Formerly it was - and in some works it still is - placed in Limnornis with the curve-billed reedhaunter which lives in the same general region and habitat, and thus shares some adaptations with L. rectirostris. But L. rectirostris seems closer to the typical spinetails than to the curve-billed reedhaunter. The modern treatment thus makes Limnornismonotypic also. As using monotypic genera if not really necessary is eschewed by most taxonomists today, the straight-billed reedhaunter is liable to be placed in Cranioleuca if further studies verify its placement.
Behaviour
As with many of its relatives, rather little is known about its reproductive habits. In the south of Uruguay, a juvenile was observed in mid-January.
Distribution and habitat
This bird is found in north-easternArgentina, south-eastern Brazil and Uruguay. In its range it is essentially limited to marshy areas in the pampas and planalto grasslands. In Brazil, it is found in Rio Grande do Sul and Santa Catarina States. In Argentina, it occurs in Entre Ríos and the extreme north-east of Buenos Aires Provinces. In Uruguay, it is limited to the erastern and southern part of the country. In recent years, it has been recorded in Cerro Largo, Canelones, Maldonado - where Charles Darwin recorded it in 1833 at Laguna José Ignacio and Laguna del Diario, where it can still be found -, Rocha, San José and Treinta y Tres Departments. From Lavalleja Department, there are only records from 1994, but the bird probably is still found there. This species prefers marshy and swampy localities between sea level and 1,100 m ASL, and also occurs in more wooded habitat if suitable wetlands are nearby. A key feature of prime habitat is an abundant growth of caraguata, such as E. pandanifolium which it utilizes particularly in upland localities. It is sometimes claimed that the two reedhaunters differ in microhabitat preference, occurring sympatrically in the same region but not within the same locality, but this seems to be incorrect.
Status and conservation
The straight-billed reedhaunter may be common in suitable habitat, but globally its numbers are decreasing and is becoming rare due to habitat loss. Significant threats are pollution and the draining of wetlands, particularly for construction on the outskirts of larger towns. Invasive willows as well as plantations of eucalypt and pine are also detrimental to habitat quality. The species occurs in some protected areas, such as Área Protegida Quebrada de los Cuervos in Uruguay's Treinta y Tres Department.
