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September morning K.P. McFarland

Field Guide to September 2023

It can happen almost anywhere. On a cool, foggy morning, when fall warblers drop from their nocturnal migratory flights into your backyard. Or along a big river some evening when you notice Common Nighthawks moving south. Or on a hilltop when the Broad-winged Hawks circling above and Monarchs gliding southward convince you that summer is indeed coming to a close. Here is your field guide to some life on the move in September.

By Vermont Center for Ecostudies September 14, 2023
Brook Trout in Sherman Brook (Huntington, VT) shared with the Vermont Atlas of Life on iNaturalist Richard Joyce
Brook Trout in Sherman Brook (Huntington, VT) shared with the Vermont Atlas of Life on iNaturalist Richard Joyce

Brook Trout

By Jason Hill

September marks the beginning of migration and spawning for our native Brook Trout (Salvelinus fontinalis) populations. Shorter days and water temperatures dropping below 50°F prompt many Brook Trout to move into headwater streams. The males’ orange bellies take on a richer hue as they search for females and the redds (nests) that the females excavate on the gravel bottom. Males aggressively defend females and their redds and fertilize the eggs as they are deposited. Females then cover the eggs with gravel, and the eggs will remain buried until they hatch during the winter.

In New England, the Brook Trout spawning period typically lasts through November. Warmer summer temperatures delay the onset of spawning; a 1.8°F increase in mean maximum daily air temperatures results in a one-week delay that autumn. Following spawning, the adults often seek out deeper residual pools for winter, highlighting the importance of habitat heterogeneity and the need to maintain stream connectivity.

Learn more about Brook Trout from the Vermont Fish & Wildlife Department, and add your sightings to the Vermont Atlas of Life on iNaturalist.

White-footed Deermouse observation shared with the Vermont Atlas of Life on iNaturalist ( meltraros
White-footed Deermouse observation shared with the Vermont Atlas of Life on iNaturalist ( meltraros

Of Mice and Mast

By Michael Hallworth

The transition from the dog days of summer to cool autumn nights is upon us. That means that trees are preparing for winter by reabsorbing nutrients from their leaves, and soon their true colors will shine through. Many breeding birds, a few migratory dragonfly species, moths, and the iconic Monarch Butterfly are preparing to migrate south. On the other hand, resident animals are busy preparing for the colder months that lie ahead. For example, small mammals are beginning to cache acorns, beech nuts, and cones to dine on during the cold winter.

Finding acorns, beech nuts, and cones in the forest is easier in some years than others. Tree masting events, or the synchronous production of fruit across large areas, is a phenomenon influenced  in part by summer temperatures. During masting events, many individual trees produce abundant fruit in the fall. There may be a three- to nine-fold increase in nuts and cones during mast years. When nuts and cones are plentiful, many small mammals take full advantage of the bounty. With their coffers full of food, small mammals survive the winter at higher rates and have more young. The connection between mast and mammals is so close that some small mammals increase their reproductive effort in anticipation of a mast year. The population response of small mammals to mast is noticeable and can even make news headlines.

You can help track these events. Add your observations of tree seed crops and small mammal sightings to

Monarch nectaring on Joe Pye Weed before migration.  K.P. McFarland
Monarch nectaring on Joe Pye Weed before migration. K.P. McFarland

Monarchs on the Move

By Kent McFarland

Judging by the observations from around the region, this summer hasn’t been a productive year for Monarchs. However, we are still hopeful for a good migration southward. When hawks are moving, Monarchs are, too. Both ride air currents southward. The Monarchs barely flap a wing. On days with unfavorable winds, you can often find them fueling on flower nectar, especially in large fields of Red Clover. By the time cold air settles into the Northeast, the Monarchs are well on their way to Mexico.

Each winter, in a small area in the transvolcanic mountains near Mexico City, the entire population of Monarchs from east of the Rocky Mountains wait in the trees. This mountainous area consists of small peaks ranging from 7,800 to 11,800 feet in elevation. Oyamel Fir trees (Abies religiosa), a species closely related to the Balsam Fir (Abies balsamea) found on the mountaintops here in northeastern North America, dominate this region. In 1984, a study found that there may have been as many as 60 overwintering sites that the butterflies can use, but now there are just a few each winter. A site may contain up to 4 or 5 million monarchs per acre and cover as little as one-tenth up to eight acres of fir forest. The butterflies arrive from the north in November to late December and hang out on the trees, metabolizing fat reserves built up during migration. Remarkably, they actually gain weight on migration and arrive on the wintering grounds with fat reserves for the winter, unlike songbirds, which require enormous fat stores to burn on migration.

The winter generation lives up to eight months, while the successive spring and summer generations are lucky to live five weeks. It takes up to six generations of spring and summer Monarchs to produce the final “super-Monarch” that migrates to Mexico in autumn and then back to the southern United States in spring.

