Army ants and Antbird.
These organisms have commensalism which is an association between two organisms in which one benefits and the other derives neither benefit or harm.
Army ants (legionary or marabunta) travel on the forest floor and stir up many insects lying in the path. The ant follower birds (Antbird) follow these army ants and eat the insects that try to escape from them. Birds are helped by catching their prey easily and the ants are unaffected.
There are over 200 species of army ants that exist worldwide. They are found in the southern United States, but are more common in Central America, South America, Africa and parts of Asia so of course you would find the birds where you find the ants.
Where I found info/pics: http://www.buzzle.com/articles/examples-of-commens...
Wiki, and Google/Google images