Abstract
Extreme weather is becoming more pronounced, making phenological patterns less predictable. Among the potential consequences, extreme weather may alter relationships of migratory birds with their seasonal food resources and thus impact valuable ecosystem regulating services (e.g., bird predation of herbivorous insects). Our goal was to quantify the effect of an extremely warm spring on these relationships in a U.S Midwest oak savanna. Average regional temperatures in the spring of 2009 coupled with record highs in 2010 (8°C warmer) were the basis of a natural experiment for addressing our goal. In both springs we documented tree flowering and leaf‐out phenology, related these to arrival and foraging behavior of the three most abundant migratory wood‐warbler species (Parulidae), and quantified the effects of migratory bird foraging on insect density, size, and herbivory using a branch exclosure experiment. In 2009, the dominant tree species at our study site, eastern black oak (Quercus velutina), flowered in mid‐May and the wood warblers foraged heavily in the savanna during this time. Branches from which birds were excluded exhibited a trend toward higher insect density, larger Lepidopterans, and greater flower damage than control branches. In 2010, tree phenology was four weeks earlier than in 2009 and the wood warblers were nearly absent from the savanna (83% fewer), likely because peak food availability preceded their arrival in mid‐May. Insect density was 83% greater in 2009 than 2010. However, in 2010, 81% of sampled leaves exhibited substantial damage (>25% of leaf‐area removed) compared with 27% of leaves in 2009, presumably due to a lack of herbivorous insect regulation by birds. Our results suggest that the extremely warm spring of 2010 altered linkages between migratory birds and their invertebrate prey that are typical during years of average weather, which likely affected habitat use and the delivery of ecosystem regulating services.
Highlights
Extreme variations in spring weather in the Northern Hemisphere are becoming more pronounced (Meehl and Tebaldi 2004)
Our results suggest that prolonged extreme spring temperatures altered relationships of spring migratory birds and their seasonal food resources, which in turn affected habitat use and the delivery of ecosystem regulating services
In a year when late winter and spring temperatures were similar to the long-term average, we observed patterns of coincidence between presence of both short- and long-distance migratory wood warblers and black oak tree flowering and leaf emergence
Summary
Extreme variations in spring weather in the Northern Hemisphere are becoming more pronounced (Meehl and Tebaldi 2004). A particular concern is that large deviations in weather from typical conditions may alter formerly synchronous phenological relationships 2015), such as coincidence between the arrival of migratory birds at stopover and breeding habitats and peak abundance of ephemeral food resources (Visser and Both 2005, McKinney et al 2012). The arrival of migratory birds at stopover sites (i.e., habitats used for resting and refueling during migration) typically matches peaks in herbivorous insect abundance (Graber and Graber 1983, Alerstam et al 2003, McGrath et al 2008). Phenological synchrony between avian migrants and herbivorous insects likely has broader implications for ecosystem function than solely improved body condition (Bauer and Hoye 2014)
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