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Search the site for 'synsacrum' and you will find a load of observations of these pelvic bones as skulls, with some useful information in the comments. They are the most confusing items - which is odd, as every Xmas turkey and Tesco frozen chicken has one! Check it out next time you are on poultry-carving duty (or ask a friend if you are veggie) - the bit of the back where the leg joins the spine.
recording wildlife with The Recorder's Year on www.hbrg.org.uk/TRY.html.
You could edit the observation into 'Birds'.
Lat/Lng: 55.2497, -1.5553
OS grid ref: NZ283951