One of the data points we take during prep is skull ossification. Baby birds start out with a thin single layer skull and as they age they develop a thicker two layers skull. Most orders of birds have a fully ossified skull when they fledge from the nest but passerines take a bit longer, so it can give us a lot of information about how old a bird is.
This Fox Sparrow died by hitting a window in December, and from the outside appeared identical to an adult bird, but because of the presence of a bursa near the cloaca and this partially ossified skull, I know it was a first year bird.
Incredibly, banders can determine skull ossification on a live bird by looking through the thin skin. I'm not entirely sure how they pull that off, because I typically have to remove the brain and look through the skull in the light to feel confident that I'm actually looking at a partially ossified skull. I do so many non-passerines that I don't get enough practice.
In the first photo, I circled the window section that appears transparent compared to the rest of the skull. I also included a diagram of the skull ossification process under the break.
