Extended Data Fig. 8: The function of tertial feathers.

In many modern birds, the tertial feathers (black arrows; red feathers, inset) are morphologically distinct. These feathers function as protection for the remiges from abrasion when the bird is not flying and are particularly developed in species that feed on the ground, such as (a) wagtails (Motacillidae; pictured, Eastern Yellow Wagtail Motacilla tschutschensis; photo by Y. Kiat), and (b) many shorebirds (Charadriiformes; pictured, Buff-breasted Sandpiper Calidris subruficollis; photo by I. Davies, ML110171881, adapted with permission from Cornell Lab of Ornithology | Macaulay Library). These feathers may also help close the gap between the secondary feathers and the bird’s body, thereby improving aerodynamic performance. Additionally, the tertials may be used for display and visual communication, as observed among some cranes (Gruiformes), such as the elongated tertials in the (c) Blue Crane (Anthropoides paradiseus; photo by M. McCloy, ML136331631, adapted with permission from Cornell Lab of Ornithology | Macaulay Library), or the prominent white tertials contrasting with the dark wing and body plumage in the (d) Pale-winged Trumpeter (Psophia leucoptera; photo by T. Palliser, ML62921801, adapted with permission from Cornell Lab of Ornithology | Macaulay Library).