Biology of the Yellow-shouldered Blackbird–Agelaius on a tropical island

Abstract

The Yellow-shouldered Blackbird (Agelaius xanthomus) nests in mangroves, scattered trees in pastures, and in suburbs. In the most frequently used habitat, mangroves, birds use isolated cays or scattered trees in salt pans. Nests are either open cups placed on branches or are built in cavities.

The breeding season starts with the spring rains in April and May. These rains stimulate new vegetative growth, which may result in an increase in the numbers of insects on which the blackbirds feed. Most young are in the nest during the relatively dry summer period and fledge before the fall rains begin in September or October.

Pairs form 6-10 weeks before egg laying within groups of birds that visit nesting areas of previous years. During the mate acquisition period males display at old nest sites, which the females visit. Males follow and guard females. The extended period of affiliation before mating may be advantageous to the female if it assures male loyalty, as male parental care is important in a species with such widely separated feeding and nesting sites. All males and females studied during a 2-year period were monogamous.

The average clutch is three eggs. The incubation period is 12.5 days, and hatching is asynchronous. Only females incubate and brood, but some males deliver food to their incubating mates. Males guard the nest when the female is away. Both sexes deliver food at the same rate. Delivery rates are like those of North Temperate icterids. The growth pattern of A. xanthomus is the same as that of Red-winged Blackbirds (A. phoeniceus). After fledging, young follow adults for at least 24 days.

Nest success (proportion of nests producing at least one young) was 46% (54 nests). Nests on offshore cays had higher success than those in adjacent mainland areas, because they were parasitized less by Shiny Cowbirds (Molothrus bonariensis). Pairs nesting in cavities fledged about three times as many young per nest as did those nesting in the open. Mortality during the egg stage outweighed that during the nestling stage, mainly because of disruption by cowbirds.

The average distance between nests in colonies was 16 m. Two pairs occasionally nested in the same tree. Nesting aggregations seemed to result from active attraction among birds, rather than common response to limited nest sites. Communal mobbing was a conspicuous behavior of the nesting groups. The space around the nest consistently defended against conspecifics and other species was small, extending 3 m in any direction. Males defended the nest vicinity more often than females.

The species forages mainly in the upper strata of trees, obtaining arthropods by probing and gleaning epiphytes, leaf clusters, and the surfaces of branches and trunks. This use of foraging behaviors more typical of orioles and woodpeckers may reflect the scarcity of competitors in habitats where the blackbirds are common, in combination with lack of suitable ground-foraging sites (marshes) in Puerto Rico. Most food brought to nestlings is gathered up to 2 km from nests, but commuting pairs do not follow each other to scattered foraging sites. The Yellow-shouldered Blackbird feeds on the ground as do other Agelaius, but unlike its marsh-dwelling congeners, when ground-feeding it gathers mostly vegetable material rather than arthropods. When delivering to nests, adults carry more than one item in their beaks. They also regurgitate both plant material and arthropods to young.

The sex ratio is equal, although winter feeding flocks often held significantly larger proportions of one sex. This difference is due to the greater feeding site specialization of females, perhaps related to male dominance at some feeding stations.

Annual survival of adults was 82.4%, and of juveniles 65.5%. No difference was found between the annual survival of males and females. The individuals considered were permanent residents in the study area, although they occupied large, home ranges, with an average distance of 1.5 km between successive recaptures.

The species uses at least 26 distinct displays and vocalizations, a larger repertoire than described for other monogamous icterids. Unlike polygamous icterids, A. xanthomus has few intersexual differences in the quantity or quality of its displays. Each sex has only one unique display, and the remaining displays and vocalizations are shared by both. Structural characteristics of the action patterns and sounds of A. xanthomus make them useful for transmitting information over short distances, in competition with the displays of close neighbors.