Your sentence is great and very clear! Let's look at how we can make it perfect:
Original: Dolphins are smart! They even talk by using clicks and whistles.
This sentence is grammatically correct, but to make it sound a little smoother, you can say:
Dolphins are smart! They even talk using clicks and whistles.
Here, we simply take out the word 'by' before 'using' to make it flow better.
Great job talking about dolphins!