Budgie worming under cap

[KiwiAndBubblesBudgies1]

A few weeks ago, my female budgie, Bubbles, got a strange habit. I normally let my budgies loose from their cage, either in a tent or directly in the house. They always stick near their cage and I discovered their obsession with chewing on clothes. Bubbles in particular loves my cap, so I often leave it on top of their cage so she play with it when she wants to go out (I leave their door open so they can go in and out). At first, she just climbed on it, bit it, and dragged it around. But then she started worming under it. I’m not sure why and whether it’s bad for her as she might squish herself, but she seems intent on doing it… here’s a picture