Look who came visiting!

This lil un was found on our balcony. How it landed there, clueless. The lil sparrow's mother is unable to take it away. So have placed it in a cardboard box till it learns to fly. The mother is constantly chirping around.

What can I feed it till it remains my guest? I certainly would not want it to starve. Though the mother is around, she is away for quite long periods.