Marie Read / American avocets (Recurvirostra americana) two during aggressive (territorial) encounter, Bear River Migratory Bird Refuge, Utah, USA, May.

Alan Murphy/ BIA / Baltimore Oriole (Icterus galbula) males fighting, Texas

Alan Murphy/ BIA / Baltimore Oriole (Icterus galbula) males fighting, Texas

Thomas Marent / Boulenger's Green Anole (Anolis chloris) male displaying, Tatama National Park, Colombia

Winfried Wisniewski / Hippopotamus (Hippopotamus amphibius) in territorial display, Mkhuze Game Reserve, KwaZulu-Natal, South Africa

Sean Crane / Northern Elephant Seal (Mirounga angustirostris) male calling in territoral display with breath visible, San Simeon, California

Alan Murphy / Clay-colored Thrush (Turdus grayi) and Great Kiskadee (Pitangus sulphuratus) pair fighting, Texas

Thomas Hinsche / Ring-necked Pheasant (Phasianus colchicus) male in terriorial display, Texel, Netherlands

Suzi Eszterhas / Hyacinth Macaw (Anodorhynchus hyacinthinus) pair fighting, Pantanal, Brazil.Sequence 3 of 4

Mark Raycroft / Elk (Cervus elaphus) bull approaching car and tourist in territorial display, North America, sequence 2 of 3

Vincent Grafhorst / Hippopotamus (Hippopotamus amphibius) bull marking territory by flicking dung, Chobe National Park, Botswana

Vincent Grafhorst / Hippopotamus (Hippopotamus amphibius) bull charging in territorial display, Botswana

Vincent Grafhorst / Hippopotamus (Hippopotamus amphibius) bull charging in territorial display, Botswana

Vincent Grafhorst / Hippopotamus (Hippopotamus amphibius) in territorial display, Botswana

Vincent Grafhorst / Hippopotamus (Hippopotamus amphibius) bull in territorial display, Khwai River, Botswana

Vincent Grafhorst / Hippopotamus (Hippopotamus amphibius) bull in territorial display, Khwai River, Botswana

Cyril Ruoso / Alpine Marmot (Marmota marmota) two adults fighting for territory, France

Cyril Ruoso / Mountain Gorilla (Gorilla gorilla beringei) large silverback continuing to display after attacking a tourist, Virunga Mountains, Rwanda

Tui De Roy / Greater Flamingo (Phoenicopterus ruber) pair in territorial display while two other feed, Galapagos Islands, Ecuador

Michael & Patricia Fogden / Southern Grasshopper Mouse (Onychomys torridus) male singing on its territory, Chihuahua Desert, Mexico

Sumio Harada / Red Squirrel (Tamiasciurus hudsonicus) mother protecting territory near her nest from other squirrels, Rocky Mountains, North America

Flip Nicklin / Narwhal (Monodon monoceros) males sparring over carcass of dead female, Baffin Island, Canada

Mark Raycroft / White-tailed Deer (Odocoileus virginianus) buck scent-marking, North America

Mark Raycroft / White-tailed Deer (Odocoileus virginianus) buck licking scent-marked branch, North America

Mark Raycroft / White-tailed Deer (Odocoileus virginianus) buck licking scent-marked branch, North America

Marie Read / Yellow-bellied Sapsucker (Sphyrapicus varius), male drumming, long exposure, New York, USA, April.

Marie Read / Red-winged blackbird (Agelaius phoeniceus) male calling and displaying, backlit, showing steamy breath on a chilly spring morning, Ithaca, New York, USA, April.