How do we know that at least some Monarchs from the Northeast actually make it to Mexico? Tracing unique chemistry in their wings tells us that, on average, about 15% of the overwintering population comes from the Northeast. And we also know directly from Monarch tags. Many of us have been tagging adults during fall migration in cooperation with Monarch Watch. Using small tags like tiny bumper stickers with unique identification numbers, volunteers capture and place them on Monarchs’ wings in the fall. With millions of Monarchs on the wing, the odds of a recapture are very poor. Here in Vermont, we have had a few lucky finds. Seven Monarchs tagged in Vermont have been found in Mexico!

Known recoveries of Monarchs tagged in Vermont from Monarch Watch.

You can also track Monarch movement online as people like you report sightings to Journey North,, and iNaturalist. Whether you find eggs or caterpillars, see them nectaring or actively migrating southward, you can add your sightings to help get a picture of this amazing migration across the continent.

A 24-hour national mosaic animation of radar imagery featuring large numbers of birds taking flight. Circular blue and green features represent bird migration; precipitation appears as irregular bands. (Base reflectivity radar imagery from October 16th-17th, 2017, 12pm-12pm EST.) BirdCast
A 24-hour national mosaic animation of radar imagery featuring large numbers of birds taking flight. Circular blue and green features represent bird migration; precipitation appears as irregular bands. (Base reflectivity radar imagery from October 16th-17th, 2017, 12pm-12pm EST.) BirdCast

Ghosts on the Screen

By Kent McFarland

When radar was first deployed, operators called them “ghosts,” mysterious echoes seen on radar screens on clear nights. It turns out that these blips on the screen were often migrating birds, insects, or bats. In addition to detecting and depicting meteorological phenomena, our weather radar network can be used to watch and track birds’ movements. Since the first units were placed along the Gulf Coast in the 1950s, ornithologists and birders have become increasingly aware of the power of using radar to study bird migration. And it’s available at your fingertips.

It’s called BirdCast. They turn weather radar data into information on the numbers and flight directions of birds aloft and eBird data on the ground in order to expand our understanding of migratory bird movement. You can visit the site and see the migration forecast and live migration maps. Visit their migration data dashboard to see what has already migrated overhead or what is passing during the night, as a live data feed begins each night after sunset. Last night, with the passage of a cold front that provided favorable winds for migration, over 1.7 million birds passed through Vermont on migration, most of them at 9 PM! Tune in to the night sky and discover what is passing you by.

Ruby-throated Hummingbird in flight Michael Sargent
Ruby-throated Hummingbird in flight Michael Sargent

Humming Along

By Kent McFarland

Frequency of Ruby-throated Hummingbirds reported on Vermont eBird checklists.

Frequency of Ruby-throated Hummingbirds reported on Vermont eBird checklists.

When flowers stop blooming, and insects stop flying, it means Ruby-throated Hummingbirds go south. Some adult males start migrating as early as mid-July, but the peak of southward migration is August and early September.

Most  hummingbirds at your feeders in September are migrants and not the same birds you’ve watched all summer. Since they all look alike, it’s difficult to know for sure; however, banding studies have shown the turnover. It’s  a common misconception that you must take down your feeders and force hummingbirds to leave. In the fall, the birds at your feeder are already migrating, so keep them up as long as you can; maybe a wayward rare species will find them!

So where exactly do these hummingbirds all go? There is some evidence that many travel around the Gulf of Mexico during fall migration rather than cross it as they do in spring. Most of them will spend the winter in Mexico and Central America. Weighing a mere two-tenths of an ounce, they’ll make the long  trip back in April and delight us once again with their bright colors and active flights after our long, white winter.

3620, , broad-wing-kettle, A kettle of Broad-winged Hawks / © Bryan Pfeiffer, , image/jpeg,, 860, 579, Array, Array Bryan Pfeiffer
A kettle of Broad-winged Hawks Bryan Pfeiffer
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Broad-winged Hawk migrating


By Bryan Pfeiffer, VCE Research Associate

September is the time to pack a lunch, climb a mountain, and witness thousands of hawks gliding south over hills blazing with fall foliage. If only this romanticized rite of autumn were so easy. If only migrating hawks didn’t appear like specks of pepper cast to the winds, the birds of prey preying on a birder’s self-confidence. So here’s some advice for fledgling fall hawkwatchers.

Step one: find a mountain. You’ll want a high perch from which to see lots of sky. However, not just any mountain will do. Go to established hawk-watching sites (more on those later), where the topography conspires to funnel southbound hawks in large numbers. Up there, you’ll also find birders willing to share their hawkwatching wisdom.

Next, check the forecast for a classic fall day. Cool north winds and full or partial sun fuel hawk migration. By late morning, sunshine warms the valleys, releasing thermals with hawks along for the lift. At least that’s the idea; even on perfect days, the hawks may not cooperate.

But when they do, it often goes like this: You finally spot your first hawk…then another, and another, then three more. As you brush your binoculars back and forth, up and down across the sky, you’ve discovered a kettle–hundreds of hawks, specks of pepper to be sure, but some of them drifting close enough to identify.

Identification Rule Number One: It’s a broadwing. The Broad-winged Hawk is the most abundant and reliable fall hawk in the Northeast (with rare exceptions – see John’s comment below). Small, stocky, and circling on flat wings, broadwings are themselves an event. Although they move south from late August into November, their numbers often surge from September 10–20, when broadwings constitute the vast majority of hawks we see in the skies. Here is your opportunity to learn and enjoy Broad-winged Hawks from every angle. And from that blizzard of broadwings, your next challenge is to find birds that are not broadwings.

