Answer: stabilizing selection. Robins are apparently not ably to raise more than 4 chicks with much success. This is probably because of the size of the birds and the amount of food that two adults can provide. By comparison, most penguins can only raise one chick at a time, due to the size of the chick and the amount of food it requires. While they stabilized on different numbers, both are forms of stabilizing selection which maximize the fitness of the species in their environment.
Step-by-step explanation:
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