[ Hopefully, this subject has not already been covered on Tweeters in an edition
I missed. ]

In perusing the latest "Bird Watch Canada" (from "Bird Studies Canada" Port
Rowan, ON) I found an article on Varied Thrush population cycles. There has been
some Tweeter discussion about surprising appearances of this beautiful bird,
along with corresponding window-strike incidents, so I thought this might be of
interest.

The article is adapted from "Population Cycles in the Varied Thrush" by Jeffrey
Wells, Kenneth Rosenberg, Diane Tessaglia, and Andre Dhondt, 1996 Canadian
Journal of Zoology, 74:2062-2069.

Basically, using Feeder Watch, Christmas Bird Count, and Breeding Bird Survey
data, from 1988 through 1995, the authors demonstrate a "dramatic biennial (once
every two years) cyclic change within most of the species' wintering range."

The cyclic change, when expressed in percentage of feeders visited, shows a
remarkable alternating percentage of approximately 30% to 60% from year to year.
1995 is shown as one of the "high" (60%) years but, as an infuriating
consequence of digestion to article size, they don't state if that means the
winter of 1994-1995 or 1995-1996. So I don't know if we are in (1996-1997) a
high or low incidence year - but I would bet on the former based on our
experience.

Likewise, the abundance rationale (local or widespread population changes) is
quite opaque in the article, so I won't repeat it here, but it appears to
suggest not changing gross populations but rather changing dispersion patterns
within the bird's normal winter range.

However, an interesting theory arises. It seems that oak mast (meat of fallen
acorns) is one of the bird's most prevalent food sources, and many oaks are
known to have a biennial production pattern.

Another article in the same newsletter asks a very pregnant question: "Can
JetSkis and Loons Co-exist?" The question is not answered, but they are
collecting data.

Jerry Blinn


E-mail from: Jerry Blinn, 11-Feb-1997