Birds have light and fewer bones, unlike mammals. Mammals have strong but many bones when compared to birds. When it comes to flightless birds, they have different bones from the birds that can fly. Flightless birds like penguins do not have hollow bones. Their bones are filled and solid which makes them swim in the water easily.
Other flightless birds have marrow-filled bones like emu, kiwi, ostrich, cassowary, rhea, elephant bird and moa. They fall under the name ratites.
Penguins lost their ability to fly due to evolution and change in environment. Not every flightless bird has solid bones. The extinct flightless bird “dodo” is one of the examples which have mostly hollow bones.