Very good question! Birds will come any feeders with the high-energy, high-fat foods that provide them with the crucial nutrition to survive. Whether it has a shell on it shouldn't effect the number or variety of birds that frequent a feeder.
Benefits of shell free bird seed
The most obvious benefit of shell free bird foods is that the birds are able to eat the food faster and then move to a safe location to digest. Predators won’t have that extra few seconds of distraction while they crack the seed to pounce. There is about twice as much seed in a bag without shells because you aren’t buying the shells. This means it should last twice as long in the feeder. And because no waste birdseed has no hulls, the seeds don’t sprout weeds under the birdfeeders. With less mess, rodents won't be attracted to the area.
A good no-mess blend may also contain nuts or hulled millet; beware blends which promise a "waste-free" experience but contain filler ingredients such as cracked corn, canary seed, or wheat. While these ingredients may not have a shell, they appeal to a very limited group of birds and will often be rejected by the species commonly found in our yards.
Both of the no-mess blends we offer have sunflower seeds and peanut pieces without the shell as the first ingredients. Sunflower for all the seed eating birds like finches and peanuts to attract all the bug eating birds like chickadees. Peanuts have a high calorie/ounce ratio due to its high fat and protein content to attract a variety of interesting birds.
Our #1 selling Wild Birds Unlimited No-mess LM has a Little Millet (LM) added to attract all the ground feeding birds like the juncos, sparrows, and doves. The hull is removed from the white proso millet so it can’t grow. It is 100% edible.
Our Wild Birds Unlimited No-mess NM has a chopped- up Nutty Mix (NM) added to attract more of the forest birds to the feeder. The Tree nuts include, but are not limited to, pecan, walnut, almond, hazelnut, cashew, pistachio, and Brazil nuts. Birds that would head for the peanuts and tree nuts are chickadees, woodpeckers, nuthatches, jays, wrens, robins, bluebirds, warblers and more.
I would recommend both blends highly! We sell at least a couple of tons a week.