DIET SELECTION IN AMAZONIAN ANTWRENS - CONSEQUENCES OF SUBSTRATE SPECIALIZATION

I used an observational and experimental approach to investigate the d ietary consequences of substrate specialization in six species of Amaz onian antwrens (Myrmotherula). Three species (leucophthalma, haematono ta, and ornata) foraged exclusively at curled dead leaves suspended ab ove ground, axillaris and longipennis foraged on live foliage, and hau xwelli was a substrate generalist, feeding at both live and dead folia ge. Diet composition of all species was qualitatively similar, with so ft-bodied orthopterans consistently the most important prey type. Dead -leaf specialists took other prey roughly in proportion to their avail ability in dead leaves, whereas diets of live-leaf foragers differed g reatly from prey available on live foliage. Dead-leaf specialists also ate larger prey and especially larger orthopterans than did other ant wrens. Substrate generalization in hauxwelli was associated with highe r diet breadth and greater heterogeneity among individuals, compared w ith substrate-restricted foragers. Diet breadth was negatively correla ted with prey size across all six species. When tested in outdoor cage s, live-leaf foraging and generalist species showed little interest in dead- or live-leaf substrates, whereas all dead-leafers repeatedly in spected and manipulated dead and curled leaves in the absence of food. All foraging groups showed a similar degree of selectivity of prey ty pes in feeding trials. Dead-leaf specialists did not differ from other species in their preference for orthopterans of different colors, alt hough specialists were better able to catch and handle the largest kat ydids (>30 mm). Individual hauxwelli (the generalist) showed elevated levels of exploration at dead leaves with food reinforcement, suggesti ng short-term plasticity in search behavior. I conclude that substrate specialization in these birds involves fundamental differences in sea rch behavior, but is not accompanied by equivalent changes in prey sel ectivity or preference. Dead-leaf specialists search for suitable subs trates and then inspect them for hidden prey, taking prey roughly in p roportion to their availability. In contrast live-leaf foragers search directly for prey and select suitable items from the wider array of a rthropods available on live foliage. Substrate-restricted foraging may reduce diet breadth and promote diet stereotypy in both groups, where as the single most important factor promoting specialization on dead l eaves may be the predictable abundance of relatively large orthopteran s.