The British comic star splashed out £20,000 on the robotic menagerie after seeing how much Katy who turns 25 on October 25 enjoyed looking around a similar attraction at The Grove entertainment complex in Hollywood.
A source said: Russell took Katy to see Where The Wild Things Are at the Grove’s cinema.
She was really taken with the petting zoo temporarily set up next to the Farmer's Market. There are real animals in one section and then this amazing fake zoo for kids.
"You can milk the cow and ride on the pony. It's very fun.
Katy was keen to get close to the animals which are models brought to life using robotics but was worried paparazzi might take pictures of her.
Russell immediately decided to treat her to her own version for her birthday to ensure her big day is extra special.
The source added to Britain’s The Sun newspaper: While she was looking at a goat being unloaded Russell got the contact details of the event organisers.
"He has arranged for the zoo to arrive at Katy's Los Angeles home for her birthday.
Russell is currently visiting Katy in America, with the pair enjoying several nights out with friends over the weekend.
They visited Los Angeles trendy H.Wood nightspot, where Katy entertained her 34-year-old boyfriend by performing a raunchy routine to her hit I Kissed A Girl.
A source said: The tune came on and Russell was beaming with pride. Katy took advantage of the attention and she and a female friend started giving Russell a lap dance.
"Towards the end of the song the girls pulled each other close for a kiss. Russell's eyes lit up. But at the last second they pulled away and started laughing.
Katy and Russell were first romantically linked after the MTV Video Music Awards last month when they were seen kissing at Lady Gaga's after-party at New York nightspot Avenue.
