Low reproductive success of the endangered Ibera Seedeater in its only known breeding site, the Ibera Wetlands, Argentina

Subtropical grasslands are highly susceptible to habitat conversion and number among South America's most threatened ecosystems. The grasslands of northeastern Argentina have been identified as a priority conservation area for threatened capuchino seedeaters because they constitute the main breeding area of these migratory birds. The Ibera Seedeater (Sporophila iberaensis) is a newly described species in the Ibera Wetlands in Argentina whose biology is still poorly understood. The endangered species inhabits grasslands but has only been reported to breed in the Ibera Wetlands ecoregion of northeastern Argentina. To explore the species' association with grassland vegetation, we studied the breeding biology (clutch size, hatching success, and fledgling production) of the Ibera Seedeater and the main parameters that influence nest survival and nest-site selection. We conducted nest searches and banded adults and nestlings in Ibera National Park during the breeding seasons of 2016-2018. The breeding season of the Ibera Seedeater was highly synchronous and the cumulative probability of nest survival was 0.16. The daily nest survival rate decreased as the breeding season advanced, survival was lower for nests supported by Rhynchospora corymbosa than Paspalum durifolium, the two main nest substrates, and the main causes of nest failure were nest predation and strong winds. Additionally, the population exhibited male-biased site fidelity and a low female return rate. In contrast to other capuchinos, whose breeding biology is associated with upland grasslands, the Ibera Seedeater nested exclusively in flooded lowland grasslands on marsh plants. Thus, effective lowland grassland management is key to maintain the vegetation structure required for reproduction in the Ibera Seedeater.