Donald M. Jones / American Cliff Swallow (Petrochelidon pyrrhonota) gathering mud for nest building, Montana

Donald M. Jones / American Cliff Swallow (Petrochelidon pyrrhonota) trio gathering mud for nest building, Montana

Matthew Studebaker/ BIA / American Cliff Swallow (Petrochelidon pyrrhonota) collecting mud for nest, Wyoming

Phil Savoie / Cliff swallow (Petrochelidon pyrrhonota) carrying nesting material over the Madison River, Yellowstone National Park, Montana, USA. July.

Phil Savoie / Cliff swallow (Petrochelidon pyrrhonota) carrying nesting material over the Madison River, Yellowstone National Park, Montana, USA. July.

Phil Savoie / Cliff swallow (Petrochelidon pyrrhonota) carrying nesting material, a feather, to the Gallatin River, Montana, USA, July.

Phil Savoie / Cliff swallow (Petrochelidon pyrrhonota) in flight carrying nesting material, Nez Perce Creek, Yellowstone National Park, Montana, USA, July.

Phil Savoie / Cliff swallow (Petrochelidon pyrrhonota) nesting colony, Yellowstone National Park, Montana, USA, May.

Phil Savoie / Cliff swallow (Petrochelidon pyrrhonota) gathering mud for nest building, Yellowstone National Park, Montana, USA.

Marie Read / Cliff Swallow (Petrochelidon pyrrhonota) adult in flight, Bear River Migratory Bird Refuge, Utah, USA, May.

Gerrit Vyn / Great Horned Owl (Bubo virginianus) adult female roosting in a cliff covered in Cliff Swallow nests (Petrochelidon pyrrhonata) Sublette County, Wyoming, USA, June

Gerrit Vyn / Cliff swallow (Petrochelidon pyrrhonota) at nest. Sublette County, Wyoming, USA. June.

Gerrit Vyn / Cliff swallow (Petrochelidon pyrrhonota) at nest. Sublette County, Wyoming, USA. June.

Alan Murphy / American Cliff Swallow (Petrochelidon pyrrhonota) flying, Texas

Charlie Summers / Cliff Swallow (Petrochelidon pyrrhonota) Aurora Parkway, Aurora, Colorado, USA. May.

Charlie Summers / Cliff Swallow (Petrochelidon pyrrhonota) banking in flight. Aurora, Colorado, USA. May.

Benjamin Knoot / American Cliff Swallow (Petrochelidon pyrrhonota), California

Jacob S. Spendelow / American Cliff Swallow (Petrochelidon pyrrhonota), Alaska

Marie Read / American Cliff Swallows (Petrochelidon pyrrhonota) flock visiting muddy puddle to gather mud as nesting material. Mono Lake Basin, California, USA, June.

Marie Read / American Cliff Swallows (Petrochelidon pyrrhonota) forced copulation attempt at muddy puddle where they are gathering mud as nesting material. Mono Lake Basin, California, USA, June.

Marie Read / American Cliff Swallows (Petrochelidon pyrrhonota) forced copulation (attempt at muddy puddle where they are gathering mud as nesting material. Mono Lake Basin, California, USA, June.

Marie Read / American Cliff Swallows (Petrochelidon pyrrhonota) forced copulation attempt at muddy puddle where they are gathering mud as nesting material. Mono Lake Basin, California, USA, June.

Marie Read / American Cliff Swallow (Petrochelidon pyrrhonota) fluttering its wings while it gathers mud in its bill to use as nesting material. Mono Lake Basin, California, USA, June.

Marie Read / American Cliff Swallow (Petrochelidon pyrrhonota) fluttering its wings while it gathers mud in its bill to use as nesting material. Mono Lake Basin, California, USA, June.

Marie Read / American Cliff Swallow (Petrochelidon pyrrhonota) fluttering its wings while it gathers mud in its bill to use as nesting material. Mono Lake Basin, California, USA, June.

Marie Read / American Cliff Swallow (Petrochelidon pyrrhonota) in flight. Mono Lake Basin, California, USA, June.

Sebastian Kennerknecht / American Cliff Swallow (Petrochelidon pyrrhonota) in partial mud nest on cliff, Santa Cruz, Monterey Bay, California

Sebastian Kennerknecht / American Cliff Swallow (Petrochelidon pyrrhonota) in mud nest on cliff, Santa Cruz, Monterey Bay, California

Donald M. Jones / American Cliff Swallow (Petrochelidon pyrrhonota) gathering mud for nests, southwestern Montana

Tom Vezo / American cliff swallow in mud nest {Hirundo / Petrochelidon pyrrhonota} Utah, USA

Tom Vezo / American cliff swallow mud nests {Hirundo /Petrochelidon pyrrhonota} Utah, USA

Tom Vezo / American cliff swallow at mud nest {Hirundo / Petrochelidon pyrrhonota} Utah, USA

LYNN M. STONE / Cliff swallow nest colony on cliff {Petrochelidon pyrrhonota} South Dakota, USA

Tom Vezo / American Cliff Swallow (Petrochelidon pyrrhonota) colony of mud nests, Bear River, Utah

Donald M. Jones / American Cliff Swallow (Petrochelidon pyrrhonota) gathering mud for nest building, Montana

Donald M. Jones / American Cliff Swallow (Petrochelidon pyrrhonota) gathering mud for nest building, Montana

Matthias Breiter / American Cliff Swallow (Petrochelidon pyrrhonota) resting on wire during rain storm, delta of Stikine River, Alaska

Barrie Britton / American cliff swallow attempted rape {Hirundo petro- chelidon pyrrhonota} USA Nebrask

Jack Dykinga / Cliff swallows (Petrochelidon pyrrhonota) skimming a small pond for insects, after monsoon rains transformed the Sonoran Desert, Arizona, USA.

Jack Dykinga / Cliff swallows (Petrochelidon pyrrhonota) skimming a small pond for insects, after monsoon rains transformed the Sonoran Desert, Arizona, USA.

Jack Dykinga / Cliff swallow (Petrochelidon pyrrhonota) skimming a small pond for insects, after monsoon rains transformed the Sonoran Desert, Arizona, USA.

Jack Dykinga / Cliff swallows (Petrochelidon pyrrhonota) skimming a small pond for insects, after monsoon rains transformed the Sonoran Desert, Arizona, USA.

Charlie Summers / Cliff swallow (Petrochelidon pyrrhonota) flying low over a lake, reflecting the brown hues of the hillside,North Park, Colorado, USA.

Charlie Summers / Cliff swallow (Petrochelidon pyrrhonota) gathering dead grass to take back to its mud nest, North Park, Colorado, USA.