Synsacrum

This stylised bird skeleton highlights the synsacrum


However concerning "non-avian dinosaurs":
http://www.reinhold-necker.de/seite14.html

The sacral enlargements of non-avian dinosaurs [non-maniraptor dinosaurs] do not share much structural similarities with the lumbosacral specializations of modern birds. Furthermore these species were quadrupedal and had a long tail, i.e. there was no need for a sense organ of equilibrium. It seems that early birds with a pygostyle and a well-developped synsacrum show structural specializations which resemble that of modern birds. However, there is a need for more details and more examples of well-preserved fossils.