Marie Read / Wood Ducks (Aix sponsa), females behave aggressively toward a male (second from right) that is trying to join them on a floating log, autumn, New York, USA, October.

Marie Read / Canada geese (Branta canadensis) two during territorial fight in early spring, Ithaca. New York, USA

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

David Pattyn / Alpine ibex (Capra ibex) adult males fighting in alpine landscape, Valsavarenche, Gran Paradiso National Park, Aosta Valley, Italy, May

Alan Murphy/ BIA / Northern Caracara (Caracara cheriway) pair fighting, Texas

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

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

Markus Varesvuo / Jay (Garrulus glandarius), two fighting in mid-air with another observing. Norway. October.

Markus Varesvuo / Jay (Garrulus glandarius), two fighting in mid-air with another observing. Norway. October.

George Sanker / Arctic tern (Sterna paradisaea), two fighting in mid-air. Machias Seal Island, New Brunswick / Maine, Canada / USA. July.

Eric Baccega / Lioness (Panthera leo), aggressively rebuffing male lion banks of the Luangwa river, South Luangwa National Park, Zambia

Staffan Widstrand / Wild Wonders of China / Pere David's deer / Milu (Elaphurus davidianus), two stags testing their strength in Hubei Tian'ezhou Milu National Nature Reserve, Shishou, Hubei, China