The pattern, pace and extent of the evolutionary radiation of modern birds (Neornithes) by the end-Cretaceous (65 Ma) has long been debated. Well-dated, taphonomically understood and phylogenetically constrained fossil birds from both sides of the Cretaceous-Paleogene (K-Pg) boundary are required to quantify the shape of this radiation, but have largely been lacking. Here we report on a large collection of fossil birds from the Lower Eocene of Denmark (ca. 54 Ma) that includes three-dimensionally preserved, articulated specimens from carbonate concretions as well as skeletal imprints and feathers. These birds are from a marine diatomite sequence (the Fur Formation), a low-energy deep-water preservational environment unique to the Cretaceous and Paleogene avian fossil record. We present taphonomic and palaeoecological information gleaned from these birds that in combination with phylogenetic data have implications for unravelling avian survivorship across the K-Pg boundary as well as for the pattern of the neornithine evolutionary radiation.