ABSTRACT

Abstract. In the Upper Midwest of the United States, fire suppression has resulted in succession of savanna and forests that differ in both plant community composition and vegetation structure from their condition prior to Euro-American settlement. Furthermore, variations in weather affect spring phenological events and potentially alter synchronous relationships of migratory songbirds with their seasonal resources. Our goal here was to understand how annual variation in phenology of four tree species-northern red oak (Quercus rubra), eastern white oak (Q. alba), sugar maple (Acer saccharum), and red maple (A. rubrum)—affect foraging behavior of migratory songbirds during spring migration. Oaks currently have poor regeneration, whereas maples have good regeneration in forests in the Upper Midwest. A typical temperature regime in 2009 coupled with a record warm winter and early spring in 2010 provided a natural experiment for addressing our goal. In the spring and early summer of 2009 and 2010, we monitored migratory songbird foraging behavior and collected data on tree flowering and leaf-out phenology for 160 replicate trees of the four study species at the Kickapoo Valley Reserve in southwest Wisconsin. In 2009, 15 species of migratory wood-warbler (F. Parulidae) arrived at the stopover study area in late April and were present

until late May. Birds foraged heavily on flowering northern red oak and, to a lesser extent, on flowering eastern white oak and sugar maple. Red maple was not preferred by wood-warblers. In 2010, the arrival date and duration of stay among the 15 species of wood-warblers was similar to 2009, yet the frequency of use of the four tree species was reduced by 60%. Northern red oak, sugar maple, and red maple achieved summer condition 2 to 3 weeks earlier in 2010 than 2009, but these tree species were not preferred by the wood-warblers. Instead, eastern white oak, which flowered from early to late May, was the preferred foraging substrate in 2010. Our findings suggest that the flowering and early leaf-out phase of trees provides important resources to migrant wood-warblers that are apparently absent from trees that are more phenologically advanced. Our results also suggest that managing for heterogeneity in tree species, including early and late flowering species, as well as maintaining early successional tree species in the landscape, may be an important consideration in maintaining wood-warbler population levels under a variety of climate conditions.