Also, I think it's good to point out that のは always needs a context. For example, if, out of the blue, I said:
今朝食べたのはパンです。 "The thing I ate this morning was bread."
That would be a little strange. The grammar is perfect, but it is a bit unnatural. The reason is because の is a stand-in for some kind of noun.
If we have a context, using のは is perfect and natural.
A: 最近食べているものは何ですか？ "What things are you eating recently?"
B: ええと、今朝食べたのはパンです。"Umm... the thing I ate this morning was bread."
Person A uses the noun もの, and in the reply, person B uses the particle の to mean もの.
In a nutshell, のは is most natural in a Q and A situation, when you want to reference the noun that the asker used in your reply:
What animals do you see out the window?
The one I just saw was a bird.