The handy thing about the songbird’s fall migration is that sometimes the birds come to you. You're not likely to find southbound shorebirds or geese in the typical Berkeley back yard. But we do get a steady trickle of warblers, vireos and flycatchers in late summer and early fall, and now and then a surprise. Last weekend I had a brief look at a female western tanager in the island mallow near the garage: a midsized yellow-green songbird with two yellow wingbars and a large pale bill; as far as I can recall, she was a new addition to the yard list.
Western tanagers have always seemed like one of those quintessential Sierra birds. But they're not confined to the high mountains; the Contra Costa breeding bird atlasers has found them nesting in the Berkeley Hills, as well as the Diablo Range. Most likely the tanager in the yard came from the northern part of the species' range, which stretches as far as the Chilkat and Skagway rivers in Alaska.
Males are unmistakable, with their blazing red heads. That color, it turns out, comes from an unusual pigment called rhodoxanthin, which seems to be sequestered from insects they eat. I have to wonder if males and females have different dietary preferences; not sure anyone has investigated that. The other North American tanagers—summer, scarlet, and hepatic—get their reds from a more common type of carotenoid pigment, as do male house finches.
The female in my yard was about on schedule for her species. A few precursors reach Southern California as early as mid-July, but the peak of the California migration spans late August through late September. Stragglers have been seen as late as November. At least in southern British Columbia, adults head south first. They fly by night, most traveling singly or in pairs but sometimes in groups of up to 30. They're suspected of being high fliers, since their nocturnal flight calls have never been documented.
Some western tanagers only travel as far as the coastal region from Santa Barbara to San Diego, where about 75 are documented each year in the Audubon Christmas Bird Counts. They're attracted to flowering eucalyptus trees, feeding on nectar and insects. (Eucs can be dangerous to smaller songbirds like warblers, gumming up their beaks; it's not clear if the tanagers are also affected.)
The bulk of the migrants, though, are headed for western Mexico (from southern Baja California and central Sonora southward), Guatemala, and Costa Rica. In Mexico they frequent the pine-oak woodlands of the Sierra Madre, and sometimes high-altitude fir forests. Wintering tanagers in Costa Rica use a wider range of habitats, including deciduous forests, scrub and second-growth, and open areas.
Like many neotropical migrants, western tanagers adopt a different set of behaviors on their wintering grounds. Solitary during the nesting season, they often join mixed-species foraging flocks in winter. Flocking provides more eyes to search for food and detect predators. Northern birds often team up with resident species. In Oaxaca, three tanagers were observed associating with house finches and several warbler species in a fruiting fig tree. Primarily insect-eaters on their breeding grounds, these birds may eat more fruit in winter if it's available. Costa Rican sojourners glean insects from branches and flycatch from the crowns of trees.
In a real sense, songbirds that winter in Mexico and points south are returning to their evolutionary roots. We like to think of them as “our” birds, but most of these groups originated in the tropics and still spend half their lives there. The tanagers' ancestors expanded their breeding ranges northward to take advantage of the spring flush of insects at higher latitudes and the lower incidence of nest predation.
The catch, of course, is getting from Point A to Point B without flying off course, slamming into a high-rise building or radio tower, or being nailed by a hawk or cat—and then finding the right habitat still there at the end of the journey. With massive clearcutting for poppy and marijuana plantations, the Sierra Madrean forests are in grave trouble, and so are its specialist bird species. E.O. Wilson says we've lost at least 70 percent of the neotropical dry forest where many winter migrants winter.
Western tanagers, fortunately, seem flexible in their requirements. They'll raid ripe figs, take seed from feeders, even pick through trash in logging camps. With luck, we'll have these striking birds around for a while.