Birdseed mixes are usually too high in fat and too low in vitamins (especially when the mixes are high in millet and cracked corn) to meet the long-term nutritional needs of a laying flock. They're also way too low in calcium. Remember, modern domestic chickens lay lots of eggs year-round, while wild birds may lay only a few eggs a year, seasonally. So seed mixes designed for wild birds just won't make a good, well-balanced diet for your flock.
Still, as an occasional treat, your chickens will love it! And many seeds are high in SOME vitamins and amino acids that your birds do need. Even better, some research shows that a diet that contains certain whole grains and seeds may be associated with chickens being able to better stave off salmonella infections. You can see those studies here:
The Influence of Whole Wheat Feeding on Salmonella Infection and Gut Flora Composition in Broilers
Influence of Housing System, Grain Type, and Particle Size on Salmonella Colonization and Shedding of Broilers Fed Triticale or Corn-Soybean Meal Diets
So feel free to offer them wild birdseed. In moderation, as a treat, it can even be quite healthy!