[Exolent[jNr]]

You have to use that function to loop through all chicken entities.
Also, you need to specify quotes around the classname since it is a string, unless you have a variable chicken holding that string of "chicken".

Anyway, here is an example:

new entity = -1;

while ((entity = FindEntityByClassName(entity, "chicken")) > 0)
{
    // Do something with entity
}