There is a world of difference between authentic native seeds and commercial wildflower seed mixes. Though they were created because of popular demand, mixes are not usually helpful either for wildlife or for the environment.
Most wildflower mixes on the market contain Asian and/or European flower seeds that crowd out true native North American species. Non-native wildflowers then grow and displace the native plants that provide food and shelter for our birds and butterflies and other wildlife.
Books, such as David Tallamy's Bringing Nature Home, explain the critical link between native plants and native wildlife. Tallamy describes how native insects cannot eat alien imported plants. So when our native plants disappear, our native insects disappear too and this impoverishes the food source for our birds and other mammals.
Gardeners can make a significant contribution to bio-diversity if our home gardens include, for example, host native plants for our butterflies and moths. It is fine to also include some benign imported species that provide additional nectar sources as long as we have a healthy mix of natives and non-invasive non-natives.
It is safest for home gardeners who seek to provide a healthy ecosystem if they purchase their wildflower seeds and plants from nurseries that specialize in natives. This ensures that we do our bit to protect and nourish our wildlife and protect the environment.