Let’s say you’re a bird. Your breeding area this spring and summer abounded with budding plants, fresh fruits, nutritious insects—plenty for you and the kids you raised. But what about the coming fall and winter? To paraphrase the Clash, should you stay or should you go?
In San Diego County, a number of species of birds migrate in the fall, at least to some extent. Some just move down from higher altitudes (e.g., American Robin, Mountain Chickadee, Dark-eyed Junco). Others, including Black-headed Grosbeak, Ash-throated Flycatcher, and Hooded Oriole, travel south to winter in Arizona, Texas, or northern Mexico. Still others, such as Yellow Warbler and Cliff Swallow, fly thousands of miles to Central and South America. And really—why not just flap on down to the tropics for the winter? Sounds pretty good to me.
Well, migration is a mighty risky prospect. You expend vast amounts of energy to get from your breeding grounds to your wintering grounds. En route, food can be hard to find and rest, even harder to come by—especially if you hit bad weather or you have to fly over large bodies of water (e.g., the Gulf of Mexico). When you get there, exhausted and starving, the year-round residents may have already staked out the best territories—and they’ll fight you tooth and nail (beak and claw?) to keep them. Indeed, migrating birds have annual adult survival rates of about 50%—meaning that only about half of the adults that migrate north in the spring will be alive the following spring. Migration can be the death of you, quite literally.
Then why would you ever migrate from the tropics to breed in transitional or temperate areas such as San Diego County? If you live year-round in the tropics, you have an annual adult survival rate of about 80-90%. Lucky you. But year-round tropical residents have much lower annual reproductive success rates than do birds that breed in more temperate areas, whether they migrate north or reside full-time in transition/temperate zones. So tropics-living birds raise fewer young each year than do those that breed in more temperate zones—in large part because appropriate nesting areas are in shorter supply there.
If you’re in a northern area where all necessary resources—water, food, shelter—exist predictably year-round, you’ll likely stick around all year. In our area, jays, scrub-jays, thrashers, woodpeckers, and a few warblers will adapt their diets to make it through the colder, rainy winter. Even as most birds wrap up their 2017 breeding activities, the 2018 breeding season for a few year-rounders already peeks out from off-stage. (Listen carefully after dark and you’ll begin to hear Great Horned Owls calling and dueting as they pair up and establish territories for raising their young this coming winter.) Some species—Yellow-rumped Warblers, White-crowned Sparrows, many species of waterfowl and shorebirds—even trade their far-northern breeding grounds to winter here. Compared to tropics-dwelling species, you have a lower annual survival rate if you live in an area such as ours all the time, in the range of 35-60% (even lower for those wintering in the mountains). The wet, cool winters here can kill you. But if you survive to the next spring, you’ll probably raise more young than any full-time tropical resident will. Ya pays yer money and takes yer chances…
For a classic image of fall migration, picture Canada Geese in a V formation, winging noisily southward across the sky. Indeed, some birds do migrate during the day—raptors, cranes, swifts, and swallows, among others. But the majority of species migrate almost exclusively at night, which offers several advantages. Birds have more time during the day for feeding. The atmosphere at night tends to be more stable and better for flight. The generally cooler air at night reduces stress. Avian navigation at night relies on a variety of skills, including using the stars, sensing changes in the earth’s magnetic field, and perhaps even smell for some species. Of course, a full moon offers a bonus for nocturnal migrants—and for humans interested in observing this phenomenon. Aim a pair of binoculars at a full moon this time of year and you might catch glimpses of the silhouettes of songbirds, quietly passing by the face of the moon—rather like E.T. on Elliott’s bicycle.
In The Edge of the Sea (1955), Rachel Carson wrote:
“There is symbolic as well as actual beauty in the migration of the birds… There is something infinitely healing in these repeated refrains of nature, the assurance that after night, dawn comes, and spring after the winter.”
Sentiments to keep in mind as another breeding season begins to disappear in the rearview mirror.