Answer:
C
Step-by-step explanation:
A, B, and D are all ways that the population can be limited. The number of predators in the area affects how many of the bluebirds will be eaten/ killed. The number of nesting sites affects how many bluebirds have the proper environment to reproduce safely, and this creates compitition between the birds, and only those that get adequate food can survive. that leaves us with C: the type of soil.