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

ABSTRACT 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 Iberá Seedeater (Sporophila iberaensis) is a newly described species in the Iberá Wetlands in Argentina whose biology is still poorly understood. The endangered species inhabits grasslands but has only been reported to breed in the Iberá 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 Iberá Seedeater and the main parameters that influence nest survival and nest-site selection. We conducted nest searches and banded adults and nestlings in Iberá National Park during the breeding seasons of 2016–2018. The breeding season of the Iberá 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 Iberá 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 Iberá Seedeater. LAY SUMMARY Capuchino seedeaters are austral migrants strongly associated with subtropical grasslands, which are among the ecosystems that suffer the highest rates of habitat conversion. The Iberá Seedeater is a recently described, endangered species that has a poorly understood biology, restricted breeding range, and faces unknown threats. We located and monitored nests, banded adults and nestlings, and took vegetation measurements to assess nest-site selection between 2016 and 2018 in Iberá National Park. Breeding site fidelity was male-biased, and nest survival was relatively low, decreased as time of breeding advanced, and differed among nest substrates. Breeding occurred exclusively in flooded lowland grasslands, suggesting that the species could be sensitive to changes in vegetation structure and water levels.