A few weeks ago I noticed that my backyard bird feeder was running low on seed – time for a refill. The feeder wasn’t completely empty, however: At the bottom there remained a large pile of one particular type of seed that the local birds had evidently passed over in search of other, tastier morsels.

The seed in question was round and reddish brown, about half the diameter of a pea, with a black dot at one end. It was familiar enough – you see it in just about any bag of bird seed mix you can buy at the store – but I realized I didn’t know what it was, or why the birds wouldn’t bother with it.

A quick Google search brought a quick answer. The mystery seed was called milo, the grain harvested from the sorghum plant. In the United States it’s typically used for livestock feed and ethanol production, but some of it goes to human and pet consumption as well. It’s also a common ingredient in wild-bird seed mixes, including the one I’d been pouring into my feeder.

There’s just one problem: Most common backyard birds won’t eat it. In its backyard bird feeding guide, for instance, the U.S. Fish & Wildlife Service notes that no birds appear to like it.

Cornell University’s Project FeederWatch, which recruits backyard birdwatchers to assist in annual bird population counts, recommends that feeders avoid “mixtures that have a high percentage of less-appealing ‘filler’ seeds such as red milo.” Even the Wild Bird Feeding Industry (WBFI), a trade group for bird seed manufacturers, warns on its website that seeds such as milo are “less attractive to birds.”

Unattractive as it may be, milo makes up the lions’ share of many inexpensive commercial bird seed mixes. In testing a number of seed mixes available at local stores, I found that in several of them, milo accounted for well over half of the mix by volume. In one, the proportion was nearly three-quarters.

To arrive at these numbers, I purchased six different commercial seed mixes from several retail outlets where I live in northern Minnesota, including a Walmart, the Tractor Supply and Hugo’s, a large grocery chain. I took tablespoon-sized samples from each one, separated each sample by seed type, and approximated the volume of each seed type using a set of common kitchen measuring spoons. For sanity-checking purposes, I ran multiple samples on several of the bags to ensure that measurements were consistent across samples.

Bird feeding is a popular activity. An annual study commissioned by the WBFI found that nearly half of American households purchased wild-bird feed at least once in 2016.

That study found that price was the top factor people considered in purchasing bird feed, followed closely by the ability to attract a wide variety of birds.

As a result, “economy seed mixes” – such as the stuff I’d been putting in my feeder – are the No. 1 seller among different types of bird seed, accounting for fully one-third of the total market, according to the WBFI study. Single-seed products, such as sunflower, made up 26 percent of the market, “premium” seed blends accounted for 21 percent, and suet cakes and other products captured another 21 percent of the market.

Economy mixes tend to have a lot of milo in them for the simple reason that milo is dirt cheap. According to the USDA, in 2017 100 pounds of sorghum for grain (e.g., milo seed) cost about $5.65. That’s less than one-third of the price of, say, 100 pounds of sunflower seeds, which cost $17.60 that year.

A pair of seed mixes offered under the Pennington brand and sold at Walmart plainly illustrate the difference between an economy and premium blend. Pennington’s “Classic” formula is about 46 percent milo seed by volume, according to my rigorous kitchen-counter testing protocol. The remainder is made up of white millet, wheat and a smattering of sunflower seeds.

The brand’s “Waste Free” offering, however, contains no milo. It’s more than 50 percent shelled sunflower seed by volume, with the remainder distributed among millet, corn, canary grass, and some dried fruits and nuts.

Representatives for the companies manufacturing the other bird seed mixes analyzed in this story did not respond to a request for comment, or declined to do so on record.

The ingredient differences are reflected in the products’ price: At my local Walmart, a 3.5-pound bag of the classic mix cost $3.24, while a 2.5-pound bag of the waste-free blend was $4.83. Pound per pound, the premium stuff was more than double the cost.

Experts say knowing what kinds of foods birds will actually eat is key. “Seed mixes with lots of inexpensive ‘filler’ seeds often end up going largely to waste because those seeds aren’t preferred by most backyard birds,” said Emma Greig, project leader of the Cornell Ornithology Lab’s Project FeederWatch. But, it can depend on the backyard, she added: “Some people have dozens of mourning doves that will visit their yard, which can gobble up plenty of milo.” But research has shown that – doves, pigeons and some jays aside – most birds aren’t too interested in milo.

If most birds won’t eat milo, what do they like? In the mid-2000s, the WBFI partnered with a biologist from Millikin University to collect observations on more than 1.2 million bird feeder visits.

With the resulting data set, they were able to put together a chart of 15 different bird species’ preferences for 10 types of common bird seed. I converted that information to a numeric scale, ranging from 0 (low preference) to 2 (high). Adding up the resultant numbers for each seed type gives a good sense of its general desirability across a wide range of common species. On a theoretical scale from 0 (no bird has any preference for it) to 30 (it’s every species’ favorite type of food), red milo seed – rates a 6, the lowest score of any seed type included in WBFI’s project. Cracked corn, another popular ingredient, rates an 8.

At the top of the list, on the other hand, are three different types of sunflower seed product – fine and medium shelled sunflower seeds, as well as whole black oil sunflower seed. A different kind of sunflower seed, the white striped variety that humans often eat, ranks more toward the middle of the pack.

The take-home is pretty clear: If you’re just starting out bird feeding and want to bring in the widest variety of birds, skip the commercial mix for a big bag of black oil sunflower seed (or, if you really want to go wild, a bag of shelled sunflower). While it may cost more, it’s virtually guaranteed that none of it will go to waste.

In the end, the only thing that really matters about a given seed is whether your birds are eating it. “It is fine to try anything,” Greig said, “but pay attention to what gets eaten so you learn what works for your yard.”