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A skein of Canada Geese head south across a September sunset. © Nathaniel Sharp

Field Guide to September 2021

September is a month of transition—birds, butterflies, dragonflies, and more are beginning their southward migration while some bees and other species are emerging for the first time all summer. To the curious eye this month offers a lot of excitement and the happenings featured in this field guide are just the tip of the iceberg!

By Vermont Center for Ecostudies September 1, 2021
Common Nighthawk in flight over farm fields in Norwich, VT © Nathaniel Sharp
Common Nighthawk in flight over farm fields in Norwich, VT © Nathaniel Sharp

Nighthawk Watching

By Kent McFarland

On warm summer evenings in Vermont’s breeding season, Common Nighthawks once roamed the skies over treetops and towns. Their sharp, electric peent call was the first clue that they were overhead. At dusk, these long-winged birds flew in graceful loops, flashing white patches out past the bend of each wing as they chased insects. Those days are mostly gone now, but in August and September during migration, careful observers can still witness their spectacular flight as they whirl southward on their autumn migration.

Although most often seen migrating at dusk here in Vermont, Common Nighthawks migrate at all hours of the day, often in large flocks, on one of the longest migration routes of any North American landbird. Many travel overland through Mexico and southward through Central America, while others pass through Florida and across to Cuba southward, flying over open water to reach their wintering grounds in South America thousands of miles away. Read more on the VCE blog.

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Aster Cellophane Bee (Colletes compactus) © USGS Bee Inventory and Monitoring Lab
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Autumnal Cellophane-Cuckoo (Epeolus autumnalis) © USGS Bee Inventory and Monitoring Lab

September Specialist Bees

By Spencer Hardy

If fall is mild, the very last bees may hang on until early November, but bee activity is certainly going to decline this month. However, there are a couple species just getting started. Aster Cellophane Bees (Colletes compactus) are just beginning to emerge for the first time since last October and are frantically collecting enough pollen to support their offspring underground for the next 10–11 months. As their name suggests, they prefer asters but also visit goldenrods and other late summer composites.

Also emerging about this time, from the same nests, is the Autumnal Cellophane-Cuckoo (Epeolus autumnalis). Females of this species won’t be collecting pollen though—they’re focused on finding nests of Aster Cellophane Bees and sneaking an egg into the larder of their host. Vermont has more than 75 species of cleptoparasitic “cuckoo bees” and many of them specialize in just a few host bees. These cuckoos are generally active for a shorter period of time than their hosts and much rarer. For example, the Autumnal Cellophane-Cuckoo was first collected in Vermont in 2019, while biologists have collected its host sporadically since at least the 1960’s. However, it’s likely both have been in Vermont for centuries.

Explore all of Vermont’s bees and find out how you can help us learn more about these species through our Online Bee Guide.

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Least Sandpiper feeding on the edge of Lake Fairlee, VT © Nathaniel Sharp
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Red Knot (foreground) & Black-bellied Plover (background) foraging at Delta Park IBA in Colchester, VT. © Nathaniel Sharp

Mudflat Wanderers

By Nathaniel Sharp

Autumn bird migration in Vermont often brings to mind graceful flocks of Common Nighthawks and Canada Geese wheeling their way south, or perhaps the famously ‘confusing’ fall warblers that move through the state in huge numbers. In this land-locked state, one would be forgiven for overlooking the migration of shorebirds often associated with brackish mudflats and the seacoast, but they can be some of the most exciting and interesting birds found in Vermont this time of year.

A group that includes sandpipers, plovers, godwits, dowitchers, and others, shorebirds have a tiny window to successfully breed in the far northern tundra before making their globe-spanning migratory journeys to places as far away as South America’s tip. With such a short breeding season bookended by massive trans-oceanic migrations, “fall” migration for some shorebirds can actually start as early as July! However, September is when we can expect to see the greatest diversity and numbers of shorebirds anywhere there are mudflats (or their equivalents) in the state.

Many of the major shorebird hotspots are in the Champlain Valley. Places like Delta Park IBA and Dead Creek WMA can be magnets for these birds, especially during years when lake water-levels are low and vast mudflats are exposed. Away from Lake Champlain, look for shorebirds anywhere there might be wet, exposed mud for them to poke around in search of food. In some cases, manure pits and muddy river edges can provide just enough habitat for a migrating shorebird or two to stop for some much-needed refueling on their long journey south.

