When you feed birds, it is important to keep feeders and the ground around the feeders clean.
Feeders should be cleaned no less than every other week (I would suggest making it a weekly chore). Wash them thoroughly with hot, soapy water and allow to air dry before refilling.
You can reduce the mess under your feeder (this is under one of my sunflower feeders after just a couple of days) by feeding seeds without the hulls. This is more expensive per pound in the bag, but considering the weight of the hulls on the ground and the chore of cleaning up, the extra cost is probably worth it. However, as much as I feed, I have always felt the birds should have to do some of the work by shelling their own seeds. Seeds on the ground under the feeders get wet and moldy, and not good for the birds.
Around January I'll begin getting calls from people who tell me they have a bird (usually a dove) who monopolizes a feeder and eats constantly. We will also get emaciated birds people bring in that they insist have been eating like pigs, but just fell to the ground. We will also start getting calls from people asking about blind birds (usually finches) at their feeders.
These birds have trich (doves) or conjunctivitis (finches). These are common, highly contagious diseases that are spread by contact, often with feeders and food the infected birds have been around. This is the reason it is so important to clean your feeders and feeding areas. If you have an outbreak of trich or conjunctivitis, take your feeders down and clean them thoroughly (in this circumstance, I would also rinse in bleach water, allow to dry and then rinse in clean water), clean up the area, and do not rehang the feeders for 10 days. This will allow the diseased birds to die off and not reinfect your feeders as soon as you rehang them. This seems cold-hearted, but is necessary to keep infection from spreading.
Enjoy feeding your backard birds, but keep the area clean.