some spots of almost impenetrable black.The upperparts of the bird were uniformly olive green; there was no suggestion of wingbars on either the lower or median coverts that I can recall. The tail appeared to be short with the undertail coverts fairly long. The undertail was not observed. The supercilium was quite remarkable. In addition to being clean white, it started at the base of the bill and continued in what appeared to be a continuously increasing wedge over the eye all the way to the nape, where it terminated abruptly, giving the impression of a (thin) triangle overlaid on the curvature of the skull of the bird. The base of the triangle oriented along the side of the nape, forming an acute angle with the top of the supercilium and an obtuse angle with the bottom of the supercilium. The auriculars appeared to be the same color as the upperparts and the lower portion of the supercilium was bordered by a black eyeline.
The bill was fairly long and appeared to be fairly straight. The underparts were clean white other than described below. The throat was hard to see, but appeared to be unmarked. The upper breast was streaked in black, fairly well-defined and continuing along the sides of the breast. The flanks of the bird were marked in color difficult to describe; perhaps a rusty buff would be an apt description, and they contrasted markedly with the the rest of the underparts though the transition to white was gradual rather than abrupt. The legs were clearly pink rather than flesh colored, but did not appear to be bright pink as others have suggested. The bird was not heard to vocalize.
Analysis:
We can safely assume that this is bird is a passerine by the combination of green upperparts, contrast of the white underparts and streaking on the breast. No species of Vireonidae possesses pronounced streaking on its breast. Of the Phylloscopus warblers recorded in North America outside of Alaska, neither Arctic nor Dusky Warblers possess the streaking either. Frankly, we're left with Wood-Warblers (Parulidae and Peucedramidae), the latter of which having a sole representative in North America, the Olive Warbler, which also lacks the pronounced breast streaking observed on this individual. In Parulidae, only birds in the genus Seiurus have a combination of green backs, pale underparts and pronounced breast streaking.Ovenbird can be easily eliminated by the absence of a supercilium, so we're left with the Waterthrushes, which is where most experienced birders would have jumped directly to. Northern and Louisiana Waterthrush both are regularly occurring species in North America, but the distribution in California is quite a bit different, with only 14 accepted records in California as of November 2000 (1). Thus, virtually all waterthrushes found in CA are likely to be Northern, so identification of Louisiana Waterthrush must be done with great care.
Separation of the two species must be done by a combination of field marks since individual field marks may be equivocal. Northern Waterthrush typically has a a yellowish supercilium tapering behind the eye, chin and throat with distinct, narrow, black streaking and concolorous flanks with the underparts. Louisiana Waterthrush typically has a broad white supercilium extending all the way to the nape, chin and throat usually without streaking and flanks and undertail coverts buff, contrasting with the otherwise white underparts (2). Additionally, leg color in Louisiana Waterthrush may be brighter than Northern, though perhaps in spring only. The coloration of the flanks and the pattern of the supercilium, in combination with the likely clean white throat appear to unambiguously point towards LOUISIANA WATERTHRUSH.
Molt assessment was not attempted due to the difficulty of observation; sexing of most Waterthrushes in the field is not possible.
An excellent photo may be found here.
1) Erickson, et al. Rare Birds of California, in press
2) Pyle, Peter, Identification Guide to North American Birds, Part I, Columbidae to Ploceidae, 1997, Slate Creek PressHome
You can contact me via email.
