Slight correction to the previous post. A clicker is a conditioned stimulus (CS) that takes on the properties of a secondary reinforcer thru associative learning. Using CR will confuse the reader with the term conditioned response.
The need for a clicker in the field can be reduced by implementing a variable schedule of reinforcement. Of course a whistle can be used in place of a clicker, but there's no reason why an animal can't learn more than one CS
“Acquiring a dog may be the only opportunity a human ever has to choose a relative.” – Mordecai Wyatt Johnson