Which brings me to Rule Number Two: Abandon plumage. Skilled hawkwatchers can distinguish every eastern hawk, eagle, or falcon species by shape and flight style alone. Give us a flapping silhouette in the sky, and we’ll put a name on it. After all, immature Red-tailed Hawks don’t have red tails, and immature Bald Eagles don’t have white heads. In the same way we use shape to separate pointy-winged falcons from broader-winged hawks in flight, practiced hawkwatchers use subtler differences to separate similar species. Once you know the shape and flap of a broadwing, for example, you may more easily distinguish  Red-shouldered Hawks by their slightly longer wings with more squared tips. (You won’t see red shoulders.)

The same goes for eagles – and Rule Number Three: It’s not a Golden Eagle. Okay, it might be. But Golden Eagles are rare in the East. Most Golden Eagle reports are actually of an immature Bald Eagle, which has a huge head and beak that project well forward of the wings, more so than on a smaller-headed Golden Eagle. Bald Eagles, which are regulars at fall hawkwatching sites, often hold their wings flat or slightly drooped; Golden Eagles mostly hold theirs in a slight but noticeable dihedral angle.

Until the weather’s right and you get yourself up a mountain, here’s some homework: Consult the Hawk Migration Association of North America’s website to locate a hawk-funneling mountain near you and download free guides to hawk identification. Augment your own field guides with Hawks at a Distance, by Jerry Liguori.

Finally, Rule Number Four: Be skeptical of rules and generalizations (including mine). You may encounter migrating hawks away from mountains, even over cities. And hawks won’t always be so distant. Now and then, they glide low overhead, exposing their classic field marks (so use them when you see them). Along with grace, force, and intensity, raptors produce some of the best “ooh-ahh” moments in all birding. But even when they don’t, or even when hawks don’t show up at all–hey, at least you’re on top of a mountain.

Read more about nature and life on Bryan’s Blog.

Fen Grass-of-Parnassus flowers beckon and float like little balloons over their own cluster of basal leaves and nearby plants.  Bryan Pfeiffer
Fen Grass-of-Parnassus flowers beckon and float like little balloons over their own cluster of basal leaves and nearby plants. Bryan Pfeiffer

A Duplicitous Flower and its Rare Bee

By Bryan Pfeiffer

When I first met up with Fen Grass-of-Parnassus about 25 years ago, the attraction was, I freely admit, all physical and not the least bit intellectual or even carnal.

My fondness for this plant came by way of its long, elegant stems bearing a single symmetrical flower of white petals marked with pert, green striping. And even though it had me at those petals, Fen Grass-of-Parnassus also flashed reproductive parts surrounded by an unusual array of 15 glassy, yellow orbs.

From the day I first laid eyes on them, those fetching flowers alone were enough of a turn-on. But sometime later, in the company of Fen Grass-of-Parnassus, I encountered a globally imperiled bee and was among the few people in the world to have photographed it on its flower. That was good.

But only after that did I discover the extent to which Fen Grass-of-Parnassus courts its bee in the drama of reproduction. So compelling were the flower’s antics and deceptions that I set up two study plots—which I visited day after day, regardless of rain or mosquitoes or my mood—in order to learn more about how Fen Grass-of-Parnassus, um, you know … gets it on.

Along the way, in my quarter-century of visiting with this plant, I came to realize a kind of intimacy with Fen Grass-of-Parnassus matched perhaps only by the human capacity for inquisitiveness and joy that we reserve for one another and the sacred things we create together. All that from a plant now in bloom and its bee in flight. Here are their stories, which are my own stories as well.

You can read Bryan’s stories on his Chasing Nature blog.

Day to day gyrations of the stamens, the male parts of the flower, in an attempt to dust pollen on visiting insects. Bryan Pfeiffer
Day to day gyrations of the stamens, the male parts of the flower, in an attempt to dust pollen on visiting insects. Bryan Pfeiffer

Comments (3)

  1. Gretchen McFarland says:

    Bryan and Kent are keen observers, knowing biologists and skilled writers who never cease to interest, inform and delight their readers. Bravo! Another well-done newsletter!

  2. Larry Layne says:

    Thanks Jason, Michael, Kent and Bryan for this most informative September newsletter. Much new food for thought about things I knew something about like the brook trout, mast years, and monarch, hummingbird and hawk migrations. And the sirens’ call of the new to me marvel of the beautiful Fen Grass-of-Parnassus. Thanks for your efforts to share your love of the natural world.

  3. Lisa von Kann says:

    I was lucky enough to have a beautiful Gray Tree Frog in it’s emerald green stage visit me right outside my doorway for many weeks. This little creature would rest in the shade on my phone box that is on the side of my house next to my porch. It would sometimes be gone for several days and then reappear as if “coming home” to my little shaded spot. What a joy it was to step outside in the morning and see the stunning green there and know that it was back.

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