Your Wildlife Score grows massively as you level up and it becomes the main factor in your progress.

If you have 100 of the same eggs and you merge eggs and birds only in sets of 3, your Wildlife Score ends up at 58.

But if you merge them in sets of 5, that same 100 eggs will give you a score of 78.

As you continue merging and progressing, the scale of difference becomes even larger especially because Stage 5–8 birds contribute much higher Wildlife Scores.