Beware the Empids of Winter, Part 2

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Note: to see Part 1, click here.

Sherlock Holmes famously lectured Watson, “How often have I said to you that when you have eliminated the impossible, whatever remains, however improbable, must be the truth?” Science might strive for such certainty, but it’s a lofty goal and we usually have to settle for second best. So instead, “when you have eliminated the impossible and rejected the least likely, whatever remains best justified, must be regarded as probably true.” Not a phrase elegantly turned, but a reasonable sentiment . . . and one that applies to our December empid.

I originally noted that had I caught the questionable bird in August, I would have banded it as a Yellow-bellied Flycatcher without question. Why? Because Yellow-bellied are common in our area at that time, and the other look-alikes would have been wildly improbable. Catching the bird in December altered this calculus, as in this month Yellow-bellied Flycatchers are even more wildly improbable than the Pacific-Slope member of the Western complex. The photos below show the number of accepted eBird reports of both Yellow-bellied Flycatcher and Pacfic-slope Flycatcher for the month of December (in all years!).

Yellow-bellied Flycatcher has a paltry three reports on what we can charitably call the northeastern quarter of the continent: a single bird each on Long Island in 2009, Ontario in 2015, and Ohio in 2017.

By contrast, Pacific-slope Flycatcher has 8-12 reports: Lancaster PA in 1990; Lancaster PA in 2001; Harrisburg PA in 2012; a bird each in Connecticut and Reading PA in 2015 (the PA bird showed up 8 hours before the last sighting of the CT bird); two or three individuals in Massachussets, Philadelphia PA and Cape May NJ in 2019 (it is clear the Philly bird is different from the MA and the NJ bird(s), but it’s possible the MA and NJ sightings are of the same bird; and Massachussets in 2020. There currently also appear to be Pac-Slopes in Maryland, Nova Scotia, and North Carolina.

So, the probability seemed to be in our favor. Enter perception bias. It’s one of the hardest things to avoid in birding – letting your expectations color your perceptions. There are any number of minute differences between the Yellow-bellied Flycatcher and the Pacific-slope Flycatcher, but they are small and subject to interpretation. Both have almond-shaped eyerings, but the Yellow-bellied is said to be more rounded at the back of the eye, and the Pac-slope more pointed, for example. So how does our bird look? Judge for yourself.

People commenting on facebook and in messages were divided:

  • “The white ring surrounding the eye on a western flycatcher would come to a tear-drop point as on this bird, so this guy looks to be good for a western flycatcher.”
  • “It looks suspiciously YBFL . . . Eyering looks round, narrow and yellowish.”
  • “It looks perfectly round in the left eye, and tear dropped in the right…”

In fact, people were pretty split on the bird’s appearance overall!

  • “Looks good for “Western”. Here in _____ I’d have just marked this down as a Pac Slope.”
  • “It’s not quite yellow enough for a Yellow-bellied.”
  • “Pacific Slope is one of our most common birds at _____, and if I had this bird, I’d be going “hmmmmm, this looks different!””
  • “It looks like a longer primary projection than I’m used to on Western Flycatchers.”
  • “I’d not be quick to rule out Yellow-bellied.”

An informal tally seemed to suggest that about 75% of the East-coast commenters thought it was a Yellow-bellied Flycatcher, whereas West-coast commenters were evenly split.

Characteristics like “yellowishness” and “rounded” are notoriously subjective, and even experienced birders can be misled by their expectations. Measurements might be a more objective guide, and they are . . . when you take them without detailed knowledge of the expected ranges. For example, suppose you know that Yellow-bellied Flycatchers have an expected bill length of 7.0-9.4mm, and Pacific-slope Flycatchers have an expected length of 7.7-9.2mm. And suppose you measure the bill length at 7.4mm. If you expect or hope the bird is a Pac-Slope, it’s easy to convince yourself that you measured wrong. So you measure again. And maybe again and again, until you get the measurement that is “obviously” right. In cases like this, it’s important to take extra pains to take the measurements in as neutral a way as possible, ideally without having the expected ranges in front of you and without looking at the dial on the calipers until the measurement is complete.

So – where are we? Timing is one bit of evidence, and it favors Pacific-slope. Appearance is a second bit, and most folks seem to think it favors Yellow-bellied. Measurements are a third bit, and they were no help at all. DNA will be the most conclusive bit of evidence, but we may have to wait weeks or months for the analysis. Is there nothing else we can consider now? Ah . . . but there is.

