These birds were created in the same way we build our feathered dinosaurs.  By using the fossil evidence we're able to reconstruct these in three dimensions.  The challenges with this project: the fossil material is all flattened like roadkill, so distortion must be compensated for and removed in constructing the three-dimensional animal.  For this project, there was a mural by Velizar Simeonovski that we needed to match in regard to coloration and overall form.  The biggest challenge was the raptor because of the limited post cranial material.