The Chestnut-capped Blackbird (Agelaius ruficapillus) is widely distributed in South America, where it breeds in a variety of natural and man-made habitats. We studied the nesting of this species during two breeding seasons in rice paddies of Rio Grande do Sul, Brazil, a region where it is considered a pest on rice crops. Chestnut-capped Blackbirds build nests in rice paddies when the plants start to flower; thus, breeding phenology in this habitat is determined by sowing date. Clutch size averaged 2.5 eggs, and we found that clutch size tends to decrease the farther the nests are from the surface of the water. Incubation lasted 12.9 d on average, and hatching was asynchronous. The duration of the nestling period ranged from 12 to 17 d with an average of 13.9 days. Only 7.1% of nests were preyed upon, and none was parasitized by the Shiny Cowbird (Molothrus bonariensis). About 90% of nests produced at least one fledgling. These results suggest that the growth of the Chestnut-capped Blackbird population in Rio Grande do Sul may be due not only to the high availability of food during the nonbreeding season but also to the bird's relatives high breeding success in rice paddies.
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Vol. 76 • No. 4