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March 2019 Natural Mystery Answered

posted Apr 8, 2019, 11:19 AM by Jonathan Poppele   [ updated Apr 18, 2019, 10:54 AM ]
Our March natural mystery was a tricky one, as small bird tracks usually are. In this case, it is possible to trim down the list of possible birds quite a bit based on when and where this photo was taken—in January in an urban area of St. Paul, MN. By making a list of possible birds, and comparing that to the size ranges in the literature, we are left with just a few candidates…

I'll start out by noting that this is an unverified track photo, so we may never be able to be 100% certain on the ID, but let's see how close we can get.

Kirsten Welge drew on her experience living in St. Paul to create a list of the most likely candidates: Northern Cardinal, American Robin, Black-Capped Chickadee, White-Breasted Nuthatch, American Crow, and Blue Jay. She measured the mystery track at 1 5/8” long, eliminated the corvids, robin and chickadee based on size, then selected the Northern Cardinal over the White-Breasted Nuthatch based on morphology. As she notes, “nuthatch shows a very skinny foot and a strong claw register – not a match for our mystery track.“

This is a good process, but there are a few other birds we need to consider before we conclude that these are cardinal tracks. Juncos, finches and even a few sparrows also frequent St. Paul in January. So let's take a further look at a more extensive list of candidates.

If we look on eBird for sightings in Ramsey County in January, we can put together a list of about 15 species of smaller passerines. Using size data from Elbroch & Marks Bird Tracks & Sign we can pare the list down even farther:

In our photo, the right track in each pair appears to be 1 1/2" long. Interestingly, the left track in each pair appears longer--about 1 11/16". Perhaps this is because of the way that foot drags the snow in the hopping gait, or perhaps the left foot is splaying more. In any case, we can use the range of 1 1/2” to 1 11/16” as a starting point.

Based on Elbroch's measurements, the tracks are too large to be a wren, goldfinch, house sparrow or chickadee. Though there are no data in the literature, comparing body to similar species we can likely eliminate American tree sparrow and pine siskin, both of which would also be unusual to see on a sidewalk in an urban area of St. Paul. The tracks are too small to be robin, starling, or blue jay.

That leaves house finch, purple finch, northern cardinal, dark-eyed junco, white-breasted nuthatch and white-throated sparrow as possibilities.

As Kirsten notes above, we can eliminate white-breasted nuthatch based on morphology.

While there is no published data for house finch or purple finch track sizes, we can make some guesses based on body size of these birds and comparing them to their slightly smaller cousin, the goldfinch. Assuming that these birds are proportional, we can compare various linear measurements such as body length, wingspan, and the cube root of body weight. I used data from the Cornell Lab's All About Birds website to do this, and found that house finch can be up to 10% larger than goldfinch and purple finch can be as much as 20% larger. This suggests that the tracks of these slightly larger finches would still be under 1 3/8”, making them too small for our mystery.

The tracks seem a little on the large side for any of our remaining candidates, but if we allow for the tracks appearing a bit large because of the snow, we can consider junco, white-throated sparrow and cardinal.

Dark-eyed junco tracks show slender toes and, of particular note, a slender hallux. These images appear to show plumper toe pads, particularly in the hallux, compared to junco. Elbroch also lists the hopping strides for juncos as 1-6”, while we were told that our trail showed hopping strides of 7-10”

White-throated sparrow also have a more slender hallux than cardinals, but the difference is less pronounced. Here again, Elbroch lists the hopping stride as 6” or less, where our bird was consistently hopping over 7”. We are also on the very edge of white-throated sparrow winter range, and they are a much less common sight in these urban areas compared to cardinals.

This leaves us with the cardinal as the last bird standing. The hopping stride fits Elbroch's data, the track size appears a tiny bit on the large side, but seems reasonable, and the morphology seems the best fit. Like finches, cardinals have a fatter hallux than sparrows and juncos do--particularly the proximal pad--a feature we see in these tracks. Although it isn't definitive, the clues we have available most strongly suggest that these are the tracks of a Northern Cardinal.

An Interesting Note: Elbroch lists the size of cardinal tracks as 1 1/4" - 1 9/16", which is still smaller than the measurement for the left side tracks--but he notes that his size range comes from a small data pool. If we the same calculations we used for the house finch and purple finch suggests a maximum track size for cardinals of at least 1 9/16" (the same as Elbroch's data pool), and possibly as much as 1 11/16" (the length I measure for the left track in this photo). But cardinals have a different enough body shape from goldfinch that I don't think this is more than a curiosity.

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