Answer:
the second one
Step-by-step explanation:
The topic sentence is the most important sentence found at the introductory part of a text. It highlights and summarizes the main point of the text. Sometimes, it is the first sentence in the paragraph but this is not always the case.
In the sentence above, the topic sentence is the second one because it states an important point about birds which might be the basis of the text. More details to support this important detail about birds will be found in the succeeding sentences and paragraphs.