University of Wisconsin–Madison
Spatial Analysis For Conservation and Sustainability

Climatic extremes influence spring tree phenology and migratory songbird foraging behavior

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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.