INTERSPECIFIC DEFENSE OF PASTURE TREES BY WINTERING YELLOW WARBLERS

Yellow Warblers (Dendroica petechia) defend small territories in isola ted patches of trees in cattle pastures in the Lacandon Forest of Chia pas, Mexico. We observed interspecific aggression much more frequently than intraspecific aggression. Although aggression by Yellow Warblers was directed at a minimum of 37 species, resident and migrant alike, attacks on Magnolia Warblers (Dendroica magnolia) comprised 35% of the total observed. We found that individual Magnolia Warblers use overla pping home ranges where one warbler tends to be the predominant occupa nt of a single Yellow Warbler territory. Overlap between neighbors was promoted by the aggressive chasing of Yellow Warblers. Arthropos were twice as abundant in the canopy as in understory shrubs. Exclosure ex periments showed that birds harvested approximately 80% of the arthrop ods available in the canopy and 50% of those available in the understo ry vegetation. Yellow Warbler defense may be responsible for this diff erence in density in foliage arthropods between the canopy and underst ory. To avoid aggression, subordinate species depended on the presence of refugia, comprised of dense understory vegetation. Maintaining the structural heterogeneity of arroyo vegetation may provide critical ha bitat for many species of migratory birds in cattle pastures. Taken to gether, the interspecific aggression, intraspecific territoriality, an d high rates of resource depletion indicate that competition plays an important role in determining the carrying capacity of second-growth r emnants for migratory birds and their use of habitats.