We studied the breeding biology of the Lesser Grass-Finch (Emberizoides ypiranganus), a bird with a little-known life history, in upland grassland of the Atlantic Forest (Mata Atlântica) Biome in southern Brazil. We collected data for 93 nests over two breeding seasons (2012–2014). We assessed how some covariates influence the daily survival rates and affect nest survival, building models in Program MARK. Breeding begins in early October, peaks in late November, and continues until early March, lasting approximately 150 days. The mean size of breeding territories was 1.1 ha, and territories located in burned areas were smaller than those in unburned areas. Only the females build the small open-cup nests, in clumps of grass (mainly composed of Andropogon lateralis, Schizachyrium tenerum and Sorghastrum setosum) at 36.2 cm above the ground. The clutch size is three (67%) or two eggs (n  =  52), and laying occurs on consecutive days. Incubation is performed by the female and lasts 13.7 days. The mean period of on-bouts was 73.5 min and the mean period of off-bouts was 39 mins. The hatching is synchronized and the hatching rate was 94%. In most nests, both the female and male fed the nestlings (biparental care). The frequency of visits/hr to feed the young did not differ with nestling age, but the length of visits was longer when the nestlings were younger. Nestlings fledged after 9–12 days. The apparent success was 42%, with 39% and 35% breeding success when calculated with the Mayfield estimate and the MARK software program, respectively. Predation was the main cause of failure, affecting 76% of the unsuccessful nests. The best models included time-specific factors (nest age and year) and nest-site features (nest height and habitat) influencing nest survival. Daily survival rates decreased through the nest cycle, and were much lower in the nestling stage. The re-nesting interval after nest failure was 2.7 days, and the mean distance between consecutive nesting attempts was 104.2 m. Up to four attempts (n  =  5) by the same female were observed in the same breeding season. Although little published information is available for this species, the basic reproductive features are very similar to those found in a population in Argentina. In Brazil, the breeding season is slightly shorter, and clutches of two eggs are more common.

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