All sunflower seeds come from the same plant, Helianthus annuus, and are safe for human consumption. On the other hand, sunflower seeds marketed for human consumption are not good for birds because of possible seasonings or excess salt.
Most sunflower seeds marketed for feeding birds are either unhulled black oil sunflower seeds or hulled seeds. Black oil sunflower seeds have a thinner hull and a higher oil content than the striped variety. This makes them easier for birds to crack and gives them a higher nutritional content. Hulled seeds are easier for small birds to eat. They do not require the birds to waste energy on removing seeds from their shells, but hulled seeds are more expensive per pound and spoil more quickly than unhulled seeds in high temperatures.