Look at Jon Skeet's article about Parameter-Passing in Java, which explains this.
In short (look at his site for a more throughout explanation):
Arrays are reference types. If you pass a reference that points to an array, the value of the reference is copied and assigned to the parameter of the function. So the parameter will point to the same array as the argument that was passed. Thus changes you make to the array through the parameter of your function will be visible in the calling function. Changing the parameter itself (b), for example by setting it to null, however, will not be noticed by the calling function, since the parameter (b) is just a copy of the argument (tiger) passed.
Integers are so-called primitive types. Passing the integer copies its value and assigns it to the parameter too. But that value is not a reference to the actual data, but is the data itself. So changes to the paramter in the function will affect the parameter (a), but not the argument passed in the calling function (bird).
