Fall migration: Keep an eye out for these birds

One of the most fascinating elements of observing birds is the phenomenon of migration. This is so alien to us that most humans register “bird migration” only as a phrase, an abstractly existing notion, rather than a real observable phenomenon, vividly and distinctly alive in our daily experience. But migration is happening all around us! Birds that were not here six months ago are here now, with a different suite of species and behaviors matching the changing of the seasons. Becoming aware of these patterns and the reasons behind them enriches your experience of the year and reveals the falseness of the proposition that California has no seasons.

Why do birds migrate anyways? The most obvious factor is food availability. In December, most of northern Canada and Alaska are not rich with the insects, fruit and seeds that most birds eat. In July, however, there is plentiful food available, and birds — with their unique abilities of flight — are the creatures for whom it is plausible to fly a few hundred or thousand miles to the north in order to enjoy it.

While coastal California is relatively rich in winter food, leading to a great net influx of birds in the fall, we can still see the seasonal changes in food availability. Even here, flying insects are more numerous in the spring and summer, and so we welcome many insect-eating birds from the south each spring, such as swallows and most of our local flycatchers. In fall, we have berries and seeds, and so we welcome berry-eating birds, like thrushes and waxwings, and primarily seed-eating birds like siskins and sparrows.

Two other benefits accrue to birds that risk the dangers of travel to go north (or sometimes to higher elevations, or away from the more moderate coasts). One is predator avoidance: being much more mobile than most mammal or reptile predators, birds can travel north to get away from the areas of highest predator density. And the other is the greater availability of space for breeding territories: while flocking in close companionship may have been fine for the winter, most birds in spring prefer to separate into pairs and maintain relatively private “households” until their children are born.

As with the changing patterns of food abundance, this sequence is also clearly visible in our local birdscape: while I might sometimes refer to different species as “summer birds” or “winter birds” as a convenient shorthand, what we are really mean are “breeding birds” and “nonbreeding birds.” After all, a classic “winter” bird, such as a white-crowned sparrow, may arrive in September (which we would typically call fall) and stay until April (well into spring). Our migratory birds of winter are here for more than just our calendar winter, but you can tell almost at a glance that they are nonbreeding birds: they rarely sing, they are rarely seen in pairs and overall they are less aggressive and combative than birds in the heat and competition of the nesting season.

So what are our migratory birds here in the Bay Area, and when do they come and go? The first waves of nonbreeding migrants reach Marin as early as June and July, when shorebirds — often the long-legged or long-billed waders of the intertidal zone — first return from their nesting grounds. This includes birds like sandpipers, curlews, godwits and yellowlegs. Other branches of the avian world have a later and more gradual journey southward, with many ducks and raptors still on the move through October, November and into December. We won’t reach peak numbers of many diving ducks such as scaups, goldeneyes and buffleheads for another month or so.

In between, those early-arriving shorebirds and those late-arriving ducks are a variety of winter songbirds that first appeared in September and have just recently become abundant. The white-crowned and golden-crowned sparrows are the most prominent of these neighborhood arrivals, announcing the autumn with their clear, musical songs. Yellow-rumped warblers pursue insects, moving in loose flocks through the treetops. Cedar waxwings search for berries, passing overhead in tight groups as they emit their high, lisping whistles. Varied thrushes subtly infiltrate the woods, and ruby-crowned kinglets subtly infiltrate the neighborhood and everywhere else, two more birds that have come down from the north to find their winter refuge here.

Jack Gedney’s On the Wing runs every other Monday. He is a co-owner of Wild Birds Unlimited in Novato and author of “The Birds in the Oaks: Secret Voices of the Western Woods.” You can reach him at jack@natureinnovato.com.