That seagull! xD great game! I like the aesthetic, good job with the runing cycle, I would tune-up the hitbox collision detection (I have problems with that too :P) but other than that, you have a nice thing going on here.
Yea I think its using a square hitbox so you might have to jump earlier then need be.
Yes, most collision boxes are fixed so it makes a little bit harder to predict. But still glad you enjoyed it : )