Recall that I was able to take a video of the bird vocalizing. A friend of the station, Nick Kachala, took the the sound file and created spectrograms that give us a sort of picture of what the calls “look” like. Here are his spectrograms . . . compare them with these spectrograms of Yellow-bellied Flycatchers, these spectrograms of Pacific-slope Flycatchers, and these spectrograms of Cordilleran Flycatchers. Which do they match the best? (Note that there are lots of different calls and sounds represented in those recordings – you’ll need to scroll through several to find a picture that “matches.”)

With this evidence in hand, and with the physical appearance favoring Yellow-bellied over Pacific-slope, we believe that this is probably a really (really really!) late Yellow-bellied Flycatcher. In some ways, that’s disappointing, but in some ways it’s also more interesting.

We’ll still send the DNA out to be analyzed (because who knows . . . maybe this is actually a Pac-slope / Yellow-bellied hybrid!), but for now, our best evidence suggests it’s a plain-old Yellow-bellied who is taking migration at a more leisurely pace.

-Andrea Patterson

Beware the Empids of Winter!

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Earlier this fall, our friends at Long Point Bird Observatory remarked on facebook that any Empidonax flycatcher seen in the area after October 1 deserves extra scrutiny. “A useful tip,” I thought to myself, and filed it away in the back of my mind, to be pulled out in case a late empid ever wanders through Braddock Bay.

Empidonax flycatchers can be difficult under even the best of circumstances. Here on the east coast, we have five regular empids: Yellow-bellied, Acadian, Alder, Willow, and Least. The west coast adds five more: Hammond’s, Gray, Dusky, Pacific-slope, and Cordilleran. While there are some subtle visual differences between most of these species, there are a few that look so similar they can’t really be distinguished in the hand. Willow and Alder Flycatchers are close enough in appearance that they are often referred to as “Traill’s”, and Pacific-slope and Cordilleran are often referred to as “Western.” Yellow-bellied and Acadian Flycatchers are similar in some ways, and – to make things more confusing – Yellow-bellied also look quite a lot like the “Western” complex. In the field, range, habitat, and vocalizations can be helpful in distinguishing them. In the hand, we are often stuck taking fiddly fussy measurements to tell the difference.

When I headed out to the station this morning, my plan was simple: open about half the nets, give them a few minutes to dry thoroughly, and then take them in for the winter. The plan was working well. With nets 1-6 all safely in grocery bags and the guylines wound and stowed, I turned my attention to the nets around the field.

I headed out on a net run to make sure the nets were clear before I started taking things down. Net 13 . . . clear. Net 12 . . . clear. Net 11 . . . clear. Net 10 . . . WTH??? A small yellow empid was hammocking in a short net between the field and the edge of the woods. “It looks just like a Yellow-bellied!”, I thought to myself. And, if it had been August, I would have banded it as a Yellow-bellied Flycatcher without a second thought. But Yellow-bellied Flycatchers move out early, and by mid-September they are scarce. So I unpacked the sage advice of our Long Point friends . . . beware the Empids of Winter! Whatever I had in my hand, it deserved extra scrutiny.

Once I had the bird back at the banding station, I took a few moments to gather my thoughts. I texted two friends – Jay Carlisle of the Intermountain Bird Observatory in Idaho, and Stu Mackenzie of Long Point. Since western empids aren’t in my wheelhouse, I needed advice on what species might be most likely, and how best to distinguish them from anything more familiar to me. The word came back almost instantly – “Western” Flycatcher was the complex to consider, and Jay noted that the primary projection (i.e. how far the longest primary flight feathers extend past the longest secondaries/tertials) in Western is much shorter than in Yellow-bellied. I pulled out Sibley and did some quick reading, and then I pulled out Pyle and did some not-quite-as-quick reading. Finally, I pulled out my “Empid ID” chart, updated it with stats for Western, and got to work.

Below are several pictures of the bird, a video with vocalizations, and a chart with the measurements I took along with the expected ranges for several of the look-alike empids. The measurements do a pretty convincing job of ruling out Acadian (which wasn’t really a big contender anyway), but don’t do much to clarify whether our bird is a Yellow-bellied Flycatcher or a Western Flycatcher.

Fortunately, we also took a feather sample and a fecal sample (he pooped on Pyle!), and we are hoping that a DNA analysis will reveal what species graced our nets today.

— Andrea Patterson

To see Part 2 of this tale, click here.

