There are two types of sunflower available for wild bird feeding: black oil sunflower and striped (confection) sunflower. Here are the key differences between the two:
Striped sunflowers are generally larger than black oil sunflowers. In fact, striped sunflowers are the same type of sunflower you would buy yourself at a convenience store (roasted sunflower seeds - like Spitz for example). However, striped sunflowers for bird feeding have been screened to select smaller seeds that are more suitable for birds.
Oil sunflowers are smaller than striped sunflower and therefore are easier for small birds to handle and contain more oil per gram than striped sunflower. In addition, oil sunflowers have a thinner shell meaning that some birds find it easier to open black oil sunflowers than striped sunflowers. That said, larger birds like jays, woodpeckers and grosbeaks love striped sunflower. Also smaller birds like chickadees, titmice and nuthatches can easily open the shells of striped sunflower seeds. However, some birds like House Sparrows and blackbirds do have more difficulty cracking open striped sunflower seeds. So if you are suffering from an abundance of these types of birds, you could try feeding more striped sunflower seed. This may reduce the number of undesirable birds at your feeders and improve the diversity of birds in your backyard.