We report the first detailed account of the breeding biology of the Pectoral Sparrow (Arremon taciturnus). We found 15 dome nests, each containing two eggs in a spherical interior chamber. Eggs were variable in color, ranging from immaculate glossy white to white heavily spotted with brown. Incubation patterns were obtained for six nests for time spans that ranged from 1 to 15 days for a total of 28 days across nests. Incubation in all nests was solely by the female, spending an average of 57% (range  =  20–65%) of daylight incubating, leaving the nest an average of 7.4 times per day (range  =  4–7) with an average trip length of 46.4 min (range  =  6–263 min.). Nest temperature averaged 29.2 ± 2.64° C when the female was incubating and decreased to 26.6 ± 2.43° C during incubation recess. Eggs in only two nests hatched and were monitored for 2 and 9 days. The male provided the young with 75% of the food. Nestlings gained an average of 2.53 g per day. Incubation, provisioning behavior, and egg coloration were similar to other species of Arremon; however, nest shape, location, and materials differ among species.