It's really situational. Don't ever feed any wildlife anything with milk, it can cause bloating so bad it can be fatal.
What you feed it really depends on what kind of bird it is. Certain birds eat berries, some eat certain vegetables, some only like insects etc. I use a soaked science diet cat food with a mix of vitamins and stuff for birds like mockingbirds and robins (in addition to insects and berries), I don't know if just any old dog/cat food would work.
Cages again are situational. I use wire cages for squirrels and juvenile opossums, but use glass reptile cages for gophers and mice. Always use t shirts or pillow cases as bedding for mammal because theit digits can get tangled up in the loops, resulting in them losing a digit.
Overall, people need to do research or call a wildlife center before picking anything up. Almost everyone has a smart phone so there is no excuse. People should not just be picking shit up and rehabbing it at home, because it will die.