| I investigated the trophic ecology of forest trees, caterpillars, and wood warblers (Parulidae) and vireos (Vireonidae) during spring migration in 2001--2003 at Trelease Woods, a deciduous forest fragment near Urbana, Illinois, USA (40°09'N, 88°10'W). I hypothesized that (1) the photoperiod-driven pattern of migration of wood warblers and vireos through the stopover is independent of earlier- or later-than-average spring warm-ups, and, in the event of trophic asynchrony, the magnitude of the trophic cascade generated by birds feeding on caterpillars is altered in the form of increased or decreased folivory of the trees during early and late springs, (2) birds exhibit preferences for tree species that relate to food resource availability (caterpillars), and (3) yellow-rumped warbler (Dendroica coronata) migration phenology and tree preferences relate to tree phenology and insect abundance. Among all three years, tree and caterpillar phenology varied in accordance with spring temperature. The timing of bird migration through the stopover did not vary. As an indirect result of trophic asynchrony in two years, the trophic cascade was altered and folivory in Juglans nigra (black walnut) and Quercus macrocarpa (bur oak) canopies was increased 100%. Birds foraged disproportionately in rare trees [(<5% relative basal area; J. nigra, Q. macrocarpa, Ulmus americana (American elm), and Ulmus rubra (slippery elm)], and avoided a dominant tree species, Acer saccharum (sugar maple). Rare tree species supported more caterpillars per leaf than Acer saccharum, and tree species preferences were associated with caterpillar abundance in one year. Yellow-rumped warblers fed in both common and rare tree species. Early migrant arrival corresponded with budbreak and the presence of an ephemerally abundant insect prey, the hackberry psyllid, Pachypsylla celtidismamma. Later migrants foraged on caterpillars. In general, yellow-rumped warbler tree species use was associated with tree phenology and caterpillar abundance. This community-level research documented the relationships between songbird migration phenology through a stopover habitat and the phenology of their prey and the prey's host plants, identified critical tree species used by migrant birds, and determined the possible consequences of climate change and the decline in the abundance of Quercus spp. and their replacement with Acer saccharum . |
