Well, several statements circulating in the eagle-cam watching world assert that saliva and nasal fluids passed from a breeding parent to a new hatchling provide important benefits, including digestive enzymes, antibodies, and moisture to enable the chick to swallow food.
I had to find out if any of the claims are valid, so I took a deep dive into peer-reviewed research on the biology of avian saliva, nasal secretions, and digestive and immune systems, as well as embryological development of a chick.