Category Archives: Migration

Monarchs Still Migrating Through Pittsburgh

Monarch butterfly in autumn (photo from Wikimedia Commons)

Though it’s nearly mid October I saw monarch butterflies migrating through Pittsburgh on Thursday and Friday October 10 & 11. Their timing seems late, but they were given a boost by August-like weather early this month.

You can follow their progress across the U.S. on Journey North’s monarch butterfly blog where you’ll find:

Today’s rain will put a damper on monarch migration in Pittsburgh but we can watch from afar as the butterflies make their way to Mexico.

(photo from Wikimedia Commons; click on the caption to see the original)

Establishing A Bridgehead

Asian lady beetles in the Netherlands (photo from Wikimedia Commons)

Now that the weather has changed unwelcome insects will invade our homes including Asian ladybeetles (Harmonia axyridis) that resemble native ladybugs but don’t act like them. Also called “harlequin ladybirds,” they overwinter indoors, make a stink, and bite when frightened.

A hundred years ago we thought this bug was a great idea and repeatedly introduced it to the U.S. to control aphids. The introduced ladybeetles never made it in the wild until a population was found thriving near New Orleans in 1988. After that they spread like wildfire across the eastern U.S. and into Canada.

Thirteen years later they became established in South America and Europe(*). By 2004 they were in southern Africa. They hadn’t been introduced. How did they get there?

A 2010 study of their genetic markers revealed that those three continents were invaded by the eastern North America population. In a move called the bridgehead effect, Asian ladybeetles in the U.S. used our continent as a jumping off point to colonize Europe, South America and Africa.

The bridgehead effect: Worldwide invasion of Asian lady beetles (map from PLOS One and

The bridgehead effect is now recognized as a method of worldwide pest invasion. The pest establishes a bridgehead — a strong position near a human transportation hub — then fans out from there. Ants, obscure mealybugs and brown marmorated stinkbugs have spread this way.

Who will be the next pest to establish a bridgehead? I hope it won’t be the spotted lanternfly.

Read more at: Bridgehead Effect in the Worldwide Invasion of the Biocontrol Harlequin Ladybird.

(photo from Wikimedia Commons; map from PLOS One article posted at Click on the caption to see the original)

p.s. (*) The European population of H. axyridis is mixed. Some were directly introduced from their native range but were not considered pests until the North America cohort arrived.

From Broad Wings to Red Tails

Broad-winged hawk on migration (photo from Wikimedia Commons)

15 September 2019:

You can tell what month it is at a Pennsylvania hawk watch by noticing the most abundant raptor.

If you’re seeing a lot of broad-winged hawks, it must be September. Broad-wing migration peaks right now; they’ll be gone by the end of the month.

If you’re seeing a lot of turkey vultures and red-tailed hawks it must be October. In the fall of 2018, 90% of the turkey vultures that passed the Allegheny Front Hawk Watch went by in October.

Turkey vulture in flight (photo from Wikimedia Commons)

Red-tailed hawks spread their migration over several months. Last year at the Allegheny Front roughly 25% were seen in September, 50% in October, 25% in November.

Red-tailed hawk at the Allegheny Front, 31 October 2016 (photo by Steve Gosser)

If golden eagles are at their most abundant, it must be mid-October to mid-November. It’s my favorite time of year at the Allegheny Front.

Golden eagle at the Allegheny Front, 2 Nov 2016 (photo by Steve Gosser)

Pennsylvania hawk watches are about to switch from broad-wings to red-tails. Count the raptors to find out what month it is. 😉

(Broad-winged hawk and turkey vulture photos from Wikimedia Commons; click on the caption to see the originals. Red-tailed hawk and golden eagle by Steve Gosser)

Take A Birding Break

Click here to watch the video

Would you like to take a birding break?

Spend a few minutes watching fall warblers in Central Park in a video by Quoteny, recorded August to October 2014.

Enjoy the birds’ delicate beauty or challenge yourself to identify them, listed them below in order of appearance.

Click here to watch the video.

In order of appearance:

  • Magnolia warbler
  • Black-and-white warbler
  • Black-throated blue warbler
  • Common yellowthroat
  • American redstart (female or immature)
  • Palm warbler
  • Hooded warbler (adult male)
  • Northern parula
  • Yellow-rumped warbler (notice that in this view it isn’t showing any yellow)
  • Red-eyed vireo
  • Blackpoll warbler (bathing)
  • Northern waterthrush
  • Ovenbird (mesmerized by a rat)

(photo is a screenshot from video by quoteny on YouTube)

Locust Swarms Act Like Facebook

Mormon cricket, Sept 2014 (photo from Wikimedia Commons)

In the American West there’s a 3-inch long katydid called a Mormon cricket (Anabrus simplex) that cannot fly but it sure can walk.