Jeff Foott / Black-tailed Prairie Dog (Cynomys ludovicianus) in territorial display, Wind Cave National Park, South Dakota

Thomas Marent / Boulenger's Green Anole (Anolis chloris) male displaying, Tatama National Park, Colombia

Thomas Marent / Boulenger's Green Anole (Anolis chloris) male displaying, Tatama National Park, Colombia

Winfried Wisniewski / Hippopotamus (Hippopotamus amphibius) in territorial display, Mkhuze Game Reserve, KwaZulu-Natal, South Africa

Duncan Usher / Ring-necked Pheasant (Phasianus colchicus) males in territorial fight, Texel, Netherlands

Duncan Usher / Ring-necked Pheasant (Phasianus colchicus) males in territorial fight, Texel, Netherlands

Duncan Usher / Ring-necked Pheasant (Phasianus colchicus) males in territorial fight, Texel, Netherlands

Duncan Usher / Ring-necked Pheasant (Phasianus colchicus) males in territorial fight, Texel, Netherlands

Alan Murphy/ BIA / Red-winged Blackbird (Agelaius phoeniceus) male calling in territorial display, British Columbia, Canada

Alan Murphy/ BIA / Arctic Tern (Sterna paradisaea) approaching in densive posture, Iceland

Sebastian Kennerknecht / Coyote (Canis latrans) pack in territorial howl in winter, Lamar Valley, Yellowstone National Park, Wyoming

Sebastian Kennerknecht / Coyote (Canis latrans) pack in territorial howl in winter, Lamar Valley, Yellowstone National Park, Wyoming

Chien Lee / Boulenger's Green Anole (Anolis chloris) male in territorial display, Utria National Park, Colombia

Mark Raycroft / White-tailed Deer (Odocoileus virginianus) buck rubbing antlers, North America

Mark Raycroft / White-tailed Deer (Odocoileus virginianus) buck rubbing antlers, North America

Alan Murphy/ BIA / Baltimore Oriole (Icterus galbula) males fighting, Texas

Alan Murphy/ BIA / Great Kiskadee (Pitangus sulphuratus) and Golden-fronted Woodpecker (Melanerpes aurifrons) female fighting, Texas

Nick Saunders/ BIA / Red-winged Blackbird (Agelaius phoeniceus) male in territorial display, Saskatchewan, Canada

Tui De Roy / Nazca Booby (Sula granti) trio facing off, Gardner Islet, Floreana Island, Galapagos Islands, Ecuador

Donald M. Jones / Red-winged Blackbird (Agelaius phoeniceus) male in territorial display, Troy, Montana

Thomas Marent / Snowy Egret (Egretta thula) pair in territorial dispute, Los Llanos, Colombia

Thomas Marent / Southern Lapwing (Vanellus chilensis) in defensive posture, Pantanal, Mato Grosso, Brazil

Thomas Marent / Southern Lapwing (Vanellus chilensis) in defensive posture, Pantanal, Mato Grosso, Brazil

Ralf Kistowski/ BIA / Pied Kingfisher (Ceryle rudis) females fighting for territory, Gambia

Ralf Kistowski/ BIA / Great Crested Grebe (Podiceps cristatus) pair fighting over territory, North Rhine-Westphalia, Germany

Gerald Haas/ BIA / African Fish Eagle (Haliaeetus vocifer) pushing off another individual in territorial display, Chobe National Park, Botswana

Steve Gettle / Red-winged Blackbird (Agelaius phoeniceus) male in territorial display, J. Clark Salyer National Wildlife Refuge, North Dakota

Murray Cooper / Andean Goose (Chloephaga melanoptera) trio in territorial fight, Argentina

Murray Cooper / Bearded Helmetcrest (Oxypogon guerinii) hummingbird in territorial display, Colombia

Murray Cooper / Collared Inca (Coeligena torquata) males in territorial fight, Ecuador

Murray Cooper / Collared Inca (Coeligena torquata) males in territorial fight, Ecuador

Murray Cooper / Collared Inca (Coeligena torquata) male and female in territorial fight, Ecuador

Richard Du Toit / Burchell's Zebra (Equus burchellii) stallion in territorial display, Rietvlei Nature Reserve, South Africa

Yva Momatiuk & John Eastcott / African Lion (Panthera leo) male and female in territorial call, Etosha National Park, Namibia

Yva Momatiuk & John Eastcott / Ostrich (Struthio camelus) males in territorial display, Etosha National Park, Namibia

Nate Chappell/ BIA / Booted Racket-tail (Ocreatus underwoodii) males fighting, Pichincha, Ecuador

