Bay-capped Wren-Spinetail

The bay-capped wren-spinetail is 13 to 14.5 cm (5.1 to 5.7 in) long and weighs about 10.5 g (0.37 oz). The sexes have the same plumage. Adults have a whitish supercilium on an otherwise sandy brown face. Their crown is rufous. Their upperparts are sandy brown to olive-brown with bold black streaks on the nape and upper back. Their wings are brown with cinnamon at the base of the flight feathers. Their tail is mostly brown and graduated with pointed feathers; the central pair have some black. Their underparts are whitish with a buff or cinnamon tinge on the breast, sides, and flanks. Their iris is dull white, whitish, or pale bluish, their maxilla blackish, their mandible whitish to pinkish with a dark tip, and their legs and feet light tan. There are some minor differences in size and darkness of plumage, but little genetic divergence, between coastal and inland populations.

The bay-capped wren-spinetail is found from extreme southern Brazil south through Uruguay into northern Argentina as far south as Río Negro Province. It also has occurred as a non-breeder in Paraguay. It inhabits reedbeds and sedges in freshwater, brackish, and saltwater marshes. In elevation it ranges from sea level to about 900 m (3,000 ft).

The bay-capped wren-spinetail feeds on invertebrates, mostly insects and spiders. It usually forages alone, gleaning from marsh grasses and sedges. There is some evidence that pairs that nest in salt marshes preferentially feed in freshwater marshes.