In an environment in which insect populations are decreasing, which bird, small insect eater, cactus eater, seed eater, bud, leaf and fruit eater and what bids population might also decrease over time? why?

The population of insect eater birds are decreasing due unavailability of food.

Explanation:

If the population of insects in an ecosystem decreases, so the organisms which feed on them i. e. insect eater birds also decreases and die due to hunger. In an ecosystem, every organism is the food of other organism, if one organism is eliminated or reduced in population, then the other organisms will also be affected.