Bay-capped Wren-Spinetail is difficult to observe, often remaining hidden within grasses. It sometimes perches between grasses while calling or scanning for predators. When disturbed it either drops into thickets or dashes into a clump of grass with a low flight.
In the Argentinean Pampas, Bay-capped Wren-Spinetails defend a small area around a clump of grasses where the nest is built. Nests can be as close as 15 m from each other and clumped in certain areas, giving the impression of a loose colony. Adults can fly long distances to forage in fresh water marshes while nesting in salt water marshes. Aggressive interactions between conspecifics have been observed close to the nest. Although this territorial system has not been described for other Furnariidae which generally breeds in multipurpose terriotries, it may be more frequent in marsh-nesting birds that experience a tradeoff between safe nesting sites and food rich areas. In some species of marsh-nesting reed warblers (Acrocephalidae) breeding sites are separated from foraging sites, as dense vegetation offers safe nesting places while areas with open vegetation are richer in food (Leisler and Schulze-Hagen 2012). A similar trade-off may force the Bay-capped Wren-Spinetail to forage far away from the nesting site.
The social mating system of Bay-capped Wren-Spinetail is monogamy with biparental care; both sexes build the nest, incubate the eggs, brood and feed the nestlings (Llambías et al. 2009). The genetic mating system, parental care of fledglings and nest defense against predators remains to be described.
Wren-spinetails exhibit a subtle sexual dimorphism. Males have a slightly more robust bill, a longer rufous cap and wings when compared to females (Cardoni et al. 2009). These differences can be the consequence of sexual selection (inter or intra-sexual competition) or natural selection (reducing competition between the sexes) or both. To test these hypotheses it will be necessary to assess the genetic mating system, courtship and territorial behavior and foraging strategies. UV reflectance has not been evaluated to date and may provide additional information on sexual dimorphism.
Social and interspecific behavior
No reports on social or antagonist interactions between Bay-capped Wren-Spinetails and other species been reported. Usually forages alone. Llambías et al. (2009) observed no cases of brood parasitism by Shiny Cowbird (Molothrus bonariensis) on 42 nests monitored during four breeding seasons in the Argentinean Pampas.
There are no reports of predation on adults. In the Argentinean pampas, most of nest failures are caused by nest predation; however, nest survival is relative high (0.51) when compared to other sympatric marsh-nesting birds (Llambías et al. 2009).