Identifying Skeletal Remains of a Common Loon
While out tracking with the Earth Tracks Widllife Tracking Apprenticeship along a stretch of beach at Lake Huron at Saugeen First Nation we came across the fairly decayed carcass of a medium sized bird.
The surrounding area was all rocky with some Common Silverweed (Argentina anserina) flowers coming up amidst the corpse, and what looked liked Canada Goldenrods (Solidago canadensis) growing around. A couple meters away there was Eastern White Cedar (Thuja occidentalis) and some Trembling Aspen (Populus tremuloides) stand nearby. The substrate was all pretty rocky on this bit of a spit out into the lake.
Most of the feathers which were touching the ground had begun mouldering and decaying, with the colour fading considerably. We could tell that there was both light and dark feathers, but mostly it seemed like their were lighter feathers on the body. Could it have been a Herring Gull (Larus argentatus)? The overall body size, as observed in this state of decay, was approximately the same size as a Herring Gull, and it would not be strange to find a common bird in this particular area.
I started to look for the skull as the bill would help me indicate the species, and when I found it I realized that Herring Gull wasn’t a correct i.d. for their bills are yellowish with a red spot on the bottom mandible near the distal end. The distal end of the upper mandible also curves downwards. This skull was black with no speacialized spotting though it did look a little worn by weather. There was no downward curved end on the upper mandible either.
Herring Gulls nares (nostrils) are thin and located midway along the upper mandible, while on the skull we found the nares were pretty large and located closer to the eyes. It was also a relatively large bird skull overall with a broad cranium and long bill. The overall length was about 15.5 cm (6⅛ in) long. Someone guessed perhaps the skull was from a Great Blue Heron (Ardea herodias), but I remembered the skull from a juvenile Great Blue I have at home is a bit longer and narrower overall.
Other possibilities were shot back and forth such as Pileated Woodpecker (kk), Belted Kingfisher (kk), and Black-crowned Night Heron (Nycticorax nycticorax). I think we checked a couple of these in Bird Tracks and Sign by Elbroch, Marks, and Boretos (2001) but none were definitive. It would have to be something to look up a little bit more, but probably at home. This meant taking some bubble wrap from Alastair, wrapping the skull, and gently packing it away in my little lunch container along with the coracoid bones for later examination.
According to Animal Skulls by Mark Elbroch (2006) mature Great Blue Heron skulls range between 19.3 - 22.2 cm (7⅝ - 8¾ in) long overall and 3.3 - 3.7 cm (1¼ - 1½ in) wide. The juvenile skull I have at home is about 18 cm (7⅛ in) long overall and 3.35 cm (1⅜ in) wide. The skull we found was again about 15.5 cm (6⅛ in) long and 4.7 cm (1⅞ in) wide (with a cranium about 3.3 cm tall), being shorter than a Great Blue, but a lot wider making for a more robust skull than the Great Blue overall. It is worth noting that in my examining I did notice some similarities between the skulls.
Both of the skulls were sporting long bills with long wide nares (nostrils). They also both had occipital complexes (bones on the back end of the skull) which protruded well beyond the cranium. The interorbial fenestra (hole in the bone between the eye sockets) was also large in both skulls. But there were also some noticeable differences. The nares on the new skull were longer, and there was a hole which wasn’t present in the Great Blue, on the lower mandible below and behind the formerly mentioned hole.
Additionally, on the top of the newly found skull, there are two symmetrical grooves which run from just posterior of the end of the bill, along the anterior edge of the frontal bone and the rim of the orbits (eye sockets) down to the post-orbital processes (small protrusions behind the eye sockets). These two grooves also each have a small hole in them close to the bill. I didn’t notice these grooves and holes in the field but I did photograph them.
The grooves on the top of the skull along the edge of the frontal bones were never apparent in any of the Night Heron images.
I had originally thought this skull was from a Black-crowned Night Heron, to the degree that at one point I had written a paragraph in this post where I wrote that I was feeling fairly certain. But I couldn’t reconcile a couple of things so I dug deeper and as I did so, I began feeling less and less secure in that identification. I ended up getting into some of the textbooks I have when I came across images of the coracoids of Black-crowned Night Herons and they just didn’t match up. I ended up taking a pause from writing this up and decided to look for a different possible species.
I was just throwing darts at this point. With dozens of tabs open, scrolling through hundreds of photos of skulls, sternums and coracoids, and a growing mess of texts piled around me on every surface within reach of the couch, while fragile delicate bones sat mutely waiting on the cushion next to me waiting to be recognized… I was having a ton of fun but I also wanted to figure out the mystery.
The revelation came in a couple of ways. I was flipping through my own photos from the day of the outing when I noticed I had taken a pretty good photo of the sternum. I noticed first how it wasn’t shaped like a heron of any kind and was oddly sharp looking. I then studied the photo of the synsacrum which was very narrow, laterally compressed compared to many other birds. These were clues I held close as I started flipping through the bone collections of birds at the Idaho Virtual Museum. When I came across the entry for the Common Loon (Gavia immer) things started to click. There were photos of the skull, with the grooves running along the top. The bill shape and nostril lengths looked just right; the sternum had that same sharp look and the synsacrum looked right on as well.
I visited another source, Skullsite.com (developed by The Experimental Zoology Group of Wageningen University). Their measurements for Common Loon are:
Length : 164 mm
Length (cranium) : 66 mm
Width (cranium) : 49 mm
Height (cranium) : 38 mm
The skull we found:
Length : 155 mm (6⅛ in)
Length (cranium) : 64 mm
Width (cranium) : 47 mm (1⅞ in)
Height (cranium) : 33 mm (~1¼ in)
Now, I am again feeling pretty secure about this identification but I wrote this before in regards to the Black-crowned Night Heron. But what about the coracoids? Do they fit under close inspection?
