Chinese artists perform on stage during the opening ceremony of the International Horticultural Exposition 2011 at the Exposition Park on April 28, 2011 in Xi'an, Shaanxi Province of China. The World Horticultural Exposition 2011 will be held in Xi'an from April 28 to October 22. (Photo by ChinaFotoPress/Getty Images)

A protester sprays paint on the window of a bank during a rally against Expo 2015 in Milan, Italy, April 30, 2015. The Milan Expo will open in the city on May 1. Officials are counting on some 20 million visitors to the six month-long exhibition of products and technologies from around the world. (Photo by Stefano Rellandini/Reuters)

A Siberian eagle owl, which can grow up to 71cm tall, is among a collection of 60 birds of prey at SMJ Falconry near Oxenhope, West Yorkshire, England. (Photo by Charlotte Graham)

Jan Agha, 49, an Afghan hunter, tries to catch his crane at a field in Bagram, Parwan province, Afghanistan on April 10, 2019. As the early morning light breaks over the plain north of Kabul, bird hunter Jan Agha checks his snares as he has done for the past 30 years, hoping to catch a crane, using a tethered bird to lure others down to the nets. (Photo by Mohammad Ismail/Reuters)

A robin perches on a spade handle in a garden on a rainy day in Lixwm, north Wales. Researchers said this week that when European robins are subjected to human-produced noise, their behaviour changes. (Photo by DGDImages/Alamy Live News)

A brown-hooded kingfisher sits on a branch with a freshly caught frog in it's beak. Riaan Marais who took the photo said, “We were doing a boat safari on the Rufiji river in Selous Game Reserve, Tanzania when I spotted the bird with the frog on an overhanging branch on the river bank. It most probably caught the frog in the reeds along the river’s edge as this is where these frogs stay. The kingfisher must have caught the Common Reed Frog just before I took the image, as the frog was still alive, I had a brief moment to take the image before the bird flew off with the frog”. (Photo by Riaan Marais/Solent News and Photo Agency)

The Spanish photographer Xavi Bou digitally combines sequential pictures of birds to create a single image, or chronophotograph, that reveals the shapes of their flight paths against Catalonian skies. His work shows the variety and beauty to be found in the daily activities of the local birds, including spiralling storks, swooping starlings and giddy swifts. Here: European herring gulls at Llobregat Delta. (Photo by Xavi Bou/Rex Features/Shutterstock)

A bodyboarder rides a wave at Mackenzies Bay on May 1, 2011 in Sydney, Australia. A strong east-south-east swell has developed along the NSW coastline with waves between 6ft and 8ft expected to hit the coast for much of the weekend and into next week. (Photo by Cameron Spencer/Getty Images)