We hardly notice Mormon crickets until millions swarm and march across the landscape, advancing more than a mile a day like a Biblical plague of locusts. Naturally scientists wondered what makes them do it.

Swarm of Mormon crickets heading eastward through a culvert on Halfway Highway, Oregon, circa 1939 (photo from Wikimedia Commons)

A 2006 study discovered that the swarms are driven by hunger and fear. The crickets are hungry for protein and salt so they start to migrate, but the bugs are cannibals and will eat each other if they can. Those at the back of the pack pursue the front line to catch and eat them. Fear forces the crickets to keep track of their neighbors and constantly on the move.

Despite the danger from behind, individuals sometimes double back into the crowd. If enough of them do it the whole swarm changes direction. How does this happen? What signals such a change?

In 2011, a study of locust swarming behavior at the Max Planck Institute worked on a computer model to predict the bugs’ behavior. Nothing adequately mirrored the swarms’ movements until researchers applied a social networking model that predicts the movement of human opinion on Facebook and Twitter. Somehow the locusts walking in one direction convince others to walk in the same direction. As researcher Gerd Zschaler remarked,

We concluded that the mechanism through which locusts agree on a direction to move together (sometimes with devastating consequences, such as locust plagues) is the same we sometimes use to decide where to live or where to go out: we let ourselves be convinced by those in our social network, often by those going in the opposite direction.

We don’t necessarily pay more attention to those doing the same as us, but many times [we pay more attention] to those doing something different.

Swarms of locusts use social networking to communicate, Science Daily

Our crowd-based opinions move like swarms of locusts.

Swarms of locusts act like Facebook.

p.s. If you’ve never seen a swarm of Mormon crickets click here for a National Geographic video. Read more about locust swarming behavior in Science Daily.

(photos from Wikimedia Commons, click on the captions to see the originals)

Watch Songbird Migration Online

BirdCast on 3 Sep 2019, 4:50a (screenshot from Live BirdCast map)

5 September 2019

Songbird migration is underway across the continent but we can’t see it happening outdoors because the birds travel at night. However, we can watch them online.

Radar can see birds in flight so Cornell Lab of Ornithology’s BirdCast uses national radar data to plot where, when and what direction the birds are moving. Click the BirdCast link to see the most recent video. Watch sunset (red bar) sweep across the continent and the birds start to move, then sunrise (yellow bar) sweep across and migration stops for the day. The date above the map is a pulldown menu for selecting prior nights.

The BirdCast screenshot above was taken on Tuesday 3 September on a night when many birds left Pittsburgh on their way to the Gulf coast. At 4:50am you can see them lighting up the BirdCast map from southwestern Pennsylvania to Mississippi.

Before BirdCast existed, I watched weather radar for a snapshot of current bird activity. BirdCast filters the weather map so you see only the birds. The Weather Service does not so you’ll want to check out this vintage article — Watch Migration On Radar — for a quick tutorial on how to read the map.

The screenshot below was taken from Great Lakes weather radar on the same date and time as the BirdCast snapshot at top. Notice the differences!

Great Lakes weather radar, 3 Sep 2019, 4:50a (screenshot from the National Weather Service)

Wisconsin and Lake Michigan are brightly colored on the weather map because of heavy rain. That same area is a dark spot on BirdCast because birds don’t migrate in a storm. BirdCast also shows no birds moving in Florida; Hurricane Dorian was there.

Last night, Sep 4-5, the wind was from the north and skies were clear west of the Appalachians. BirdCast and weather radar both show birds on the move from Pennsylvania and Illinois to the Tennessee and Mississippi Valleys.

BirdCast and Great Lakes radar, 5 Sep 2019, 4:10am (from BirdCast and NWS)

It’s a good day to go birding.

(screenshot maps from BirdCast and the National Weather Service Great Lakes)

Monarchs On The Move

Monarch butterfly flying away in Iowa, Sept 2018 (photo from Wikimedia Commons)

In the past week I’ve noticed many monarch butterflies in flight and they’ve all been heading south.

When you see a monarch, pay attention to the direction it’s heading. Right now is the peak period for them to pass through Pittsburgh on their way to Mexico.

Monarch butterflies are on the move!

(photo from Wikimedia Commons; click on the caption to see the original)

Broad-winged Migration

Broad-winged hawk at Bent of the River, Connecticut, Sept 2016 (photo from Wikimedia Commons)

Every year, beginning in late August, broad-winged hawks (Buteo platypterus) head south on a 4,500 mile journey from their nesting territories in North America to their winter grounds in Central and South America.  It’s a journey many of us witness at Pennsylvania hawk watches. 

Unlike other hawks, broad-wings usually travel together. Though not in organized flocks they cue off each other to find the best travel conditions. This brings them together on migration.

