male fighting fish will built the nest for the female to reproduce..after builting the nest,this will attract the female fish to produce the egg inside the bubble..after that,the male will take care...
in dark places,a box is perfect, a good idea is to put newspaper on the bottom
in the wild however, they nest in tree cavities or dig out a tree stump. their nests are usally unlined.
Depends on what's going on with the birds. If they have eggs or babies, the reason is obvious. If no eggs/babies, some birds like to "hide out" and/or enjoy the comfort and security of enclosed...