September 2020 Natural Mystery

posted Sep 14, 2020, 9:31 AM by Jonathan Poppele   [ updated Sep 14, 2020, 9:32 AM ]

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August 2020 Natural Mystery Answered

posted Sep 7, 2020, 12:04 PM by Jonathan Poppele   [ updated Sep 14, 2020, 8:42 AM ]

We received a range of guesses for our August natural mystery including muskrat, beaver, otter and raccoon—all of which are common on the muddy banks of the Minnesota River in Ft. Snelling Sate Park. Congratulations to Bren White and our generous patron Mike Holtz who correctly identified the species and which foot of four left this particular track.

This is the right front track of a river otter, This is the right front track of a river otter, Lontra canadensis.

As Mike shares:
"At first glance it looks like raccoon, but I think the carpal pad registering at the bottom of the track is helping tip it towards otter. The size is right, and the location certainly fits for otter."

Mike is exactly right here. And as he points out, the carpal pad is a key clue. Let's go through some of the other possibilities and see how we can distinguish this track from each:

Raccoon palm pads are more fully fused than mustelid palm pads and show less definition. The leading edge of the palm usually forms a smooth arc, rather than showing distinct lobes. In this track we can see several distinct, round palm pads. Raccoons do not have a separate carpal (heel) pad, seen in this track. Raccoon toes tend to register more uniformly along their length, and rarely show a bulbous tip.

Beaver have five toes on their front feet, but their toes taper from the palm to the tip, rather than appearing bulbous at the end. Their nails are more stout (they are a burrowing animal), and they have a different arrangement of pads. As in other rodents, beaver front feet have two heel pads. One of these is a carpal pad, and the other is the metacarpal pad at the base of toe 1. These two pads register roughly side-by-side and have a similar size and appearance. In this track, the metacarpal pad at the base of toe 1 is fused with the palm pads and the carpal pad registers as a separate heel. Finally, while toe 1 on beavers is more developed than in many rodents and has a claw, it is still much less developed than the other toes.

Muskrat hind feet are smaller, lack well developed palm pads, and don't have any pads on their heel. Their toes are lack a bulbous tip and are more slender. Though the fringe of hair can make them appear wide, they typically appear widest at the base.

Most often, otter tracks do not show the length of the toe this clearly—but while uncommon, this presentation isn't exceptionally rare. In this track, the structure of the palm pad and the distinct heel pad are some of our clearest clues that we are looking at a mustelid track. The size, habitat, location, and crisp tracks showing no signs of fur on the foot leave us with otter as our best candidate.

Congratulations again to Bren & Mike, and thanks to everyone who sent in guesses!

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August 2020 Natural Mystery

posted Aug 10, 2020, 10:34 AM by Jonathan Poppele   [ updated Aug 10, 2020, 10:34 AM ]

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July 2020 Natural Mystery Answered

posted Aug 10, 2020, 10:25 AM by Jonathan Poppele   [ updated Aug 10, 2020, 12:06 PM ]

A few years ago, I ran across a large collection of barred owl pellets in Ft. Snelling State Park. In dissecting them, I found nearly 20 small mammal skulls. Eighty percent of those skulls were from this species. By some combination of them being so common, their habitat being so exposed, and perhaps them being so tasty, they appear to be an extremely common prey animal for barred owls. Congratulations to Mike Holtz and one anonymous contributor for correctly identifying these common, tiny skulls to species. And an honorable mention to Joe Conrad who identified the genus based on this species' analog in the UK.

These are the skulls of meadow voles, Microtus pennsylvanicus.

Joe Conrad and Mike Holtz gets us started with our analysis. Joe began by thinking about the context--that these skulls came from an owl pellet:

"Field Voles [ed: Microtus agrestis, a Eurasian species] are abundant and common prey for owls. I presume that there is a similar vole in North America. So I searched for vole skulls. The features look quite similar to the Field Vole. Kind of striking lower jaw shape."

Mike began by comparing measurements, then looking at teeth and mandible shape:

"I started with a chipmunk, just to pick a random starting point. The sizes for chipmunk skulls and mandibles seemed higher than what's in the picture. Dropping down in size from there, the teeth on the mandible don't match for shrews. Looking at mice vs voles, the sizes matched pretty close for both, but the shape of the mandible and the teeth seemed to line up nicely with meadow vole."

