Let's look at your sentence: "Hummingbirds can hover to sip flower nectar, fly backwards and upside down."
This sentence is mostly correct, but to make it clearer and grammatically perfect, it's better to add a little punctuation. Here's a good way to write it:
"Hummingbirds can hover to sip flower nectar, and they can also fly backwards and upside down."
We added "and they can also" to connect the ideas smoothly so it sounds nicer. The original sentence is understandable, but this version is clearer.