Diet selection in Amazonian antwrens: consequences of substrate specialization

Abstract

I used an observational and experimental approach to investigate the dietary consequences of substrate specialization in six species of Amazonian antwrens (Myrmotherula). Three species (leucophthalma, haematonota, and ornata) foraged exclusively at curled dead leaves suspended above ground, axillaris and longipennis foraged on live foliage, and hauxwelli was a substrate generalist, feeding at both live and dead foliage. Diet composition of all species was qualitatively similar, with soft-bodied orthopterans consistently the most important prey type. Dead-leaf specialists took other prey roughly in proportion to their availability in dead leaves, whereas diets of live-leaf foragers differed greatly from prey available on live foliage. Dead-leaf specialists also ate larger prey and especially larger orthopterans than did other antwrens. Substrate generalization in hauxwelli was associated with higher diet breadth and greater heterogeneity among individuals, compared with substrate-restricted foragers. Diet breadth was negatively correlated with prey size across all six species. When tested in outdoor cages, live-leaf foraging and generalist species showed little interest in dead- or live-leaf substrates, whereas all dead-leafers repeatedly inspected and manipulated dead and curled leaves in the absence of food. All foraging groups showed a similar degree of selectivity of prey types in feeding trials. Dead-leaf specialists did not differ from other species in their preference for orthopterans of different colors, although specialists were better able to catch and handle the largest katydids (>30 mm). Individual hauxwelli (the generalist) showed elevated levels of exploration at dead leaves with food reinforcement, suggesting short-term plasticity in search behavior. I conclude that substrate specialization in these birds involves fundamental differences in search behavior, but is not accompanied by equivalent changes in prey selectivity or preference. Dead-leaf specialists search for suitable substrates and then inspect them for hidden prey, taking prey roughly in proportion to their availability. In contrast live-leaf foragers search directly for prey and select suitable items from the wider array of arthropods available on live foliage. Substrate-restricted foraging may reduce diet breadth and promote diet stereotypy in both groups, whereas the single most important factor promoting specialization on dead leaves may be the predictable abundance of relatively large orthopterans. Received 25 November 1991, accepted 27 May 1992.