You seem to be running an old version of Greenfoot - 1.5.1's out now so upgrading would be wise! Yup, as Builderboy said it's the age old image problem (image names are case sensitive on the gallery, but they may not be on your computer.) In your case, it's the world backdrop causing the problem - if you change your "back.jpg" in your world class to "back.JPG", it seems to work fine. As for why I recommended upgrading, 1.5 and onwards will catch this case sensitive issue at runtime and tell you exactly what the case needs to be - pretty much eliminating this problem (you're not alone, it was one lots of people fell for, including me once!) Some programs like MSPaint seem to love saving with upper case extensions, but usually they're typed in lower case, which is usually the cause of the problem.

This game is pretty good. Seeing through the development stage, its good to see it uploaded to a website. Well done dude :-D

If you're looking for other things to do, some delay on the bird's weapon of choice might be helpful - or perhaps some restriction so he can't attack constantly. Or how about he has to eat food and then wait a while first... ;)