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Wood Thrush Declines



It is very hard to understand population declines of
Neotropical migrants like Wood Thrush because of their
complex life cycle.  Increases in mortality can occur
during breeding (nests or adults), both migration
events, or wintering.  Short term trends can indicate
punctuated mortality (such as several cold fronts
during the peak of trans-gulf migration) while long
term trends can indicate gradual mortality (such as
increased  domestic animal predation and cowbird nest
parasitism).  Drought and extremely wet weather has
been shown to decrease (or increase, depending on the
species) survivorship  of nests.  Determining causes
of population declines requires long term monitoring
not only of numbers, but also a miriad of factors that
can affect mortality.  Then, complex regression
techniques must be used to tease out these causes. 
Using only counts of adults makes it difficult to
determine causation because many factors are
correlated and there is little data on nest survival
(see BBIRD at http://pica.wru.umt.edu/BBIRD/). 
Distinguishing between juveniles and adults during
counts can help but is not practical.  

Acid rain is thought to disrupt calcium uptake in
birds.  However, for acid rain to affect a couple of
species is suspect.  

Your later thoughts tend to imply that the drought
last year and the wet one this year may have resulted
in a decline of Wood Thrush for this year's count.  I
think that that explanation is as good as any with the
limited amount of data at our disposal.  This is what
the folks at Patuxent are paid to do.  Speculation on
population trends, short or long term, is just that.

As a related aside, A PhD student at NCSU was able to
show increased nesting success of Wood Thrush in the
Smokies by an experiment that supplemented some
breeding Wood Thrush with meal worms.  The
supplemented birds had significantly (statistically)
greater nesting success than non-supplemented birds. 
The hypothesis goes that, with increased food there is
increased nest vigilance, nestling feeding and,
therefore, increased survival.  

By the way, does anyone know where I can get some
shade grown coffee?

Kendrick Weeks
Fuquay Varina

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