 Yellow-belliedAcadianWesternOur Bird
Wing60 – 7265 – 8056 – 7267
Tail46 – 5552 – 6250 – 6355
Wing – tail12 – 1912 – 216 – 1512
Bill from nares7.0 – 9.49.2 – 10.17.7 – 9.27.4
Bill width4.8 – 5.65.3 – 6.35.0 – 5.85.4
p6 emarginated?usually yesnoyesyes
Longest p – longest s10.3 – 17.513.3 – 23.58.6 – 17.112.9
Longest p – p62.2 – 6.75.2 – 9.30.2 – 4.44.4
p6 – p101.9 – 6.3-2.9 – 1.74.7 – 9.85.4
p9 – p55.8 – 11.58.6 – 14.42.9 – 9.87.8
p10 – p50.8 – 5.1 -4.4 – 0.3Almost identical
Ranges that do not match our bird are shaded in red. Note that many of these measurements are difficult to take. I was as precise and as careful as possible (in some cases, taking a measurement two or three times), but differences are often in tenths of millimeters, and even a little error might throw a measurement into one species or the other.

A Golden Swamp Warbler Visits the Station

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Wednesday September 2, 2020: 19 new birds of 14 species; 13 recaps. New species: Blackpoll Warbler and PROTHONOTARY WARBLER!!! We have banded a total of 46 species this season, 24 of which are warblers. Bird of the day was Gray Catbird with 4 new bands, followed by a tie for second between Yellow-bellied Flycatcher and Bay-breasted Warbler, each with 2.

May 22, 2015 was Ryan’s day to sleep in. He rolled in to the station just as the netpickers were getting back from their first net check. As he put his bags down on his desk, Marilyn called back to him . . . “Ryan, I have a bird for you!” The next thing I heard was Ryan’s quiet declation (in a voice of complete disbelief), “You’ve got to be kidding me.” It was a Prothonotary Warbler only the third in the station’s history.

And today we caught the fourth.

Photos: Prothonotary Warbler (hatch year, female)

As I pulled the almost too-yellow bird from her bag, my reaction was exactly the same as Ryan’s was. You’ve got to be kidding me! Is this for real? Is this actually a canary and everyone is pranking me? Is this REALLY a Prothonotary Warbler? I was gobsmacked! It didn’t take long for reality to set in, but the excitement lasted all morning.

Photo: Prothonotary Warbler Nest Box
(credit: Erik Johnson, Louisiana Audubon)

Prothonotary Warblers mostly breed well south of BBBO, but there are some small breeding populations 60 miles to the east and to the west, near Montezuma National Wildlife Refuge and Iroquois National Wildlife Refuge. In their core range in the southern US, these birds like swampy woodlands but they have become acclimated to riparian forests further north. Unlike most warblers, Prothonotaries are cavity nesters! Just as we build Bluebird Trails and put up Purple Martin condos, people have set up nest boxes for Prothonotaries throughout their range and a conservation group has grown up to explore best how to protect this charismatic species. Such work is not without its challenges . . . if you look closely at the base of the nextbox in the photo (taken at Palmetto Island State Park in Louisiana), you’ll see a 6′ gator lurking in the water!

Thanks to Peggy, Sue, Ann and John for sharing the day.

-Andrea Patterson

Northern Par-ooh-la-la!!!

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Tuesday September 1, 2020: 57 new birds of 14 species; 23 recaps. New species: Philadelphia Vireo and Northern Parula. We have banded a total of 44 species this season, 22 of which are warblers. Bird of the day was Magnolia Warbler with 20 new bands, followed by American Redstart with 6.

The winds were from the south and southwest all night long, but somehow birds must have been encouraged to take to the night skies, as today we had our busiest day yet this season! Most of the difference was made up by a big jump in Magnolia Warblers (6 yesterday, and 20 today), but both vireo and catbird numbers were up as well.

A delightful surprise came in the form of three Northern Parula – two young females and one young male (pictured below). These tiny warblers are a bright blue-gray with a vivid yellow throat and chest, and they are heavily accessorized with a yellow-green backpack, white wing bars and eye crescents, and a tawny-colored necklace. They are plump, perky birds that always bring a smile.

They also bring controversy. How DO you say their name? It seems the preferred pronounciation by most folks is Puh-ROO-la, but some folks say PAIR-oo-la. Here at the station, we tend to be in the first camp but with a twist . . . if one looks extra fancy we might call it a Par-ooh-la-la, but mostly we say Puh-ROOOOOOOOOOO-la!