Now let's talk about those teeth. Our ridiculously knowledgeable anonymous naturalist explains:

"Voles are herbivores, so they have miniature cow's teeth built to withstand all of the coarse plant material they grind up. Like a cow, the maxilla (upper jaw) is wider than the mandible (lower jaw), allowing the animal to chew with a significant amount of lateral movement. (It's better to grind your fibrous food than pound it.) You can see zig-zag ridges of dentin showing on the occlusal surfaces, pointing almost perfectly at right angles to the midline of the skull (allowing upper and lower molars to slide sideways over each other during chewing). The sides of the teeth are also deeply and regularly corrugated, and an individual molar appears block-shaped from gumline to chewing surface. These are characteristics of teeth that continue to erupt over the animal's lifetime. The continual growth enables the animal's upper and lower teeth to maintain contact even as they wear away over time.

Contrast this against Mus spp. [ed: Old World mice, such as the common house mouse], which have molars that resemble a human's. More discrete molars (when viewed from the side), often with a visible 'neck' at the bone-tooth interface, and more irregular "mountains" on the chewing surface. They are seed eaters more than plant eaters, so they do not have a diet that requires constantly-erupting molars.

It isn't a member of the shrew family either; these tiny little predators have molars that resemble tiny dog or cat molars, with sharp shearing peaks."

So it's a vole skull. But how can we distinguish which species of vole? There are four species of voles and lemmings found in east-central Minnesota: the Meadow Vole (Microtus pennsylvanicus), the Prairie Vole (Microtus ochrogaster), the Southern Red-Backed Vole (Myodes gapperi, formerly classified as Clethrionomys gapperi), and the Southern Bog Lemming (Synaptomys cooperi).

As our anonymous naturalist notes: "Based on range distribution, a vole in the Twin Cities area is most likely to be M. pennsylvanicus, the meadow vole." And since we are looking at a minimum of three skulls that all came from a single owl pellet, this is likely an extremely abundant species. The Meadow Vole is the most common and abundant vole in this part of the state. But there are some details on the skulls that can help us confirm this guess.

Myodes species such as the Southern Red-Backed Vole have narrower and more rounded zygomatic arches (cheek bones) than Microtus species. Also, the angular process (the lower-most rearward extension on the lower jaw) is shorter and more blunt. Synaptomys species such as the Southern Bog Lemming have more robust zygomatic arches that extend much wider than the brain case. Their angular process is also shorter and more blunt than in Microtus. The differences between Meadow Vole and Prairie Vole skulls are subtle. Perhaps the most distinguishing feature is the structure of the cheek teeth. According to Elizabeth & Charles Schwartz in The Wild Mammals of Missouri, Meadow Voles have five "islands of dentine" surrounded by enamel on the second molar of their upper jaw, while Prairie Voles show four.

Congratulations again to Mike, Joe and our anonymous contributor.

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July 2020 Natural Mystery

posted Jul 21, 2020, 11:20 AM by Jonathan Poppele   [ updated Jul 21, 2020, 11:23 AM ]

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June 2020 Natural Mystery Answered

posted Jul 8, 2020, 12:50 PM by Jonathan Poppele   [ updated Jul 22, 2020, 9:32 AM ]

We received a number of guesses about last month's Natural Mystery. Everyone who wrote in correctly identified this as the carcass of a semi-aquatic mammal. But there were a range of guesses about exactly who this unlucky critter was. Congratulations to Jeremy Britzius, Emily Goldberg & Kirsten Welge who correctly identified the species, and thanks to everyone for their thoughtful analysis of this mystery.

This is the carcass of an American Beaver, Castor canadensis.

Emily gets us started with with a basic analysis:

"It's clearly a mammal, and its fur has the dense, slick look of a water-dweller; in Minnesota, that leaves us with otter, beaver, or muskrat. Its front feet look small and don't seem strongly webbed (though it's hard to tell given their pose), which makes otter less likely, especially since otters are just less common in general."

Cheri Stockinger also notes that “Otters have long narrow slinky like bodies and short claws on wide stubby paws.”

That leaves us with muskrat and beaver—the two most common responses. Both of these aquatic rodents have relatively small front feet with long, stout claws that are well adapted for digging. Muskrat and beaver are quite different in size, but as Emily notes, “It's hard to judge size in this photo.” Distinguishing between the two from this photo is unquestionably tricky and many people who wrote in admitted they were essentially guessing.

