Spacing and Timing in the Nesting Ecology of a Tropical Blackbird: Comparison of Populations in Different Environments

Populations of the Yellow—hooded Blackbird Agelaius icterocephalus (Icteridae) in northern South America fall into two patterns, according to the seasonal duration of nesting and the spacing of males' territories and nests. These differences among populations of the same species result from interacting adaptations for exploiting food resources and reducing nestling mortality from predators and brood parasites. All populations breed in marshes, where males defend territories within which they build nests. In moderately seasonal areas, like Trinidad and Surinam, males' territories were 15—50 m in diameter, active nests were 2—10 m apart, and egg laying continued for most of the rainy season. A male often had a succession of females nesting in his territory, but rarely had more than one or two active at any one time. In an extremely seasonal area, the llanos bajos of central Venezuela, conditions were suitable for nesting blackbirds for no more than 2 mo at the end of the rainy season. Blackbirds migrated to this area several months before breeding, males established territories 4—8 m in diameter in discrete colonies, and females laid eggs within a 3—wk period. A male's territory had as many as five nests active at one time. Many small colonies were soon abandoned after massive brood parasitism by Shiny Cowbirds Molothrus bonariensis; a large colony nested successfully. Cowbirds parasitized 33% of nests in this large colony in Venezuela, the same as in populations in Trinidad and Surinam. Male blackbirds chased cowbirds out of their territories and thus restricted opportunities for uninterrupted searching by female cowbirds for blackbird nests. Blackbirds began incubation with the first egg, so a cowbird had to lay her egg within 1 or 2 d to have any chance of success. In Venezuela, cowbirds failed to respond immediately to the sudden availability of blackbird nests in the large, successful colony. Predation accounted for many nest failures, particularly in Trinidad and Surinam, but starvation of some nestlings, especially those hatched last, occurred in the majority of nests that escaped predation. Females feeding young brought only one item to the nest each trip, an indication that food was difficult to find. They increased their rate of feeding and selected larger items as the nestlings grew. Food for nestlings consisted almost entirely of herbivorous insects, primarily Orthoptera, collected in marshes often several hundred metres from the nest and always outside the male's territory. In Venezuela, suitable insects were sparse and widely distributed in marshes. This situation and the lack of cooperation by females in locating food suggest that aggregated nests probably had disadvantages for foraging. Males helped to feed nestlings at one, or rarely two, nests in their territories. Although males contributed less than the females, nests that received male help on >1 d before day 8 of the nestling period had heavier young on day 8 than did nests that received less male help. Tropical marsh—dwelling Agelaius experience greater nest failure from both predation and starvation of nestlings than do temperate marsh—nesting blackbirds. The staggered hatching of nestling Yellow—hooded Blackbirds, as a result of incubation starting with the first egg, leads to brood reduction, which could have advantages for parents when they confront an unpredictable, sparse food supply, but could also result from parent—offspring conflict over the optimal brood size. Nest building by males permits females to spend all of their time on the best feeding areas until immediately preceding egg laying, presumably an adaptation to sparse food resources at a distance from safe nest locations. The seasonal restrictions on breeding in the Venezuelan Ilanos make Yellow—hooded Blackbirds more vulnerable to brood parasitism by cowbirds, except in large, dense, synchronized colonies. Coloniality reduces brood parasitism as a result of the increase in male vigilance permitted by smaller territories. Blackbirds also obtain a 'head start' on cowbirds as a result of synchronized laying. Coloniality possibly also reduces predation by solitary predators that detect nests at close range, such as mammals and snakes.