Our August natural mystery proved to be a real challenge--and it is clear that some of you embrace challenges. Congratulations to Kirsten Welge for successfully identifying the original owner of this skull.
This is the skull of a Dark-eyed Junco (Junco hyemalis). Once again, I'll turn it over to Kirsten, for her excellent interpretation:
1" length. This is a tiny skull.
Very large eye sockets. The vertical bony partition between the eye sockets is delicate and mostly open, with a slender horizontal bar towards the top of the sockets.
Beak length is about 1/3 of total skull length and continues the downward slope of the anterior braincase.
The nostrils are teardrop shaped, about 1/3" the length of the beak, and placed high on the rostrum.
Provenance: Urban St Paul, in pile of dark, weathered feathers under a linden tree.
Process of Interpretation:
Skull size pointed me towards a small passerine.
With the location and associated dark, weathered feathers, I immediately thought of dark-eyed juncos. I've regularly seen these urban winter residents flocking and feeding on the ground in similar locations.
I pulled out Elbroch's Animal Skulls: A Guide to North American Species, and flipped to the "Life Size Bird Skulls" table (p 179).
Using skull size, we have six candidates. I compared features seen in the illustrations to our mystery skull:
Cliff swallow (1-1.1" length): this would be a very odd place to find a cliff swallow carcass, as this species frequents bridges, not grassy tree-lined medians. Additionally, this species' skull is more streamlined (vertically flattened), and the beak does not exhibit the strong downward slope seen in this specimen.
Yellow warbler (1-1.2" length): The beak is roughly half the length of the skull, and the cranium looks more streamlined than our specimen.
American goldfinch (.8-1" length): This species is also very common in this area. However, the partition between the eye sockets is nearly opaque, with only a small partial-width hole towards the posterior of the sockets. The feather pile would also not match the plumage of this species.
Ruby-crowned kinglet (.9-1.1" length): The beak is long, straight, and nearly half the length of the total skull, with long nostrils. The partition between eye sockets is even more open than in the mystery skull.
Black-capped chickadee (.9-1.1" length): The beak looks short and stout, measuring less than a third of the total skull length. The partition between sockets is also opaque for the lower half of the sockets, unlike the mystery specimen.
Dark-eyed junco (1-1.2"): Illustrated features match the mystery specimen. Plumage and behavior would also match this species."
Exactly right. Kirsten deftly avoided an easy pitfall in her interpretation--presuming that the date the skull was found correlates with when the bird was present. Sign often persist longer in an environment than tracks do, and this can be especially true of skulls. Under the right conditions, skulls may appear nearly unchanged for months or even years. This bird may have died last fall and been hidden under leaves and snow until its remains were spotted in June.