Photo: Northern Parula (hatch year, male)

Thanks to Gayle, Gary and Carolyn for making the day run smoothly.

-Andrea Patterson

Summer’s End

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Monday August 31, 2020: 36 new birds of 14 species; 10 recaps. New species: Ovenbird and Chestnut-sided Warbler. We have banded a total of 42 species this season, 21 of which are warblers. Bird of the day was a tie between American Redstart and Black-throated Blue Warbler each with 7 new bands, followed by Magnolia Warbler with 6.

Oh what a chilly morning as we opened the nets today! At just 11oC (52oF), we were grateful for clear sunny skies and little wind. We just kept moving until the sun peeked over the tops of the trees, and then things warmed quickly . . . although I think more than one of us basked in the sun like a snake to soak up all the last rays of summer.

Yesterday’s moth-eaters continued to be the story today. Seven American Redstarts were joined by several other Setophaga today, including two Cape May Warblers, six Magnolia Warblers, one Chestnut-sided Warbler, and seven Black-throated Blue Warblers . . . all of which, do in fact eat moths (or at least eat moth caterpillers).

Photos: Cape May Warbler (hatch year, female)

Cape May Warblers are considered to be “spruce budworm specialists”, and their populations track the prevalence of this forest pest. Spruce budworms are the larva of any of nearly 40 species of moth, all in the genus Choristoneura. The larva feed voraciously on the needles of fir and spruce trees, often completely defoliating them. Several successive years of defoliation can kill the tree, leading to forest devastation (not to mention a big economic impact for the logging industry). There are outbreaks every so often, in which the density of the larva may exceed 100 larva / 45cm branch tip (source: Wikipedia).

Chemical controls have been used for many years, often with debilitating effects not just for the natural ecosystem but for humans as well. Cape May Warblers and other spruce budworm specialists provide a level of biological control that seems to be sufficient in most years, but in years of severe outbreaks, the predatory birds simply can’t keep up with the overwhelming food supply.

So yes – Cape May Warblers love moth larva, which may comprise 50% of their diet in the summer months. But, they also have a perhaps surprising habit of sipping nectar from flowers (thanks to their straw-like tongue) and of eating fruit, especially in the non-breeding season when those sources may make up 1/3 of their diet (source: All About Birds). If you’re lucky, you might even be able to entice a migrating Cape May Warbler into your backyard by offering fruit and sugar-water!

And about the name . . . Cape May Warblers really have no special association with Cape May, except that a specimen collected there was used by Alexander Wilson to describe the species. It’s scientific name, Setophaga tigrina, means “tiger moth-eater” and that idea is reflected in both the French name (Paruline tigrée – tiger warbler) and in the Spanish name (Reinita atigrada – brindle or tabby warbler). Which name do you prefer?

Thanks to Lydia, Marilyn, Chita and Janet for great help today.

-Andrea Patterson

Day of the Moth-eaters

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Sunday August 30, 2020: 33 new birds of 16 species; 18 recaps. No new species today. We have banded a total of 40 species this season, 19 of which are warblers. Bird of the day was American Redstart with 5 new bands, followed by a three-way tie for second between Red-eyed Vireo, Veery and Magnolia Warbler, each with 4.

American Redstart; photo taken Aug 24, 2020

The breeze continued this morning, making banding possible but not ideal. When the wind is too strong, it isn’t safe for the birds to be in the nets (while humans might enjoy roller-coaster rides, birds don’t!), and so we carefully monitor how each net is reacting to both the direction and force of the wind. Luckily, we often find that the leaves on the treetops can flutter and the top branches sway, while at ground level scarcely anything moves. Such was the case today.

American Redstarts are one of our banders’ favorite warblers. They are instantly identifiable to species (at least once they’ve escaped their mousey juvenal plumage), and at least the adult males are easy to age and sex. The young birds and the adult females may present a bit of an aging-sexing challenge, but once you figure out the age, the sex is often pretty easy too.

These insectavorous birds used to be in a monotypic genus (i.e. a genus comprised of just one species) called Setophaga, which means moth-eaters. Recent genetic work shows that they are closely related to the Hooded Warbler, two species of parula, and all of the formerly-Dendroica warblers (such as Yellow Warbler and Magnolia Warbler). So now the genus has more than 30 species! Because there are conventions in how we name birds, however, the genus continues to be called Setophaga as that name was introduced in 1827 and the other generic names weren’t introduced until later.

