It's not a skull, it's a synsacrum!

Reading wildlife tracks is one way to know who’s been around, but our neighbours of other species leave other signs as well. There’s scat, fur, feathers, nests, and dens, as well as more. . .unusual finds. 

Photo: A synsacrum (pelvic bone) of a bird from PEI. Photo credit: Beth Knox, used with permission.

Beth Knox took this fantastic photo of a bone she found in woods along the Cardigan River, PEI. It’s about 12 cm (4.75 inches) long; you can see White Pine needles in the background to give a sense of scale. These bones turn up occasionally, and you could be forgiven for thinking that it’s some sort of weird skull. 

It may look like evidence of extra-terrestrial life, but the owner of this bone was very much down-to-Earth. This is a synsacrum, a structure which today is found only in birds. Historically, it was also found in dinosaurs, giving us further evidence that birds are modern-day relatives of these prehistoric creatures. 

A synsacrum is part of the base of the spine. Those two parallel rows of holes are partially fused vertebrae, and the top of bone in this photo points toward the tail end of the bird. What may look like eye-sockets just below that are the sockets of the hipbones. To give this some context, at the base of our spine there is a sacrum (connected to the pelvis) and coccyx (commonly called the tailbone). In birds, that’s roughly where you find the synsacrum. 

Birds have many special adaptations to enable flight: hollow, lightweight bones; a keeled breastbone to allow attachment of powerful flight muscles; and a fast digestive system so food isn’t “on board” any longer than necessary, among others. Fused vertebrae such as those of the synsacrum help keep the body stiff and stable during flight without the use of muscles, which would add weight. 

The synsacrum isn’t the only area of fused bones in a bird’s body. The familiar “wishbone” is a fused collarbone, giving added support for the wings. Fused hand bones in the wing provide rigidity for flight. Most of the backbone is also fused, providing strength and stability at the cost of flexibility: birds can only bend at the neck, not throughout the torso like mammals can. 

The only outstanding question is: what bird is it from? Based on the size, likely a Great Blue Heron, which is consistent with other remains found in the area. All birds have a synsacrum, but usually only those of larger birds – Herons, Cormorants, and Geese, for example – are found.

Thank you to Beth Knox for this photo and permission to use it. A cool part of PEI Untamed!