Flax Sometimes found in mixes, this seed forms a jelly-like coating when it is wet. Suet Suet can come with various seed, insect and fruit ingredients. Many of these ingredients are filler and would not be eaten by birds if offered by themselves. Millet, corn and oats are commonly used and are not eaten by suet-eating birds that generally prefer seeds high in oil such as Sunflower and peanuts. Suet with insects and fruit is excellent but costs more than those with oily seeds.
Some suet even has flavoring which is totally unnecessary.
If you have House Sparrows eating suet, then you have a problem because they do not eat suet but will eat the large amounts of millet some suet has as filler. Suet should be placed away from the seed feeder so there are fewer disturbances for Woodpeckers and Bushtits. If Starlings and Jays discover the suet, there are feeders that will exclude them. These feeders require birds to hang upside down which Jays and Starlings do poorly. Some newer ones have wire screen to keep out larger birds.