I looked up the ingredients to P/D dog food...
This is made by Science Diet

.

and

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It already has quite a bit of corn in it....corn is the first
ingredient...which means it is highest concentration!

Most canned dog foods have grains in them already.
So I am confused.

Are you going to get a cage now?

I bet the little guy was near death when you found him/her.
Now she/he is perking up and getting more normal.

I used toilet paper ripped into squares as a lining in the nest
for my sparrows. You just remove one layer as it becomes soiled. But soon you will need a cage.(I had the whole nest because it was rescued during a building being demolished. The workers handed the nest to me since I was just standing there watching. The sparrows were just hatched and had no feathers even!)
Also I used to give my birdies a bath in the sink. They loved that! shallow water, and a bit of splashing, and washing of the feet etc. Once Maxie wants to perch, a cage will be much better than a box. I had a parakeet as a kid, so I figured these birds would like a bath too, and they did!