Timing of breeding as a determinant of nest success of the vulnerable Chestnut Seedeater (Sporophila cinnamomea) in grasslands of southern South America

The breeding traits of Sporophila seedeaters have been relatively well studied in recent years; nevertheless, a group of ten species in the genus, known as southern capuchinos, remain understudied. That is the case with Chestnut Seedeater (Sporophila cinnamomea), a species vulnerable to extinction, which breeds in the grasslands of southeast South America and, after reproduction, migrates towards the Cerrado region in central Brazil. Here, we investigated breeding ecology and calculated average clutch size, productivity, the sex ratio of nestlings, and estimated nest success. Then we tested (1) whether there is a relationship between the number of active nests and environmental variables, (2) whether the nestling sex ratio deviates from the 1:1 ratio, (3) whether clutch size varies between breeding seasons, and (4) whether the nest success is related to starting date, nest age, plant support, nest height from the ground, and clutch size. During two breeding seasons (October-March 2018-2020), we monitored 98 nests. We generated survival models with five interacting covariates to assess the survival of the nests. We recorded the entire breeding period for Chestnut Seedeater, which was estimated to be 4.6 months, similar to other migratory seedeaters. Clutch size did not differ between breeding seasons. The sex ratio of nestlings was not significantly different from the 1:1 ratio. Nest success was 31%, and predation was the leading cause of unsuccessful nests (83%). The daily survival rate was 0.95 +/- 0.01. The main predictor of nest survival was the covariate starting date. These findings, added to other aspects of the species' natural history described here, may help illuminate the ecology and behavior of Chestnut Seedeater and other southern endangered capuchinos, and grassland-dependent species of South America.