They may have entered Noah's Ark two-by-two, but counting the animals at London Zoo is a much harder task.

On Thursday (January 3) keepers began the mammoth annual job of counting all the zoo's creatures.

London Zoo is home to more than 17,500 animals, birds, reptiles, fish and invertebrates, but some are much harder to count than others.

"The penguins and the meerkats aren't too bad but flocks of birds, especially in our free flight aviaries, they can be a little bit challenging, but this is about reconciling our records," said Zoological Director David Field.

"Every day the keepers come in and they check and count their animals and we just need now to make sure all of our data is correct, because it is not just about how many animals we've got, we also need to make sure who we have got," he said.

Zoos can play a vital role in captive breeding to help safeguard wild creatures for future generations.

"We often hear problems about animal's populations going down, worries about endangered animals and we are worried, but there is good news on the front. With the work of zoos, the conservation work of zoos, the conservation work that we do with many other governments, etc, there are chances that we can save habitat and save wild and the zoos will have animals to put back into the wild," he said.

All the data collected is sent to a global database to keep track of not only what animals are in captivity in responsible zoos, but also information about their lineage for potential future use in breeding programs.