What would happen if the population of the bird species shown in the ecosystem above where to set lee decrease?

In an eco-system, all animals and insects are important. Since there is no picture, I'll descibe what would happen. If a keystone species went missing or extinct, shortly after the rest of the ecosystem will callapse. The bird population would decrease because lack of food, shelter or environment, then those who feed on birds will slowly decline, then worms or insects that the bird was keeping in check, overpopulate. This can have disaterous consequences, not only for this specific ecosystem but for any.