Science - in an ecosystem, birds depend on insects to survive. if someone put pesticide to control?

The answer would be a decrease or increase in number of population but since pesticide was being introduced just to kill the insects so there would be a slight change in the dietary of the birds and a decrease of population but most of the birds eats only grasses so they will still lay eggs instead they will suffer from hunger with which adaptation may involve. In this case, there will be a balance of population since detrimental was not really the cause of hunger for some birds but its on adaptation.