Results of Experimental and Natural Food Reductions for Breeding Black‐Throated Blue Warblers

Abstract

We examined effects of natural and experimentally created reductions in food abundance on the reproductive ecology of Black—throated Blue Warblers (Dendroica caerulescens). The study was carried out between 1982 and 1985 on four 30—ha plots of temperate deciduous forest within and near the Hubbard Brook Experimental Forest, New Hampshire, USA. Experimental food reduction using aerially sprayed Bacillus thuringiensis created significant differences in caterpillar biomass between one sprayed and two unsprayed plots throughout the breeding season in 1983. Where caterpillar abundance was reduced, Black—throated Blue Warblers made significantly fewer nesting attempts and diets of neslings included fewer caterpillars. Clutch size, hatching success, and numer of young fledging per nest did not differ among the food reduction site and controls. Also, the reduced number of nesting attempts per pair on the food reduction site in 1983 did not significantly lower production of young per pair. No detectable differences in caterpillar biomass between sprayed and unsprayed sites were created in 1984 or 1985 because natural caterpillar abundances were already low. Natural declines in food abundance for Black—throated Blue Warblers occurred from 1982 through 1985, primarily because of a decrease in caterpillar abundances. Significant reductions in number of young fledging per nest, nestling growth rates and survival, and number of nests attempted per pair corresponded with the natural decline in food abundance. Of those measures, a reduced number of nesting attempts per pair lowered annual production the most. When effects of food limitation were calculted separately from those of nest predation, food limited annual breeding productivity to below that needed to balance annual mortality in at least one of the four years of this study. These findings indicate that neotropoical migrant bird species are probably limited periodically by food when breeding in north—temperate habitats.