A number of hitherto undescribed fossil bird remains have been recovered from the Lillebælt Clay Formation of central Denmark, which is early-middle Eocene in age (~50 to 43 mya). The core of the material consists of fossils acquired through the Danish ‘Danekræ' fossil treasure trove legislation. Almost two-thirds of the fossils are isolated skulls preserved three-dimensionally in clay ironstone concretions; bird fossils of this age and degree of preservation are extremely rare in an international context.

A preliminary investigation has revealed the presence of at least one odontopterygid, a member of the extinct 'pseudo-toothed birds' and the first representative of this group known from Denmark. Other taxa present include remains of Lithornithidae and a new taxon possessing a massive, psittacid-like beak.

The Lillebælt Clay Formation birds are temporally placed just after the Early Eocene Climate Optimum, a period of elevated temperatures resulting from rapid greenhouse warming. Comparison of the new bird fauna with the recently revised fauna from the older (54 mya) Fur Formation of Denmark, represents a unique opportunity to investigate the effect of the prehistoric greenhouse warming within a single zoological group in a clearly delimited biogeographic area.