[2] The skulls of halcyornithids bear a prominent shelf of bone above the upper margin of the eye, the supraorbital process, comparable to that present in falcons and other birds of prey.

One proposed function of the supraorbital process is mechanical protection of the eyeball from injury, for example, such as might be caused by struggling prey animals being killed with the beak.

[4] A high diversity of halcyornithid birds is found in the Eocene London Clay in England, from sites near Walton-on-the-Naze and the Isle of Sheppey.

Of the specimens collected from that location, almost none do not bear minor morphological dissimilarities to others, indicating that there were likely many species living in the area, comprising an adaptive radiation.

[5][1] Other halcyornithid-bearing sites in Europe include the Geisel Valley of Germany, which has produced the most recent evidence of halcyornithids, of Middle Eocene age, the Fur Formation in Denmark, and the Egem in Belgium.