The seed distribution tower of an air seeder has a distribution head that splits a single, primary stream of conveying air and seeds into a plurality of secondary streams for delivery to openers that deposit the seeds into the ground as the seeder advances. The outlets through which the secondary streams exit the head are constructed as modules that can be individually installed and replaced without disassembling the entire head. Shut-off valves and/or flow sensors may or may not be incorporated into certain or all of the modules to provide optimum user flexibility.