Nashville Warbler, Magnolia Warbler, Tennessee Warbler, American Redstart, Blackburnian Warbler Tom Grey
What to Look for Now—Timely Birding
Fall 2020
Matthew Dodder
SCVAS Executive Director
Season Premier
At this time of year I normally tell people about fall migration. During August to early September that usually means Shorebirds. In last year’s installment of W2L4 I did just that: https://scvas.org/what-to-look-for/fall-2019
Yellow Warbler—Bill Walker
This fall though, I want to talk about Warblers and their journeys. If you are familiar with Roger Tory Peterson’s “Confusing Fall Warbler” section of his field guide, you know that heart-quickening moment when you see an unfamiliar Warbler and wonder if it could possibly be a rarity. Peterson’s color plate certainly gives us hope, since they all look so much alike! A second look reveals telltale differences between these confusingly similar birds.
But let’s not get caught up in the quest for rarities yet. Fall sees thousands of Warblers in our area. Some, like Orange- crowned Warbler are present year round, but the fledged young pad their numbers now. As well, Yellow Warbler
can be common during this time. Its eye ring (the most prominent facial feature this bird shows), subtle wing bars and yellow(ish) tail make Yellows easy to identify. Annoyingly though, the young may not look especially yellow. Townsend’s Warbler will reappear for the remainder of winter, and shortly after it, the abundant Yellow-rumped Warbler. Can you spot both “Audubon’s” and “Myrtle” varieties?
Lost?
Palm Warbler—Bill Walker
So what about the rare ones? How do we explain their vagrancy? It’s a common assumption that the rare Warblers we see here in autumn, species like Blackburnian, Blackpoll and American Redstart are here by accident—that they are lost, and don’t really belong here. These non-breeding birds show up in the Bay Area each fall and are often referred to as “Eastern Warblers”.
A quick look at Blackpoll Warbler’s range map reveals something interesting though. The breeding range is not so much eastern as it is northern. If you trace a line from northern central Canada to northern South America where Blackpolls winter you see a gently curved path passing though the Bahamas, Cuba and the Caribbean. This is the long voyage Blackpolls take every year... twice in fact. What might happen if the bird’s navigation system was somehow off? It might send the bird along the California coast instead.
Mirror-image misorientation is a theory proposed by Dave DeSante in the 1970s after his work on Southeast Farallon Island. While stationed there, he and other researchers monitored hundreds of “Eastern” as well as “Asian” Passerine vagrants, and he understandably wondered, why are they here? His PhD was based on this question, and the answer involved reverse charting these wayward birds’ probable
route to California. He noticed that if an imaginary mirror were placed from north to south from the birds’ breeding range, the reflected image mimicked their expected journey. By this logic, these birds were not lost at all. They did not lack direction. Their brains—their guidance systems—had simply been mis-wired. They were exactly where their auto pilotstold them to be—a reflection of their normal path. Further research suggested that many long-distance migrants show signs of this condition, and that these misoriented travels seem to correlate with age groups (younger birds often exhibiting it more). So, could it be these misoriented journeys are somehow part of a pioneering strategy? Part of the species’ DNA? Remember also that the northbound journey of neotropic birds is usually more direct, driven by the urgency to breed, but the southbound journey is much less hurried— perhaps the best time to lose one’s way... on purpose.
ID Checklist
Let’s say you see a Warbler in fall you don’t recognize. First, break it down. Features will present themselves fleetingly, and never in order. All things being equal though, I like to determine quickly whether it has wing bars. Wing bars are hard to hide and they tend to show up quickly. The presence or absence of wing bars eliminates half of the species you need to consider.
Magnolia Warbler—Bill Walker
Next I try to get a look at the bird’s face. Does it show a supercilium (consider Orange-crowned, Tennessee) or a strong eye ring (consider Nashville)? Each answer narrows your pool of options. Maybe it shows eye crescents (consider MacGillivray’s), or maybe it shows no discernible facial features at all... Whatever you observe, it will be important in identifying your bird.
Somewhere in there I catch a glimpse of the bird’s breast. Does it look yellow or white or tan (consider Palm)? Is it streaked, and if so, are those streaks heavy, thin, crisp or smudgy (streaks suggest a lot of birds, but eliminate many too)? I do what I can to notice if the tail shows any white, and whether that white falls on the outer feathers evenly, or if it is isolated to squares (consider Magnolia).
Like everyone, I’ve gotten used to looking up at Warblers. From that low vantage, their tail pattern is often visible, and in many cases it is diagnostic. Is it olive, gray or white? Does it show both dark and light areas? Is there a distinct pattern visible on the tail or even the undertail coverts? And while you’re staring up at your bird, are those undertail coverts yellow or white?
Home Away
During migration, Warblers don’t always show up in habitats that reflect their nesting haunts. Some spend their winters in vastly different biozones, eating very different foods from one season to the next. So what is home actually?
Home is actually where the bird is. After all birds don’t carry anything and don’t own property. They may travel great distances every year and so everywhere could be called their home, their habitat. Still, you are not likely to see a Yellow Warbler scrambling around in mud between pickleweeds nor a MacGillivray’s way up in a tree. Their general behavior is usually consistent, whether ground foraging or treetop in persuasion, tail bobbing, walking, etc. But clearly their habitat changes like any train passenger’s view through the window while en route. The traveller will disembark, perhaps to enjoy a layover in a spot well suited to the travelers’ needs. In the case of Warblers, that spot might be any grove of trees located near the flight path, or “railway.” Alder woodland, walls of flowering eucalyptus, creekside willows, and patches of fennel where insects are found—these all provide generous menus for weary travelers, and opportunities for eager birders. Among the proven locations to find fall Warblersare Ulistac Natural Area, Sunnyvale Baylands Park and any riparian stretch, willow grove or fennel patch you can find. I plan on being there!
So get out there this fall, visit the places Warblers might have stopped to fuel up, bring a field guide and keep an open mind. Not every confusing fall Warbler will be rare, but that still leaves a lot of room for confusion and possible discovery.
