On October 01, 2011 Marcin Kotjka (MK) located a darkish Wood-Pewee (henceforth WP) at "the Willows" - a tiny scrap of willows, reeds and bushes around a small freshwater pond amidst coastal grass/scattered low scrub, 50 yards east of Highway 361 on Mustang Island, Nueces County Texas, about 4 or 5 miles south of Port Aransas. MK noted in addition to the overall duskiness, a very dark mandible*, and took a series of photographs; CLICK HERE to view images of this bird. The next morning Mel Cooksey (MC) located a dark, dark-mandibled* WP at this location; only this time it was vocalizing - sounding to MC like a Western WP (henceforth WWP). MK was also present and obtained some photos of this bird, while MC obtained video footage which included audio of some of its vocalizations; CLICK HERE to view images/spectrograms and hear audio clips of this bird.
Eastern WP (EWP) is a common migrant throughout the Texas coast, breeding from the east Texas Pineywoods west through north-central Texas and throughout the Edwards Plateau in the center of the state. WWP is a summer resident in the higher mountains of west Texas, and a scarce migrant west of the Pecos River and in the western part of the Texas Panhandle. Despite breeding as far east as south-central Manitoba, WWP is extremely rarely-documented in central and eastern Texas - largely due to ID problems associated with the expected EWP.
Subsequent examination of MK's photographs show that the bird he photographed on October 01 is not the same as the one he photographed on October 02 (although MK feels that the Oct 02 bird was also present on Oct 01 without him realizing at the time that two birds were present). Both birds are dusky (but in slightly different ways) and both have almost all-dark mandibles* (but with different patterns). The location was thoroughly searched on at least two subsequent days without any sign of this type of WP.
In the early afternoon of October 11 Martin Reid (MR) visited the Willows and quickly found a dark WP with an extensively dark mandible*. MR obtained a series of photographs, but it remained silent. MR returned to the location in the evening, again finding the WP quickly - but this time it was vocalizing. MR heard the bird make 4 or 5 vocalizations during about ten minutes at the spot; they sounded to him like WWP-type notes. MR obtained video footage of the bird and twice captured audio of the bird vocalizing - but only when the bird was not in the video frame (it tended to call upon landing); CLICK HERE to view images/spectrograms and hear audio clips of this bird. During the intervening period between visits to the Willows, MR had visited other Port Aransas sites, including the Port A. Birding Center. At this location he found another darkish WP with an extensively dark mandible*, and obtained photos; CLICK HERE to view images of this bird. MR did not see any typical EWPs on October 11, however he did hear the typical two-note vocalization of EWP coming from a location near to but not at the Willows.
Thus there seemed to have been four dusky, "dark-mandibled"* WPs (two calling like WWP) at Port Aransas across a period of eleven days. Note also that there was a late-September record of a calling WWP from coastal Louisiana (Justin Bosler).
UPDATE, October 16, 2011: Today Sheridan Coffey and I located another darkish, dark-mandibled WP at Port Aransas. It was slient and ignored both WWP and EWP playback... CLICK HERE to view images of this bird.
* Pyle, in "ID Guide to N.A. Birds, Part 1" provides an illustration of WP mandible patterns from below (Figure 142 on page 215): they show decreasing amounts of distal duskiness from left to right across four examples. The text states that the 3 leftmost patterns exhibit the range for WWP and the 3 rightmost patterns exhibit the range for EWP. Put another way, the middle two patterns can be found in both species, but the leftmost pattern (which is more than 3/4 dusky) is outside the range of EWP, while the rightmost pattern (which is pale except for a very small dusky tip) is outside the range of WWP. As with all attempts to limit the range of a morphological feature, I expect that extremely rarely, an EWP could have duskiness approaching or matching the illustrated leftmost pattern in Pyle's Figure 142 - but keep in mind this is meant to show the darkest limit for WWP, so an EWP like this should, in theory, be quite exceptional.