Nesting biology of the Hudsons Canastero (Asthenes hudsoni) and the Bearded Tachuri (Polystictus pectoralis), two threatened and poorly known birds of the Pampas grasslands

The accelerated rate of grasslands transformation has led to noticeable population declines of many grassland bird species. Although several species of the Pampas grasslands are of conservation concern, the information on their nesting biology is generally sparse. During three consecutive breeding seasons (2018-2019 to 2020-2021), we studied nesting attempts of two of the most threatened grassland bird species in central-eastern Argentina: the Hudson's Canastero (Asthenes hudsoni) and the Bearded Tachuri (Polystictus pectoralis). We found 16 nests of Hudson's Canastero and seven nests of Bearded Tachuri in natural grasslands. In both species, the modal clutch size was three eggs, and predation was the main cause of nest failure. Incubation and nestling periods lasted 15 and 11.5 days for the Hudson's Canastero and 14.3 and 12 days for the Bearded tachuri. Daily Survival Rates were 0.90 and 0.97, resulting in a cumulative survival of 4% for the Hudson's Canastero and 39% for the Bearded Tachuri. We found that the breeding biology of the two threatened grassland bird species is strongly tied to tall native grasslands and suggest that both species could be sensitive to changes in breeding habitat, particularly in the grassland vegetation structure.