Marie Read / American Avocet (Recurvirostra americana), pair copulating, Bear River Migratory Bird Refuge, Utah, USA, May.

Danny Green / Atlantic Puffin (Fratercula arctica) courtship display, rubbing bills, Skomer Island, Wales, UK, May

Brent Stephenson / Australasian gannet (Morus serrator) crossing bills as they display to each other at their nest site, Cape Kidnappers, Hawkes Bay, New Zealand, November.

Brent Stephenson / Australasian gannet (Morus serrator) pair crossing bills as they display to each other at their nest site, Cape Kidnappers, Hawkes Bay, New Zealand, November.

Marie Read / Horned puffins (Fratercula corniculata) pair interacting by touching bills while perched on cliff ledge, St. Paul Island, Pribilofs, Alaska, USA, July.

Rolf Nussbaumer / Sword-billed Hummingbird (Ensifera ensifera) male feeding from passionflower (Passiflora mixta), Papallacta, Ecuador, Andes, South America. January.

Brent Stephenson / Adult Black-billed gulls (Larus bulleri) with bills open, calling noisely whilst displaying at the breeding colony. Miranda, Auckland, New Zealand, October. Endangered species.

Brent Stephenson / Pair of Australasian gannets (Morus serrator) crossing bills during courtship display. Cape Kidnappers, Hawkes Bay, New Zealand, November.

David Tipling / Carved ivory of a Helmeted Hornbill (Rhinoplax vigil).

LYNN M. STONE / Royal Terns (Thalasseus maximus) in foreground with Caspian Tern (Sterna caspia) pair in background, note much more slender, more orange bills of Royal Terns, in breeding plumage, on shoreline of Tamp...

Laurent Geslin / Female blackbird (Turdus merula) catching worms on an urban lawn in London, UK

David Tipling / Huli Wigmen with Blyth's Hornbill bills as necklace decoration, Tari Southern Highlands, Papua New Guinea, August 2011