The term “thistle seed” is not botanically precise, referring to the seeds of various flowering plants within the large Asteraceae family (sunflower or daisy family). Plants in this family, including true thistles, produce fruits technically called cypselae, which are commonly referred to as seeds. The common name is ambiguous because it describes two distinct products with vastly different applications: one for human health and the other for commercial bird feeding. Understanding the context is necessary to distinguish between the medicinal milk thistle seed and the oil-rich grain sold for wildlife consumption.
Clarifying the Identity of Thistle Seed
The most common sources of confusion for the term “thistle seed” are the botanical Milk Thistle seed and the commercial Nyjer seed. Milk Thistle seed is derived from the plant Silybum marianum, a true thistle species, and is primarily used in herbal medicine. This seed is typically black with a smooth, glossy surface.
Nyjer seed comes from the plant Guizotia abyssinica, an annual herb native to the Ethiopian highlands. Nyjer is not a true thistle, belonging to a different genus within the Asteraceae family. It was misleadingly marketed as “thistle seed” because small finches, which feed on wild thistle, are highly attracted to Nyjer. To prevent confusion, the Wild Bird Feeding Institute trademarked the seed as Nyjer in the late 1990s.
Milk Thistle Seed and Its Active Compounds
The primary application of Silybum marianum seed is in the production of herbal extracts, where its health properties are concentrated. The seed contains between four and six percent of the active compound complex known as silymarin. Silymarin is a mixture of several closely related molecules called flavonolignans, with the most abundant component being silybin.
These flavonolignans are responsible for the seed’s traditional use in supporting liver function. The complex acts as an antioxidant and anti-inflammatory agent, helping to stabilize liver cell membranes and promote the regeneration of liver tissue. Silymarin’s mechanism of action includes neutralizing free radicals and inhibiting the binding of toxins to liver cells.
Milk Thistle seed also contains a significant amount of oil, typically ranging from 20 to 35% of its composition, which is rich in fatty acids. This oil is characterized by a high concentration of unsaturated fatty acids, particularly linoleic acid, an omega-6 fatty acid. While the seed contains proteins and other sterols, the therapeutic focus remains on the extracted silymarin complex for standardized supplements.
Commercial Use in Wildlife and Bird Feeding
The Nyjer seed, commonly sold as “thistle seed” for bird feeders, is highly valued for attracting specific species of wild birds. Its high oil content, often around 30 to 40% by weight, makes it an excellent source of energy, especially during colder months. The tiny size and high-fat composition make it a favorite food for small-billed finches, such as American Goldfinches and House Finches.
Nyjer is an imported product, primarily sourced from India and Ethiopia, and is subject to strict regulatory treatment to prevent the introduction of foreign weeds. The United States Department of Agriculture (USDA) mandates that all imported Nyjer seed be heat-sterilized before sale. This process involves heating the seed to a high temperature, such as 248°F (120°C), to devitalize any potentially contaminating weed seeds and ensure they cannot germinate if spilled.
The minute size of Nyjer seed necessitates the use of specialized feeders, such as mesh socks or tube feeders with tiny feeding ports, to prevent waste. The specialized handling and the cost associated with importation and sterilization contribute to Nyjer being more expensive than common bird seeds like sunflower or millet. The thin shell and high oil content also mean the seed can spoil or dry out more quickly, requiring replacement every few weeks.