Birds see the trees inside the forest: The potential impacts of changes in forest composition on songbirds during spring migration

Since European settlement, hardwood dominated forests of the Upper American Midwest have undergone compositional changes due to fire suppression and changes in land use. It is not clear how these changes affect songbirds during spring migration. In 2009 and 2010, we quantified foraging behavior by migratory songbirds during spring migration and collected data on tree and sapling diversity in the Kickapoo Valley Reserve in southwestern Wisconsin. Furthermore, we compared the 1840s distribution of tree species (from Public Land Survey System witness tree records) with current (2010) and estimated future (sapling) tree-composition to better understand how historic and future changes in tree composition may impact migratory songbirds at spring migration stopover sites. Six tree species were selected as foraging substrates in higher proportion than they were available by eight migratory songbirds, including trees adapted to moderate shade such as northern red oak (Quercus rubra), white oak (Quercus alba), American elm (Ulmus americana), and slippery elm (Ulmus rubra), and shade-intolerant species such as big-tooth aspen (Populus grandidentata), and paper birch (Betula papyrifera). Whereas three shade-tolerant tree species were selected in far lower proportion than they were available by eight migratory songbirds, including sugar maple (Acer saccharum), red maple (Acer rubrum), and basswood (Tilia americana). We found evidence that food accessibility, as measured by a novel approach relating a bird’s attacks and search efforts to the average leaf petiole length of a tree species, was strongly inversely related with a bird’s foraging success (q = 0.96, p-value <0.001). Although tree-species composition changed considerably from the 1840s to 2010, in both time periods the forest was dominated by a mix of sugar maple and oak species. However, sugar maple saplings currently form a nearly continuous layer in the understory and there is very low recruitment of shade-intolerant or moderately shade-tolerant species, suggesting a future shift towards dominance by shade-tolerant species. Our results suggest the current trajectory of forest succession may result in future conditions that provide lower quality foraging for migratory songbirds during spring migration than they currently experience in the Upper American