So . . . do American Redstarts actually eat moths? Yes they do! They’ll eat a variety of insects which they tend to pick off of leaves and twigs, but they can also sally forth like a flycatcher to seize bugs out of the air. Overall a pretty nifty bird.

Today we welcomed Lydia and Cindy back to the crew, and it was like they had never been away. Lydia passed her NABC certification last fall with flying colors (seriously . . . everyone was really impressed!), and this season she’ll start learning the ins and outs of being bander-in-charge. So, she’ll be learning to make decisions about when to open and close nets, how many nets to open, how to monitor the weather while simultaneously making sure net runs and banding is happening in a timely way, and how to serve as a resource for less experienced volunteers.

Thanks also to Megan and Michaela for their excellent help today.

-Andrea Patterson

Another windy day

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Saturday August 29, 2020: 0 birds of 0 species, 0 recaps.

The threat of rain never materialized, but the promised winds whipped the trees about and made it unsafe to band today.

Porcupine Bird

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Friday August 28, 2020: 28 new birds of 11 species, 16 recaps. New species: Nashville Warbler. We have banded a total of 40 species this season, 19 of which are warblers. Bird of the day was Gray Catbird with 6 new bands, followed by a tie for second between Red-eyed Vireo and Magnolia Warbler, each with 4.

Canada Warbler with nanotag #69

It was a relatively ordinary August day today with a double-handful of new birds and a half-bushel of recaptures. We were lucky enough to get a Canada Warbler big enough to be radio-tagged. and so he was fitted with tag #69 and released. It will be interesting to see how long our tagged birds remain in the area. We don’t recapture most of our banded birds, and even when we do, at best you learn the mimimum amount of time the bird was lurking. With the nanotags, we’ll get really fine-grained information on exactly when the bird moves out of the area.

At this time of year, a lot of our birds are molting. Some of them are replacing feathers they grew last year or earlier this year, and some of them are still getting their first set of feathers. When a feather grows in, it is encased in a protective sheath that eventually falls off or gets preened away. Normally, the sheath is quite short, but today we caught a young catbird just getting his first set of feathers, and the sheaths on his rectrices (tail feathers) were so long they looked like porcupine quills!

Thanks to Marilyn, Peggy and Sheryl for a smooth morning!

-Andrea Patterson

Too windy to band

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Thursday August 27, 2020: 0 birds of 0 species, 0 recaps.

The forecast was pretty dicey for the morning hours, with rain and wind predicted. Out of an abundance of caution, we decided to cancel the morning rather than have our volunteers sit huddled in our education room or under the eaves (properly distanced, of course!).

Banding at a distance

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Wednesday August 26, 2020: 45 new birds of 17 species, 12 recaps. New species: Downy Woodpecker, Veery, and Tennessee Warbler. We have banded a total of 39 species this season, 18 of which are warblers. Bird of the day was Gray Catbird with 8 new bands, followed by Least Flycatcher with 5.

So I bet you’re all wondering . . . what does banding during a pandemic look like? Last spring, we ran the season with just two volunteers, but this fall we have cautiously allowed most of our “old reliables” to return. We’ve developed a rigorous set of COVID-19 protocols that require masks, social distancing, and good hand hygiene, and we have moved our banding operation outdoors.

The scribe and the bander face each other at separate tables that are set 6 feet apart, and everyone else brings a lounge chair and finds a comfy spot in the shade so that they can daydream or birdwatch between net rounds. We’ve added a sink with a foot-pump so that people can wash their hands (although we still have to cart in the water), and we’ve decided that we can’t bring Timbits or brownies to share. Other than that, it’s all pretty much the same and it seems to be working well!

Four years ago – almost to the day – we published a post about Baltimore Orioles that have abnormally colored plumage due to their consumption of invasive honeysuckle. We caught three such birds today. The carotenoids in the honeysuckle augment the normal carotenoids present in the orioles’ feathers, and so the birds have an abnormally bright polka-dotted appearance. Baltimore Orioles are snazzy enough to begin with, but these extra adornments just put them over the top.

Baltimore Oriole (hatch year, male)
abnormally colored by invasive honeysuckle

This was the Wednesday crew’s first day back. Peggy was back at the helm as BIC, and she flew through an assortment of confusing fall warblers without a single hiccup. Ann and Sue patrolled the nets, and John settled back into the scribing. With the station in capable hands, I was free to clean the main banding building and hang bird-safe tape on some of the windows. Thanks everyone!

– Andrea Patterson

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