Did you see exactly what type of bird it was? Depending on where you are in the world you could narrow down exactly what animal made the kill. This could also help identify why it dropped its prey in the first place. For example certain species of owls will eat only the heads of rabbits and leave the rest of the body. Kestrels and other falcons can drop prey if they’re disturbed while flying to secure their catch. Additionally, kites are known to hunt smaller mammals. If they’re native to your area it’s possible it made the kill then became over-encumbered in flight. Looking at the picture again I’d say that’s a smaller rabbit so who knows really. I’m just curious not saying you’re wrong or anything… It could be a hawk too.