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A few thoughts on bird food this spring:

Margy mentioned last year's freeze, which occurred around April 11.  The freeze did particularly severe damage because we had above-average temperatures in the preceding weeks in March, which sort of set spring in motion fairly early.  The result was that the April freeze did more damage than it would have in years with more "normal" phenology.  Spring insect herbivores (especially caterpillars, which are thought to make up the bulk of many songbirds' diets) appeared early too, and many were killed off by the frost.  Those that weren't killed off then had to face a shortage of food as so many leaves were destroyed.  At Tyson, where I do my work, we were experiencing our 3rd consecutive year of fall cankerworm (a geometrid, or "looper" caterpillar) outbreak, and an enormous number had just emerged when the freeze hit.  A lot of other caterpillars were in low abundance too.

I just finished an insect herbivore census yesterday on white oak, and colleagues in my lab finished an extensive survey in the Ozarks.  I had worried that insects would be largely absent because of last year's frost, but it wasn't all that bad.  A lot of caterpillars, especially "leaf-rollers," which are usually very abundant, were harder to find than usual.  But sawfly larvae (from the families Pergidae and Tenthredinidae) were very common.  Sawfly larave look and act like caterpillars, and are probably just as suitable as food for birds as caterpillars.  The crew working in the Ozarks experienced similar high numbers of sawflies, but more leaf-rollers as well.  As for the cankerworms at Tyson (which were at outbreak numbers 2005-2007), I found exactly two this entire spring!  This may not be entirely due to the freeze; typically after 2 or 3 high years, this species is brought under control by an egg parasitoid.  

So as for the birds, they haven't seemed to be struggling to find food, at least at my research site.  After the frost last year, their foraging was focused along sunny edges that were not as frost-affected, but this year the flocks are moving through continuous forest.  On some days I've noticed a dearth of bird songs, but it may be weather-related, as many birds seem to decrease song frequency on colder, cloudier, and rainy days.  And we've had plenty of colder, cloudier, rainy days over the last couple weeks!

Sorry to be long-winded about this...a disclaimer too:  my work is focused on oaks and especially white oak, so patterns may be different on other species I haven't been paying as much attention to.  Also, insect populations exhibit a lot of spatial heterogeneity, so what's going on at Tyson may be different from other sites (e.g., the cankerworms that were so abundant at Tyson have never been recorded in 15+ years of censuses in the Ozarks).  

Nick Barber
University of Missouri-St. Louis
St. Louis, MO
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