I guess formal and informal refer to two types of essay - you can write a formal essay where you avoid conversational language and such and where you need to be very professional, or you can write informally where you don't have to think about such things, it is very casual.
Answer:
1) Their nest must be close: I've seen them fly into the tree next to my bedroom window
2) Their afternoons are spent soaking up the sun and enjoying each other's company.
3) I think it's amazing that a dog and some birds spend so much of their time together.
Answer:
However
Explanation:
Despite and in spite of is used when a subject is unaffected by something
