Date of Award

1990

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Robert E. Noble

Abstract

Shiny Cowbirds were primarily located in six major areas of concentration. Of these, all but one was in mesquite woodland. Overall, cowbirds used mesquite woodland almost 75% of the time but use of this habitat varied among weekly periods and was linked to rainfall received during weekly periods prior to the observation. Habitat use of cowbirds and the time they spent eating particular prey items were most correlated with total rainfall received 2-5 weeks prior to the observation (p =.0067 and p =.0149, respectively). Major food items taken by cowbirds following periods of sufficient rainfall were caterpillars, berries and grass seeds. During periods of drought, cowbirds foraged on such food items as the leaves and inflorescences of mesquite, waste corn and other grains associated with agricultural and residential areas. Cowbirds appeared to prefer caterpillars when available. The availability of caterpillars was dependent primarily on the species, the host plant, and the amount of rainfall. Shiny Cowbirds often foraged in mixed-species flocks with Yellow-shouldered Blackbirds and/or Greater Antillean Crackles. When icterid flocks contained at least 50 cowbirds, blackbirds and grackles associated with cowbirds when caterpillars were available as prey (p $<$ 0.001). Main species of caterpillars eaten by cowbirds were larvae of the noctuid moths Mocis latipes and Melipotis ochrodes. Other species of caterpillars were seasonal and available to cowbirds for short periods of time (usually 5 days or less). On average cowbirds foraged, rested and preened, and drank and bathed about 68%, 31%, and 1% of the time, respectively. Nine incidences of allopreening were observed between cowbirds and Yellow-shouldered Blackbirds. Twenty-five incidences of sunning by cowbirds were observed. As with cowbirds, blackbirds and grackles have apparently adapted to seasonal, caterpillar outbreaks in southwestern Puerto Rico. Recent caterpillar outbreaks are probably related to an abundance of new plant hosts associated with habitat changes. Mesquite and associated exotic grasses have replaced much of the original, native savannahs. Although exotic, mesquite is probably compatible with the native ucar and should be protected and managed to benefit the endangered blackbird.

Pages

191

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