Call recognition using Digital Signal Processing
Basically it is a classification problem. You can want to classify the incoming voice.

There are many ways to tackle it. One of them would be to extract features ( by features, I mean frequency, pitch etc) and label it accordingly(if it is of bird or frog).

Then shortlist the features, which will help you differentiate between these two classes(bird or frog) and make a dictionary.

In your dictionary , it will be like this

Class
   features
                  Pitch   Frequency etc etc  


Frog        (P)  = 5     (F)=  10


Bird         (P)   =   1 (F) =       20



Whenever, your new sound comes in, you compute it features, and based on which feature it closely corresponds to (or there are many methods to do it) you choose the class.