Kirsten applied careful observation and some clever reasoning to determining the species:

"I realized I could use the duckweed for scale. I found a picture of duckweed on a muskrat. I'm noticing in this picture, the leaf size compared to the body of the muskrat is greater than what I'm seeing in the natural mystery photo. And if we want to quantify it, Texas A&M AgriLife Extension notes: 'Duckweed has 1 to 3 leaves, or fronds, of 1/16 to 1/8 inch in length.' If one leaf of duckweed is 1/16” to 1/8" in length, those feet are much larger than we'd expect for a muskrat.

Also, muskrat females show 8-10 nipples; beaver females only have four. The Natural Mystery carcass shows four. Beaver it is!"

Indeed, beaver it is. Or was, at least. I have found other sources that say duckweed fronds (apparently they are botanically distinct from leaves) can measure nearly 1/4", but even if we assume they are nearly that large, they offer enough of a scale to identify this as a beaver rather than muskrat. I'll admit, however, I did not notice the animals teats until I read Kirsten's response.

Everyone who wrote in also recognized this as the belly and front legs of the animal. The head, tail, and hind legs were all submerged. One of our contributors, who asked to remain anonymous, sent this exacting explanation of why the animal is in this posture in the water:

“Intestinal contents continue to ferment after death due to microbial activity, usually causing extensive abdominal distention (swelling due to internal pressure) and sometimes even over-extension of the back (which may be the case here since the pelvis is below the water surface). Since the thorax (upper torso) and the post-mortem ventral abdomen is more buoyant than the rest of the body, this animal is floating belly up. Head and back will be more dense relative to the distended abdomen.”

Congratulations again to Jeremy, Emily & Kirsten for identifying this carcass to species.

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Carlos Avery Story of the Day

posted Jun 29, 2020, 10:25 AM by Jonathan Poppele   [ updated Jul 22, 2020, 9:38 AM ]

With Cedar Creek Ecosystem Science Reserve still closed to all but essential research, members of the Minnesota Wildlife Tracking Project moved our Summer Survey to the nearby Carlos Avery Wildlife Management Area. Over the weekend of June 13 & 14, socially distanced trackers explored roads and trails in the WMA and brought back our stories to share. Here are a few highlights. You can see all of our observations from the weekend here on iNaturalist.

Perhaps the largest surprise of the weekend was the relative lack of mammal sign. In a typical survey at Cedar Creek, our teams often return with scores of observations of mammal tracks from more than a dozen different species. Over this weekend, we collectively identified the tracks of just 6 species of mammals, plus the sign of five more. For half of those species, we made only a single observation. Those observations included a single fox scat and single coyote scat, as well as these fresh weasel tracks that were disappearing quickly in the wind—suggesting that the weasel had come through only a few minutes before this observation was made.

As usual, our trackers located the digging signs of local fossorial mammals. We can identify pocket gopher activity like this to species, because the plains pocket gopher is the one member of the family in this part of the state. But it is not so easy to identify mole tunnels to species. We have both eastern moles and star-nosed moles around the metro area. We have similar challenges identifying other tiny mammals, such as voles and shrews, from their tracks and sign. For positive identification, we usually need a skull or a carcass. On this survey, we turned up this masked shrew corpse. Since shrews scurry about much like voles, but are apparently much less appetizing, it is not rare to find nearly intact bodies—perhaps discarded by a disappointed predator.

The one mammal that we did find abundant tracks for was the common raccoon. In particular, we found several spots where raccoons were raiding what appear to be bullsnake nests. We had found a dug-out snake nest once before on a survey at Cedar Creek. This weekend, we located more than a half-dozen. Bullsnakes are common in this area, as evidenced by the bullsnake trails we found crisscrossing many of the roads we explored. According to Moriarty & Hall's “Amphibians and Reptiles in Minnesota,” June is nesting season for bullsnakes and it seems that the raccoons were making quick work of these newly laid eggs—although in one of the excavated nests, we found an egg that was still intact.

Our team also recorded the tracks and sign of quite a few birds. Our findings included common and familiar species such as sandhill crane and American crow, but also a bank swallow nesting colony. As their name suggests, bank swallows historically formed their nesting colonies where erosion has carved sheer faces into the sandy banks of streams and rivers. The species common name, and Latin name Riparia riparia, recognize these riparian roots. Over the past century, the species has broadened its habitat to include vertical road cuts and the edges of gravel and sand mine pits, like this one. The nesting holes themselves are similar to those of kingfisher, but appear more oval. They are distinctly wider than they are tall and, like kingfisher burrows, have two "tracks" along the base where the birds feet appear to shuffle as the enter and exit.

