All birdseed is not created equal.

Not all birdseed mixes are formulated with the birds in mind. Some low-cost mixes are made purely for profit without consideration of what birds want. Many contain seeds that the birds don't like — commonly called "fillers." The birds will knock these seeds out of your feeder onto the ground, causing a buildup of uneaten seed and unwanted sprouting because not even ground-feeding birds want filler seeds. Some commercial mixes are as much as 70 percent filler seed. So, a bag of birdseed that's cheap might not be a good value — it might be mostly filler.

Common filler seeds include milo, wheat, rapeseed, oats and canary seed. Beware of the generic ingredient "grain products," which is likely to be one or more of these fillers. Look at the label on your birdseed bag. By law, ingredients must be listed. You want to see black-oil sunflower as the first ingredient listed, and if you see any of the filler grains on the label, skip it.

Most birds that come to your feeder want black-oil sunflower, so that should comprise at least half your seed blend. House finches, grosbeaks, chickadees, nuthatches and other birds prefer it. The other main ingredient should be white millet. Millet will be kicked to the ground, which is good because it is the favorite of ground-feeding birds like juncos, towhees and doves. If you live in a heavily treed area, increase the percentage of sunflower and lower the percentage of millet. My rule of thumb is the more trees, the more sunflower. A mix of roughly 80 percent black-oil sunflower and 20 percent white millet is good for areas with lots of trees.

Generally speaking, if you find some millet goes uneaten on the ground under your feeders, use a heavier sunflower mix with less millet. Adding some peanut pieces is a nice high-fat treat that your jays, chickadees, titmice and others will appreciate. But it will add to the cost of your mix.

One note about millet: If you are in bluebird country and have trouble with house sparrows taking over your bluebird boxes, eliminate or cut back on millet, which is what house sparrows tend to like best.

Quality birdseed — birdseed your birds actually eat — can be expensive. Seed price is controlled by supply and demand. Recently, grain prices have increased significantly, and there seems to be no end in sight. I've been in the birdseed business for 17 years and have never seen a more volatile market.

Why so volatile? Many complicated global factors affect the price of grain, but one main influence is the price of oil. When gas and oil prices go up as they have in recent months, so does the demand for ethanol made from corn. As more farmers switch from raising black-oil sunflower to raising corn in response to higher ethanol demand, less sunflower and other birdseed grains are grown. Less sunflower grown means higher prices. Higher gasoline prices also mean higher freight costs — and birdseed is heavy.

Cooking oil also has an impact on birdseed prices. As more and more food manufacturers move away from partially hydrogenated oils toward natural oils like sunflower and safflower, a larger share of available sunflower is being bought up for the "crush market" to make cooking oil. Also, some major grain-producers are holding back birdseed grains from the market because they believe prices have not peaked. This shortage is also affecting prices.

As prices go up, some merchants who sell birdseed, especially the big-box stores, will begin to lower the weight of their birdseed bags or will add more lower-cost filler seeds like milo and wheat to their blends. These tactics help hide the cost increase, but they may not be good for your birds.

Anne Schmauss is the co-owner of Wild Birds Unlimited in Santa Fe. She is the author, with her sisters, of For the Birds: A Month by Month Guide to Attracting Birds to Your Backyard, and writes for Birds and Blooms magazine.