Are there specific features of birds that cats/small predators are attracted to?
Not all cat predatory behavior is innate. Researchers found that cats predatory behavior for birds vs. mice depends to a significant degree on training by the mother: if the mother taught predatory behavior with birds, the kittens grew up to be better at catching birds than at catching mice and vice versa. Supportive data shows that aside from monkeys and other primates, cats are among the most adept at learning by observing the successes and failures of other animals attempting to complete tasks to obtain a reward. This might influence, therefore, a cat's response to and attraction for objects in the air or on the floor.

Blindfolded cats can hunt mice by vibration. Paul Leyhausen, a German animal behaviorist studied domestic cats (and related felines) from the 1950's through the 1970's, and much of what is known about their predatory behavior is thanks to him. 

Although their vision is not especially keen, domestic cats see well in dim light and readily perceive motion. According to Leyhausen, prey movement and the direction of movement are the only factors which innately release crouching, stalking, and catching behaviors in cats. When presented with quail, cats were far more likely to go after active quail than those stilled by tonic immobility. Cats are also more likely to go after smaller prey (mice > rats) and mice were much more likely to elicit play pawing.

Leyhausen also demonstrated that rodents were probably the natural prey of cats.

Though there has been much study of cats and predation, little is available on detail of the kind you are interested in.  My best guess for a good toy for a cat would be a smaller object that moved along the floor.

More information can be obtained from Leyhausen's book, Cat Behavior: The Predatory and Social Behavior of Domestic and Wild Cats. 

Effects of the mother, object play, and adult experience on predation in cats
 The effects of experience on the predatory patterns of cats
 Tonic immobility in Japanese quail can reduce the probability of sustained attack by cats
 The behavioral bases of prolonged suppression of predatory attack in cats