The only thing I can think of -- could your birds have been disturbed by pets or children? Or maybe in the car on the way home from your cake class? If your project was wet and then it got moved or jarred, it could form cracks. Also, if you tried to remove it from the wax paper before it was fully dry. I know that some color flow pieces can take up to a week to be fully dry. Then, there's always the chance that your color flow was "contaminated" by a trace of grease from your bag or your tip or your coupler. Cracks could be formed for any number of reasons.
As for fixing? Not sure, so I'll give you a bump!
You can't really fix them. If they are already off the wax paper, then you could re-inforce them on the back with more icing, but you can't add more to the top and have them look right. I would just make new ones. I can't really tell what caused the cracks without seeing them, but my guess would be that you made them too thin and/or your icing was too runny. Did you mix the icing for at least 7-10 minutes? Maybe it wasn't mixed well. Next time you make them, make sure that you get a nice rounded outline (don't drag your tip on the board) and really fill it in so that the icing holds a pillow shape. If you make them thicker, they shouldn't crack on you. Here's a link to the Wilton site with the recipe and some tips.
http://www.wilton.com/recipes/recipesandprojects//icing/colorflow.cfm
you can do what i did and just not add the birds!
You can make royal icing and use that the way you did the color flow. I would do them tonight, to give them time to dry. You can also quick dry them in the oven with just the light on over night.
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