Analyze the provided tally marks to determine the number of students who chose each animal (cats, dogs, birds), identify the most favorite animal, and identify the least favorite animal.

Here are the answers based on the table:

1. How many students chose cats? The tally marks for cats are |||, which represents 3 students.

   $$\boxed{\text{3}}$$

2. How many students chose dogs? The tally marks for dogs are ||, which represents 2 students.

   $$\boxed{\text{2}}$$

3. How many students chose birds? The tally marks for birds are |||| with a diagonal line through them, which represents 5 students.

   $$\boxed{\text{5}}$$

4. Which animal was the most favorite? Comparing the counts: Cats (3), Dogs (2), Birds (5). The highest count is 5 for birds.

   $$\boxed{\text{Birds}}$$

5. Which animal was the least favorite? Comparing the counts: Cats (3), Dogs (2), Birds (5). The lowest count is 2 for dogs.

   $$\boxed{\text{Dogs}}$$