Bay-capped Wren-Spinetail is a small furnariid with a long, graduated (spiky) tail, rufous cap, and streaked mantle and nape. It is a habitat specialist of fresh, brackish and salt marshes of southern South America. The wren-spinetail can be fairly common in some areas but is shy, and often is difficult to observe and easy to overlook. The wren-spinetail sometimes perches with each leg clinging to a different blade of grass, a behavior similar to that of Sedge Wren (Cistothorus platensis).
Bay-capped Wren-Spinetail is difficult to be confused with other species that inhabit the same area, as its behavior, morphology and call structure all are quite distinctive.
Bay-capped Wren-Spinetails and Sedge Wrens (Cistothorus platensis) have been observed breeding in the same area and in flight may look similar. When flushed, both species typically fly short distances between grasses; the flight of the wren-spinetail, however, is more erratic while wren’s flight is direct. When perched on grasses and at close inspection they are easily distinguished, the wren-spinetail having a rufous crown and a longer, spiky tail. Songs and calls from both species differ dramatically, the song of Sedge Wrens being more diverse and complex. Hudson's Canastero (Asthenes hudsoni) also may be confused with wren-spinetails, but is larger and lacks the rufous crown. Sulphur-throated Spinetail (Cranioleuca sulphurifera) is similar in size, but lacks the rufous crown and the black streaked nape and mantle. Other similar canasteros (e.g., Scribble-tailed Canastero Asthenes maculicauda) do not overlap in distribution with Bay-capped Wren-Spinetail.
The following description is based on Ridgely and Tudor (1994) and Remsen (2003).
Above mostly sandy brown, boldly streaked with black on nape and mantle. Rufous crown, with whitish superciliary line. Wings with cinnamon area at base of flight feathers. Long strongly graduated tail with pointed feathers, mostly brown. Below whitish, tinged dull buff flanks and sides.
Iris: dull white, whitish, pale bluish
Bill: maxilla blackish; mandible whitish to pinkish with dark tip
Tarsus and toes: light tan
Bare parts color data from Belton (1984), Ridgely and Tudor (1994), and Remsen (2003).
Total length: 14-14.5 cm (Ridgely and Tudor 1994)
Mass: mean 10.47 g (n = 9, ± 0.15 g; Llambías, unpublished data, Buenos Aires Province, Argentina)