Hey there.
Well I've read lots about demo reels on google, so i hope i can give you some good tips.
If you will ever use this reel to find a job or to participate in a project, you do realize that people to whom you'll send your reel will get lots of them, so they will not have tons of patience. In most cases, some reels are rejected after the first few seconds, if they don't intrigue the viewer, and they move on to the next one... Sometimes maybe not the best artist, but the best reel makers get the job.
So, tip #1: Make it interesting, i'd reject this reel just because i have to wait through the countdown and then the tunnel. they waste your time and don't really show anything good, so get rid of it. You should start showing your work at once, or if you really have to put in something brief for no more then a second, better even less. Like your name for example.
#2: Start with the best stuff. Your reel is not likely to be watched fully to the end, and if the beginning is not good, it will go into the trash can.
#3: Don't make them wait: you keep showing the same thing for too long. A second for one object is more than enough, unless there is something super special about it. It's better to just put as much characters as you can in it, than just to show on, no matter how good it is. I mean, I get bored just watching that dog run, and i don't have 50 more reels to see...
#4: Make it interesting, mix it up a little bit, In your video i can see: Dog, Dog, Dog, Bird, Bird, Bird, Man, make it : Dog, Bird, Dog, Bird, Man, Dog, Bird , you get the idea?. Also the white background most of the time...not that interesting, finally change the angles of the camera a little bit, now all the characters are seen from the same perspective, even better make the camera turn around them, and make them run from left side of the screen to the right, not just sprint in place.
OK, well i hope that will help. Remember, it's just my opinion.