For the same reason you do. Bluebirds eat to gain energy to complete the basic functions of life.
No, they do not.
Yes, the eat caterpillars.
No, they do not.
No, they do not eat mice. They eat insects, seeds, and berries.
Yes.
yes
Yes if they can.
no
I think that there are insects that the Spiders eat.
grass spiders eat other spiders
Eastern bluebirds are very nice birds and eat many garden pests.