Finally, our surveyors ran across a huge diversity of insects and insect sign. Most of us of course are not entomologists, or even bug collectors, so our ability to interpret what we are finding is modest. But one sighting that is worth sharing is our first confirmed sighting of antlion traps. Antlion larvae excavate conical traps in sandy soil where they capture unlucky ants and other hapless critters. They are common, wide spread, and a favorite question on CyberTracker evaluations. But they are something that our trackers rarely encounter here in Minnesota. On a couple of occasions, we have found conical depressions in the sand at Cedar Creek that looked like antlion traps, but we were unable to locate the larvae to confirm this. On this occasion, there was no doubt. We watched the larvae excavating traps and drag an ant down into the sand. We also dug one out to get a close look at this stuff of insect nightmares.

It has been a treat to explore Carlos Avery during these months when we are unable to track at Cedar Creek. But we are looking forward to returning to our usual routes before long. The University of Minnesota plans to resume in-person instruction in the fall, and we have confidence that we will be able to hold our Fall Survey at Cedar Creek – perhaps with some modifications.

Mark your calendars for the weekend of October 2-3 and stay tuned for updates as the start of the school year approaches.

Book Review: Tracking Mammals in the Northeast

posted Jun 10, 2020, 8:39 AM by Jonathan Poppele   [ updated Jun 10, 2020, 8:47 AM ]

In her book A Field Guide to Tracking Mammals in the Northeast, Linda J. Spielman tackles one of the persistent shortcomings of tracking guides. Animal tracks vary enormously in appearance. As she writes in the introduction, “real tracks frequently deviate from perfect tracks,” and “may appear like the tracks of a completely different animal.” No one illustration or photograph can possibly convey this variation. Most guidebooks include at most two or three images of the tracks for a given species. Spielman illustrates eight to ten examples for every species.

Spielman's high-quality illustrations capture the range and variety of track presentations in a way no other book does. Different drawings show different amounts of splay in the toes; tracks with and without claws, dewclaws or tarsal pads registering; and tracks that resemble those of other species. The illustrations don't show small details of track morphology as well as David Moskowitz (Wildlife of the Pacific Northwest), or Mark Elbroch (Mammal Tracks & Sign), but the range of presentations makes this book a valuable addition to the tracking literature.

Spielman's text is also first rate. Her descriptions of animal behavior are clear and accurate. Her commentary and interpretations show that she has a huge amount of "dirt time" under her belt. And despite her obvious expertise, she avoids the common pitfall of over-interpreting what she has seen and presenting conjecture as fact.

As with most regional guides, A Field Guide to Tracking Mammals in the Northeast is useful well beyond its geographic range. Most of the thirty-three mammals covered in this book are found across the continental United States.

Every book makes trade-offs, of course, and this book has its downsides. The paper is lightweight and porous. While the book is small and light enough to carry anywhere, it may not hold up well in the field. The book does not include images of the animals themselves. The track illustrations are not life sized with small tracks are shown larger than life and large tracks are reduced. There is little information about sign. And, of course, some people just find photographs of tracks easier to interpret than illustrations. With the exception of the books durability, these are minor issues. And even the durability shouldn't keep anyone from buying this excellent book.

For anyone who has puzzled over a track that just didn't look like any of the illustrations in the field guide, this guide may be what you are looking for. For the intermediate tracker, in particular, it can help avoid many common mistakes in identifying tracks in the field. And for anyone who already has a library of tracking books, I would consider this a worthwhile addition.

You can purchase A Field Guide to Tracking Mammals in the Northeast here on

Note: is an online bookstore with a mission to financially support local, independent bookstores. Founded as an alternative existing online book sellers, shares 75% of its profits with independent bookstores, affiliate publications and affiliate authors. By using the affiliate link above, a portion of your purchase go to both local bookstores and to help support the Minnesota Wildlife Tracking Project newsletter and website.

June 2020 Natural Mystery

posted Jun 9, 2020, 7:50 PM by Jonathan Poppele   [ updated Jun 10, 2020, 8:50 AM ]

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May 2020 Natural Mystery Answered

posted Jun 9, 2020, 7:46 PM by Jonathan Poppele   [ updated Jun 9, 2020, 7:48 PM ]

The all new Peterson Field Guide to North American Bird Nests by Casey McFarland, Matthew Monjello, and David Moskowitz is in press. But its release date is still nearly a year away. That didn't stop Cheri Stockinger, Kirsten Welge, Liz Jaeger, Matthew Johnson and Mike Holtz from correctly identifying the species that laid these eggs and sharing some insights into the color and why these particular eggs are so dark. An honorable mention also goes out to Maren Miller who made a compelling case for another species. Congratulations to all of you!

