I saw two birds land on the roof of our porch. One bird had a twig in its beak. The other bird had a piece of string.

Which sentence could be added to the passage to best vary the sentence pattern and affect the fluency?

I think they were a male and female robin.
Then they flew to the corner of the porch.
I sat quietly and watched as they began to build a nest.
Without a care, the little carpenters got to work on a nest.