Japanese keepers looking for a penguin on the run in Tokyo have stood down their month-long search but say they hope the bird's impending adulthood will give it away.

Staff at Tokyo Sea Life Park said Thursday they are no longer combing riverbanks every day looking for any sign of the creature, which fled captivity in early March.

"Although we believe the penguin is doing OK somewhere in a river near Tokyo Bay, we don't know what else to do after nearly a month of searching," the park's Takashi Sugino told AFP.

"Maybe it moved to an area far away from the park, in which case it's hard for us to find as Tokyo Bay is rather big."

Keepers have asked birdwatchers for help in tracking down the escapee but despite an initial flurry of news, they have received no credible information for some time.

"We hope to get fresh sightings in August, when the bird molts and its adult black-and-white feathers emerge because it will be easier for ordinary people to recognise it as a penguin," he said.

The hunt for the Humboldt penguin began after the one-year-old bird was snapped bathing in the mouth of a river.

Keepers believe the 60-centimetre (24-inch) bird made its break for freedom after scaling a rock twice its size, in an escape that has been compared with the exploits of animals in the animated hit film Madagascar.