What is the role of birds in the ecosystem? What would happen if there were no birds?

To give an idea of what we are talking, birds suppose more than 9000 different species of animal, roughly they twice the number of mammals. They are fairly more easy of notice and watch that the vast majority of mammals in fact.

I will talk about the photo in the last lines of the answer. So, if you read about these Seeds of Sideroxylon grandiflorum. yes!, the answer is finishing.

In general the main role of birds in the ecosystems is being agents of dispersion.  Their ubiquoty make them useful as bioindicators, which is not a ecological role per se, but has been very useful to spread awareness about the importance of preserving our enviroment and the disastrous consequences of the overuse of some pesticides.

- Agents of dispersal of plants, but also of other animals: thorugh their activities birds spread pollen and pollinate different species, seeds, and even animals (Even animals can be spread.   Some wading birds relocate fish eggs that get stuck to their legs, thereby aiding in fish dispersal to other parts of a river or  marsh).
Examples:
  Songbirds spreading cereal seeds.
  Hummingbirds pollinating flowers.