Many folks reading this post will remember that I am pretty excited about learning to identify bird carcasses based on the shape and size of coracoids. I know this is a very tricky practice, and often we can only get down to family groups, but even that would help differentiate between a heron, a gull, and a goose. If you want to read more about coracoids check these out (1)(2).
For a quick review of how to measure a coracoid:
First, measure the greatest length (GL) of the coracoid. The GL is determined by determining the two most opposite points and measuring from there. It’s not about trying to find the middle of the base or from a specific location on the bone, but more so just figuring out the greatest length that the bone can be measured and using that number.
Second I measure the length of the medial side (Lm). This requires a bit more precision. First you’ve got to figure out which is the side of the coracoid which would be facing towards the midline of the body of the bird. This is the medial side (medial just means “towards the middle”). Next find the bottom inside “corner” of the bone. This is called the “internal distal angle” (located on the second image with a red asterisk). Measure from the point where the asterisk is, again, the internal distal angle, all the way up to the top of the bone. This measurement is your Lm.
Basal breadth (Bb) is a bit simpler. Just measure the distance between widest points of the bone at the base. That’s it.
For the last one, the breadth of the articular facet (Bf), you’ve got to locate the shallow groove where the bottom of the coracoid would articulate (meet or join) with the sternum. This is called the articular facet or the sternal facet (I used the phrase sternal facet in my previous post on coracoids). Measure the length of this facet. What’s a facet? A facet is the smooth area where two bones come together, often bordered by ridges or protrusions which allow the surfaces to fit together snugly without shifting or slipping beyond the functional limits of the joint.
Left and right coracoids from the skeletal remains of the then unknown bird. ~7 cm (2¾" in) long.
Here are the measurements I got from the coracoids, right and left:
Greatest length :
R : 73 mm (2⅞ in)
L : 71 mm (2¾ in)
Length of the medial side :
R : 61 mm (2⅜ in)
L : 58 mm (2¼ in)
Basel breadth :
R : 39 mm (~1½ in)
L : 38 mm (~1½ in)
Breadth of the articular facet :
R : 32 mm (~1¼ in)
L : 31 mm (~1¼ in)
According to Avian Osteology by Filbert, Martin, Savage (1996) the measurements for the Common Loon are :
Length Range : 67 - 79 mm
Breadth Range : 28 - 37 mm
Now I am unsure which breadth is being referred to in the range described above, but the breadth of the articular facet would be a match. They also write that the procoracoid process strongly hooked which can be seen in the photo above, especially on the right. They mentioned a small opening at the base of the procoracoid process as well, which is evident in the hand but not shown in the photo above. I wish I could find more information to support the coracoid ID, but that might have to wait.
Image of a mounted Common Loon skeleton with coracoids in situ, anchored into the top of the sternum and bracing the humerus. Note the articulation with the furcula (“wish bone”) at the head of the coracoid. From https://sketchfab.com/3d-models/mounted-loon-skeleton-a784da638bf4450e8703819a1d14f2cb
The last thing I needed to confirm/understand/look up was those grooves in the skull above the eyes. It is often remarked that the skull can teach us more about the natural history and ecology of an animal than any other part of the body so I wanted to figure out what this characteristic was all about and what we can learn from it?
Turns out this groove is not unique to Loons, but instead may be common in other birds such as petrels, penguins, albatrosses, gulls and terns. They are sometimes called the supraorbital (“above eye”) grooves. Why would these birds share these supraorbital grooves? What do they have in common? A life by or on the sea - on saltwater! With all of the fish they eat, and water they have to drink, these birds need to expel the salt before it builds up too much and takes a toll on their systems. These grooves are actually spots where a salt-secreting gland which acts similar to kidneys sits and helps the birds expel salt from their bodies. The salt moves through these glands and then drips onto the bill where it can then drip away. For petrels, the gland drips into the nasal passages where the birds can then sneeze out the excess salt! I think this is pretty damn cool, and weird cool facts help me remember stuff better. Thank you weird salt glands!
There was so much else I could have written about from our outing; Northern Flicker (Colaptes auratus) kill site, the unknown pellets we found, the other bird skull we found, the Bald Eagle (Haliaetus leucocephalus) feathers, the Common Grackle (Quiscalus quiscula) tracks - all this within the first two hours! But I wanted to dig deep into this one find and try to learn more. I hope you all learned something too. Big thanks to the Common Loon, to Saugeen First Nation, to all of the authors who put in the time to write these books and make these websites so we can learn, and to Alexis Burnett and all who came out to learn and track together. I’m stoked to get to be a part of it.
To learn more :
Animal Skulls by Mark Elbroch. Stackpole Books, 2006.
Skullsite.com page on Nictocorax nictocorax (run by The Experimental Zoology Group of Wageningen University)
3-D image of Black-crowned Night Heron
Idaho Virtual Museum bone collections
Idado Virtual Museum 3-D image of Common Loon skull
Two Coracoid Bones blog post
Two More Coracoids blog post
Avian Osteology by B. Miles Filbert, Carry D. Martin, Howard G. Savage. Missouri Archaeological Society, Inc., 1996.
Mounted Common Loon skeleton in 3-D from RISD Nature Lab
Manual of Ornithology by Noble S. Proctor & Patrick J Lynch. Yale University Press, 1998.