Marie Read / Ovenbird (Seiurus aurocapilla), male singing in spring, New York, USA

Marie Read / Ovenbird (Seiurus aurocapilla), male singing in spring, New York, USA

Mark Raycroft / Black Bear (Ursus americanus) male scent-marking, North America

Mark Raycroft / Elk (Cervus elaphus) bull scent-marking, North America

Mark Raycroft / Black Bear (Ursus americanus) male scent-marking, North America

Richard Du Toit / Burchell's Zebra (Equus burchellii) stallion chasing rival, Rietvlei Nature Reserve, South Africa

Donald M. Jones / Elk (Cervus elaphus) bull in territoral display, western Canada

Donald M. Jones / Elk (Cervus elaphus) in territoral display, western Canada

Donald M. Jones / Black Bear (Ursus americanus) claw marks on tree, western Canada

Thomas Marent / Gelada Baboon (Theropithecus gelada) group in territoral fight, Simien Mountains National Park, Ethiopia, sequence 1 of 4

Thomas Marent / Gelada Baboon (Theropithecus gelada) group in territoral fight, Simien Mountains National Park, Ethiopia, sequence 1 of 4

Thomas Marent / Gelada Baboon (Theropithecus gelada) group in territoral fight, Simien Mountains National Park, Ethiopia, sequence 1 of 4

Thomas Marent / Gelada Baboon (Theropithecus gelada) group in territoral fight, Simien Mountains National Park, Ethiopia, sequence 1 of 4

Thomas Marent / Gelada Baboon (Theropithecus gelada) group in territoral fight, Simien Mountains National Park, Ethiopia

Thomas Marent / Gelada Baboon (Theropithecus gelada) group in territoral fight, Simien Mountains National Park, Ethiopia

Thomas Marent / Gelada Baboon (Theropithecus gelada) group in territorial fight with male, Simien Mountains National Park, Ethiopia

Sean Crane / Cheetah (Acinonyx jubatus) male marking tree, Ol Pejeta Conservancy, Laikipia, Kenya

Sean Crane / Cheetah (Acinonyx jubatus) trio marking tree with Burchell's Zebra (Equus burchellii) herd in background, Ol Pejeta Conservancy, Laikipia, Kenya

Tim Zurowski / Wrentit (Chamaea fasciata) in territorial display, California

Alan Murphy / Red-winged Blackbird (Agelaius phoeniceus) male calling in territorial display, Texas

Alan Murphy / Hooded Merganser (Lophodytes cucullatus) male in territorial display, New Mexico

Alan Murphy / Yellow-headed Blackbird (Xanthocephalus xanthocephalus) male calling durring territoral display, British Columbia, Canada

Tui De Roy / Marine Iguana (Amblyrhynchus cristatus) males battling for breeding territory, Floreana Island, Galapagos Islands, Ecuador

Thomas Marent / Eurasian Spoonbill (Platalea leucorodia) in territorial display, Cauvery Wildlife Sanctuary, India

Mark Raycroft / Elk (Cervus elaphus) bull bugling near harem, North America

Mark Raycroft / Elk (Cervus elaphus) bull approaching car and tourist in territorial display, North America, sequence 1 of 3

Mario Suarez Porras / Northern gannet (Morus bassanus) trio in territorial dispute, Saltee Islands, Republic of Ireland

Steve Gettle / American Toad (Bufo americanus) males calling to defend territory, Huron Meadows Metropark, Michigan

Duncan Usher / Great Crested Grebe (Podiceps cristatus) males fighting over territory, Germany

D. Parer & E. Parer-Cook / Cape Barren Goose (Cereopsis novaehollandiae) in fierce territorial display, Maria Island National Park, Tasmania, Australia

D. Parer & E. Parer-Cook / Cape Barren Goose (Cereopsis novaehollandiae) in fierce territorial display, Maria Island National Park, Tasmania, Australia

Filippo Nucifora / Grey Heron (Ardea cinerea) displaying at Coot (Fulica atra), Switzerland

Mathias Schaef / Common Starling (Sturnus vulgaris) pushing juvenile away from pond, Beindersheim, Germany