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The Varied Thrush

by Mike Stiles

Irruption: A sudden, dramatic and rapid increase in a bird population. Bird species will irrupt for different reasons and factors include the availability of food, suitability of climate and amount of predatory activity.

In this part of the world, the Varied Thrush has most certainly irrupted. This thrush breeds in damp, heavily forested areas of western Canada, up through Alaska, and winters down the west coast to Ventura county. In "normal" years, there are a few scattered sightings of this bird in our county, usually found in the more heavily wooded inland canyons and at higher elevations.

This fall they are everywhere. Their plaintive "police whistle" song has been heard along the coast, in backyards, and in their normal wooded haunts. A birder in Monterey County reported 750 birds in one day, a high count for that county. Mixed flocks of American Robins and Varied Thrushes numbering in the thousands were reported in Marin County.

A birder in San Mateo county had 7,120 one day, and the next had over 4,000. Here in our county, local birder Brad Schram counted over 7,000 birds in a mixed Robin/Varied Thrush flock over his house one morning, with an estimated 25% of those as the thrush. I even had four in my backyard one day, a first for my yard list.

Interestingly, the previous Varied Thrush irruption in 1994 was a precursor to a rainy year. Let's hope that this irruption is foretelling an end to our three-year drought here in California.

The bird's behavior on its breeding grounds is not well known, due to its shy nature and its use of dark, old growth forest habitat. Future forest fragmentation may be detrimental to the bird's success. It's odd "song" was aptly described by Louise Agassiz Fuertes: (it) "is as perfectly the voice of the cool, dark, peaceful solitude which the bird chooses for its home as could be imagined". Take a listen here.

The bird has an interesting migration pattern, where the northernmost birds will "leapfrog" and winter further south than the southern breeding birds. Those northernmost birds are what we are probably seeing this fall.

This time of year the Varied Thrush will switch from its normal diet of insects and other arthropods to a diet of berries and seeds. They have been known to defend a food source aggressively, displaying odd head down and wings up postures, and will dominate other seed eaters at the feeder.

Local birders are hoping that this irruption will produce other northern, montane species such as Evening Grosbeak, Cassin's Finch, Clark's Nutcrackers, crossbills and other rarities. One can only hope.