Wednesday, June 29, 2011

Evening Grosbeak

After the Badlands, the next stop on our western expedition was Missoula, Montana.   We visited friends who keep a bird feeder.  We were greeted by a pair of Evening Grosbeaks, old friends from the Minnesota North Woods.  These grosbeaks breed across Canada and the northernmost United States,  then south through the Rocky Mountians as far as southern Mexico.

Evening Grosbeaks are irruptive migrants, in the United States usually appearing in two-year cycles. Relatively recently Evening Grosbeaks have expanded eastward.  One hypothesis is that this range change is due to the planting of Box Elders in eastern cities.  Box Elder seeds are often abundant all winter, providing the grosbeaks with plenty of food.  Grosbeaks also favor Spruce Budworms, and the birds may have expanded their range after particularly big budworm outbreaks (Gillihan and Byers 2001). I have banded Evening Grosbeaks in northeastern South Dakota and saw them during the winter as a boy in Virginia.  I am told that recently grosbeaks are seldom seen in the southeast.  I am surprised that I have not seen an Evening Grosbeak in Rice County.

My mentor Roxie Laybourne once showed me a gynandromorph Evening Grosbeak.  This bird was half male and half female.  It was yellowish on the right side, gray on the left, with a sharp line of demarcation between the two.  Furthermore, the ovary was on the left side and the testis on the right.  In other words, a testis lay under the female-plumage and an ovary under the right.  This specimen and photographs of other gynandromorphs can be seen by clicking here.  I have also seen a specimen of a gynandromorph  Great-tailed Grackle, large on one half, small on the other.  I have no idea how that bird survived in the wild.

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