Answer:
The correct option is an increase in the spider and insect population.
Step-by-step explanation:
As bluebirds feed on spiders and their major source of food, hence an increase in the population or decrease in the population of the spiders will heavily affect the population of the bluebirds.
If the population of insects and spiders increases, there will be enough food source available for the bluebirds due to which their population will grow in size. On the other hand, if the population of spiders decreases, then there will be competition for food and the bluebirds population will also start to decrease.