Halcyornithidae

Halcyornithidae is an extinct family of telluravian birds thought to be related to the Psittaciformes (parrots), Passeriformes (songbirds), and to the extinct Messelasturidae. Halcyornithids have been found in various Eocene formations in Europe and North America. Widespread and diverse in the Early Eocene of North America and Europe, halcyornithids are not found in locales later than the Middle Eocene. Halcyornithids were small, arboreal birds with zygodactyl feet, with two toes facing forwards and two facing back, a trait shared with other tree-dwelling families of Eocene birds like the Zygodactylidae and the messelasturids. The skull of halcyornithids features a ridge of bone above the eye called the supraorbital process, similar to birds of prey. The relationships of the halcyornithids to other birds remain uncertain. Halcyornithids have been proposed as relatives to owls and as a lineage closer to parrots than to songbirds. Most recently, halcyornithids have been identified as the sister group of the clade including parrots and songbirds. It is also possible that Halcyornithidae is paraphyletic with respect to the Messelasturidae.

Description
Halcyornithids are recognisable by details of the skull and limbs. The tarsometatarsus, the lowermost bone of the leg, is short and shaped like those of parrots. The distal end of the tarsometatarsus has a projection of bone that supports the fourth toe, which is reversed and faces backwards. Unlike in parrots, however, this projection is not separated by a groove from the rest of the trochlea supporting the other toes. The third toe is strongly built, more so than the others. The humerus is long and slender, and the coracoid is shaped like that found in owls, and has a foramen for the supracoracoideus nerve.

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. The morphology of the head is overall similar to that of messelasturids, but in halcyornithids, the mandible is proportionally much longer.

Distribution
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. Halcyornithids are also known from the Messel Pit, in Germany, where specimens likewise exhibit diversity. 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. From the Menat in France is a skeleton tentatively believed to be a halcyornithid, which may also be of Paleocene age. In the North America, halcyornithids have been found in the Green River Formation of the US state of Utah as well as the Nanjemoy Formation of the state of Virginia.

Classification
Placement of the Halcyornithidae after Ksepka et. al, 2019.

Internal classification of the Halcyornithidae and Messelasturidae after Mayr & Kitchener, 2022.