As Matthew Johnson notes for us, these are the “robin's egg blue” eggs of an American Robin, Turdus migratorius.

Liz Jaeger summarizes the features here that are characteristic of American Robin.

“The nest has some mud on the exterior and a light lining of grass on the interior. Eggs are smooth, glossy, and oval, and have a relatively consistent color (i.e. not mottled or speckled).”

American Robins are one of a handful of songbirds nesting in Minnesota that can lay smooth, glossy, unmarked, blue-green eggs. Others include the Eastern Bluebird, European Starling, Gray Catbird, and Blue Jay.

Mike Holtz notes that the “nest structure doesn't fit bluebirds or starlings.” As Matthew Johnson noted about Eastern Bluebirds, and Kirsten Welge pointed out about European Starlings, both are cavity nesters and neither use mud in nest construction. Neither do Blue Jays.

That eliminates all the candidates except for the Gray Catbird. And Maren Miller made a compelling case for this being a Catbird nest:

"Gray Catbirds are known to lay dark green-blue eggs, approximately 1-5ish per clutch, that measure approximately 1 inch in length. Obviously, these eggs are dark green-blue (not light blue, like a Bluebird's or an Egret's; and not overtly speckled, like a House Finch's); and the clutch size of 3 eggs is certainly reasonable. Although we don't know how long each egg is, we can consider them in relation to the height of the nest. Gray Catbird nests are generally about 2 inches tall, meaning that we could stack two 1 inch Grey Catbird eggs on top of one another lengthwise to hit the top of the nest. When I "eyeball" the eggs in relation to the nest, it looks as though two eggs stacked on top of one another lengthwise would approximately equal the height of the nest—not an exact science, but again, the Grey Catbird egg hypothesis seems feasible.

Nest: If this nest doesn't belong to a Gray Catbird, a Gray Catbird should probably consider moving in here, because it's just about ideal for this bird—from the materials used to build the nest, to the nest's structure, to its immediate location and surrounding habitat. Gray Catbirds build nests with an eclectic array of materials, including twigs, straw, mud, and reeds, and the interior of their nests are often tightly woven—all of which we see in this photo. Gray Catbirds also tend to build their nest in "shrubby" locations that are close to the ground (unlike Robins, who prefer leafy trees), and the photo seems to document a brush-y habitat. Finally, Gray Catbirds are a pretty common species of bird on/around Picnic Island, so it would certainly make sense to see a GC nest with eggs in this location."

Mike Holtz
says he thinks “catbird eggs would be even darker, but I'm not sure that's enough to discredit this analysis.

As it happens, this nest was not in dense shrub, but on the horizontal limb of maple with a large wild grape vine growing along side—though that is not evident from the photo. I have been researching the nests of these two species, and haven't found anything in my study to eliminate Gray Catbird as a candidate based only on this photo. I'd love to hear from you if you have insights on this.  What I can offer is verification that this is a robin's nest—as I watched the mother return shortly after I took the photo of the eggs.

Maren also had some good insights as for why these eggs are such a dark color, explaining:

"I once read that certain birds' eggs appear green-blue because of a green-blue bile substance called biliverdin that is present in the female. A darker green-blue eggshell suggests that there is more biliverdin in the female; and biliverdin is also an antioxidant, so darker green-blue eggs indicate that the female and her brood are healthy. Male Catbirds seem to know that darker eggs are healthier eggs, which works in the female's favor: males are more inclined to stick around and help the female raise their chicks if they sense those chicks and the female bird are healthy."

Mike Holtz added to this “The eggs could be a darker blue in a first brood, when levels of biliverdin are higher in the female.

All spot on. The blue-green color in the eggs of these birds comes from biliverdin which, as Matthew Johnson pointed out, is derived from hemoglobin. Levels of biliverdin are higher in healthy females, and tend to fall off with each successive brood over the summer.

Congratulations again to Cheri, Kirsten, Liz, Matthew and Mike Holtz for correctly identifying these eggs, and another shout-out to Maren for all the great insights.

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