Although the park has foxes, Wood Mice and Brown Rats these are largely active at night and it is the Grey Squirrel is the most likely mammal for visitors to see.
The Grey Squirrel species was introduced from North America and has replaced our native Red Squirrel throughout much of the UK. Nevertheless, their athletic antics and begging for food are a constant source of entertainment to visitors.
St James's Park is a really great place to watch bats as dusk falls. About 20 minutes after sunset, dozens of Common Pipistrelles begin to forage for insects around the lake edge. Each of these bats may consume around 3000 mosquitoes and other small insects per night - so they are doing us a big favour. In autumn 2005, a Nathusius' Pipistrelle was also recorded near Duck Island, a rare find for Central London. Bats hibernate over winter - so the availability of undisturbed roosts in the buildings and trees in and around the park is of vital importance to them.