Wiley, R. H., and A. Cruz.   1980.   The Jamaican blackbird: a "natural experiment" in socioecology.   In M. K. Hecht, W. C. Steere, and B. Wallace (eds.) Evolutionary Biology, Vol. 13. Plenum Press, New York.   Pp. 261-293.


SUMMARY

From current hypotheses of relationships between the ecology and social behavior of birds and from information in the literature that the Jamaican blackbird (Nesopsar nigerrimus ) feeds on insects in montane forests, we predicted that this species should have large, exclusive territories, monogamy with dual parental care, and advertising songs consisting of frequency-modulated tones, all in contrast to blackbirds in the genus Agelaius, the closest phylogenetic relatives of Nesopsar.

During studies in montane forest in Jamaica, we documented that Jamaican blackbirds exploit sparse, evenly distributed insects in the canopy of montane forest. Seventy-seven percent of feeding stations were epiphytic bromeliads, in which the blackbirds probed for insects. The density of canopy bromeliads averaged 2900/ha and showed no significant small-scale (20 m) or large-scale (200-500 m) variation in density in two adjoining territories of blackbirds. Blackbirds generally moved rapidly between bromeliads while foraging and at intervals flew long distances. Pairs of blackbirds occupied large territories (150-360 m in diameter) with no overlap between adjacent residents' movements.

While the female incubated, her mate spent long periods perched nearby and attacked other species that approached the nest. At two nest, both sexes fed the young approximately equally throughout the nestling period. The overall feeding rate was about eight trips/hr. Parents brought one item at a time, most often dark brown Orthoptera from epiphytic vegetation.

The songs of Jamaican blackbirds consist of buzzy, wide-spectrum sounds, not unlike those typical of Agelaius species, contrary to prediction. As these sounds are not well-adapted for long-range communication in forests, this species employs two unusual strategies for territorial advertisement and long-range communication between mates: spectacular song-flights high above the forest canopy, presumably behavior retained from their marsh- nesting ancestors; and dawn activity that consists of the male flying back and forth on a beat near the center of the territory while repeating short trains of a simple sharp call. Mated birds frequently answer each other's songs; both sexes use the same song patterns.

The exclusive territories and monogamy with dual parental care confirm predictions from socioecological hypotheses for species exploiting dispersed, sparse resources. The unexpected form of territorial advertisement might be adapted only to island environments, like Jamaica, with no major predators on adult birds.

A "natural experiment," like this study of the Jamaican blackbird, lacks certain advantages of manipulative experiments with randomization but, nevertheless, avoids difficulties in purely correlational studies.