The Allegheny Front Hawk Watch, 1.5 hours from Pittsburgh, saw 119 broad-winged hawks last Saturday but will peak September 13-15 with close to 2,000. Other Pennsylvania hawk watches will count even more.

Visit to see the latest statistics and find a hawk watch near you. Plan a visit soon.

Broad-winged hawk on migration, Bentsen Rio-Grande, March 2018 (photo by Bettina Arrigoni via Wikimedia Commons)

Meanwhile, keep looking up. There’s a good chance you’ll see a broad-winged hawk overhead in the next couple of weeks.

(photos from Wikimedia Commons; click on the captions to see the originals)

Unusual Visitor In Duquesne

Yellow-crowned night-heron in Duquesne, PA (photo by Oliver Lindheim)

In August 2019 a rare bird showed up every evening at an industrial park in Duquesne, PA. The bird was outside his normal range, but this is not surprising for a juvenile yellow-crowned night-heron.

Yellow-crowned night-heron in Duquesne, PA, 18 Aug 2019 (photo by Amy Henrici)

Related to egrets and bitterns, yellow-crowned night-herons (Nyctanassa violacea) live near water and eat mostly crustaceans. Some live year round in Central and South America. Others breed in North America and migrate south for the winter as shown on the map below.

Range map of yellow-crowned night-heron, yellow is breeding range (map from Wikimedia Commons)

However, juvenile yellow-crowned night-herons are great wanderers. As Cornell Lab’s All About Birds explains, “After the breeding season, young birds often disperse to the north or west before heading to wintering grounds.” That’s how they end up in Newfoundland, North Dakota and Duquesne, PA.

At 4pm on Sunday August 18, I went to see him at the industrial park but he wasn’t there because (duh!) he’s a night-heron. So I went back at 7:20pm.

He attracted a small crowd. Five of us watched him roam the sidewalks and grass beneath the pine trees at American Textile Company. He was so unafraid of humans that he walked right past two people standing on the sidewalk. This bird is completely focused on cicadas.

Yellow-crowned night-heron in Duquesne, PA, 18 Aug 2019 (photo by Amy Henrici)

To give you an idea how close he came, here are photos from Oliver Lindheim (at top) and Amy Henrici, two of the many birders who’ve made the trek to Duquesne.

Yellow-crowned night-heron in Duquesne, PA, 18 Aug 2019 (photo by Amy Henrici)

As soon as the glut of cicadas is over, this bird will be on his way.

UPDATE 26 August 2019: This bird is gone. He didn’t show up on the evening of August 21.

NOTE: If you went to see the yellow-crowned night-heron and you use eBird, please mark his location as the new “Stakeout” Hotspot created specially for him called “stakeout, yellow-crowned night-heron at City Center, Duquesne, PA.

(photos by Oliver Lindheim and Amy Henrici)

Captive-Raised Monarchs Fail To Migrate

Monarch butterfly on zinnia at Phipps Butterfly Forest, Aug 2017 (photo by Kate St. John)

In case you missed it, a monarch butterfly migration study published in the Proceedings of the National Academy of Sciences in June 2019 has troubling news for those who rear monarchs indoors.

Eastern monarch butterflies, famous for their autumn migration from North America to Mexico, have declined 80-90% in the last 20 years. To help the butterflies many people collect eggs and caterpillars in the wild and captive-raise them to increase their chances of survival. Unfortunately this well-meaning act can damage the insect’s ability to migrate.

Researcher Ayse Tenger-Trolander at Univ. of Chicago stumbled upon this when she purchased captive-bred butterflies for her monarch migration study. To measure their autumn migratory drive she placed them in a flight simulator and noted the dominant direction they wanted to fly. Wild migratory monarchs orient South. The captive-bred monarchs chose random directions, unlikely to migrate.

To further test the butterflies, Tenger-Trolander collected wild monarchs and raised a new generation indoors, mimicking outdoor autumn conditions. Here’s what she found.

Furthermore, rearing wild-caught monarchs in an indoor environment mimicking natural migration-inducing conditions failed to elicit southward flight orientation. In fact, merely eclosing(*) indoors after an otherwise complete lifecycle outdoors was enough to disrupt southern orientation.

Contemporary loss of migration in monarch butterflies, PNAS

Chip Taylor, Director of Monarch Watch, pointed out on NPR that some captive-bred monarchs do make it to Mexico, but added that “The real reason for raising monarch butterflies is for the enjoyment, the education. [T]he idea of individuals saving caterpillars as “monarch rescue” is misguided. “That’s simply not going to work as a way to boost the population,” says Taylor. “What we really need to do is to improve the habitat.”

We’re learning that monarch migration is complex and very fragile. It’s easy to break it in a single generation.

(photo by Kate St. John)

(*) “eclosing” means emerging from the chrysalis.