//first off in your mod_* file you'll add
public int birdSpawnrate = 5;
public int spawnCount = 0;

//in one of your methods
if(spawnCount > 40){
birdSpawnrate++;
//or
birdSpawnrate == 20;
//higher number = rarer spawns
}
//or whatever, if you want it mob specific or as a whole
//then in your getCanSpawnHere method for the mob you will do
    if(rand.nextInt(birdSpawnrate) == 0)
    {
    do the spawning stuff;
    spawnCount++;
    }else
    {
    return false;
    }

//in your onDeath, or when unloaded add
spawnCount--;