Patterns of Shiny Cowbird Parasitism in St. Lucia and Southwestern Puerto Rico

Abstract

The density of Shiny Cowbirds (Molothrus bonariensis) in lowland St. Lucia and Martinique is about six individuals/km2 compared to 49/2 in lowland southwestern Puerto Rico. Their distribution in the Antilles may be related to agriculture that provides food for adults, as much as to availability of suitable hosts. In St. Lucia, four of eight potential host species were parasitized. Black-whiskered Vireos (Vireo altiloquus) and Yellow Warblers (Dendroica petechia) accounted for 90% of all parasitized nests. Patterns of parasitism did not match host availability in either SW Puerto Rico or St. Lucia. In St. Lucia only Gray Kingbirds (Tryannus dominicensis) rejected > 75% of experimental cowbird eggs. Gray Kingbirds, Northern Mockingbirds (Mimus polyglottos), and Greater Antillean Grackles (Quiscalus niger) were rejecters in Puerto Rico. The success of parasitized Yellow Warbler nests was higher in Puerto Rico than in St. Lucia. Cowbird parasitism reduced Yellow Warbler nest success in St. Lucia, but not Puerto Rico. Comparisons with Puerto Rico indicate that neither hosts nor cowbirds in St. Lucia have evolved defenses or counter-defenses during a 40-year period.