Be sure to report any shorebirds (or any birds) you see this September to Vermont eBird!

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A close-up of a Fall Webworm nest. © K.P. McFarland
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Fall Webworm caterpillar © K.P. McFarland
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Fall Webworm moths, mating on Kent's house © K.P. McFarland

Worrisome Webs?

By Kent McFarland

It’s the end of summer and they’re draped on the ends of tree limbs like some early Halloween decorations. Everyone is talking about them. Fall Webworm caterpillars are fattening up on leaves inside big bags of webbing on shade, fruit, and ornamental trees.

Some tree-owners are worried. I’ve heard of people burning the bags, spraying, and cutting the branches off. But in most cases, this is much ado about nothing. The caterpillars are feeding on leaves that are near the end of their life. They don’t feed on the buds of next year’s leaves, so in spring new leaves will sprout with no sign of damage. The caterpillars usually don’t feed on the same branch or even in the same tree each year.  It is a native moth and it has more than 50 natural predators and 36 parasites that help keep populations in check.

Fall Webworms (Hyphantria cunea) are often confused with Eastern Tent Caterpillars (Malacosoma americanum). Fall Webworm nests are found in late summer and fall, while Eastern Tent Caterpillars appear in spring. Webworm nests enclose the ends of branches and the caterpillars feed inside the nest. Tent caterpillars construct a thick nest in forks of branches and only congregate in the nest at night or in inclement weather.

Fall Webworms overwinter as pupae in the soil. In June and July, the adult moths emerge and females begin laying their egg masses. The eggs begin to hatch in mid-July. Fall Webworms use over 90 species of deciduous trees as host plants. After feeding for a month or more inside the nest, they crawl down the tree, construct a cocoon, and pupate.

See more images of Fall Webworms and share your sightings at the Vermont Atlas of Life on iNaturalist.

The yellow streamers of a Witch-hazel bloom. © K.P. McFarland
The yellow streamers of a Witch-hazel bloom. © K.P. McFarland

The Last Flower

By Kent McFarland

Yellow and spindly, it is the lonely flower of autumn. Even after the leaves have fallen and the Monarchs have left us for Mexico, Witch Hazel is acting like it’s spring outside. In barren woods this shrubby tree attracts some of the year’s final pollinators—the late-season flies and moths that have few other options for nectar. And once pollinated, the flowers take a full year or more to mature into seed-bearing capsules. That’s when the fun begins. In the fall, Witch Hazel ejects its seeds in a subtle woodland fusillade. Or, as David Taft writes in The New York Times: “The clattering seeds and popping pods fill the woods with vibrancy.”

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Painted Lady visiting Purple Coneflower © Bryan Pfeiffer
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Tagged Monarch about to be released. © K.P. McFarland

Southbound Butterflies

By Nathaniel Sharp

Weighing less than a single gram and flying on wings as thin as paper, it’s truly astonishing to think how far some of our local migratory butterflies will travel when they leave Vermont in the fall. Two of the most well-known and widely-loved of these mini migrants are the Monarch and the Painted Lady.

Monarchs use the same currents of air migrating hawks depend on to fly southward with minimal effort, and on windless days they can often be found nectaring on late-blooming flowers like red clover. This time of year, we will begin to see the “super-monarchs”—the more long-lived generation of Monarchs that will live several months and make the several-thousand-mile journey to their overwintering grounds in the Oyamel Fir groves not far from Mexico City. Prior to this fall generation, the monarchs we saw all spring and summer long have a much shorter lifespan of only a few weeks.

Thanks to their widespread popularity and rather predictable migratory patterns, Monarchs are one of the most well-studied butterflies on the continent. Researchers have been able to track their movements through wing-tags and even their chemical makeup, and community scientists have contributed huge amounts of data on Monarchs to projects like eButterfly and Mission Monarch.

Painted Ladies, on the other hand, are a little more unpredictable. Their migration takes them from Canada down to the southern US and Mexico, but unlike Monarchs they don’t congregate in a relatively small area. In spring, sometimes they’ll arrive in the northeastern US in huge numbers, and sometimes not. Where are they going in winter? Why does their abundance in our area fluctuate so much? These questions and more can be answered by submitting your sightings of Painted Ladies (along with any other butterflies you see) to eButterfly. The checklists you input into this community science database provide robust information for population and abundance estimates, and can help scientists piece together just what is going on with these migratory butterflies!

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