Don't know why they don't eat the ants. Good question. But if you watch them sitting in a bush or tree or on a powerline, just keep watching, eventually you will see them dart up into the air & then return to their perch --- this will happen over & over again. The insects are too small for us to see cause usually we aren't close enough. Now, don't quote me but if I remember correctly they eat 25% of their weight in insects. That's quite impressive! And mosquitoes are on their diet.
