@douglass – I think they’re crap sometimes. Some days I’m pretty sure they’re literally impossible without spending gems on spells or extra birds. That does strike me as a money grab on Rovio’s part, so the best thing to do is just defy them by not paying for any extras. If you’ve spent a few lives and are nowhere near the end of it, don’t keep wasting your time.
At other times, the DC is actually quite easy. Most of the time it’s in between – reasonably challenging and takes several tries, but it’s not over-the-top ridiculous. I suppose it’s expected that the difficulty level of the challenges will vary, and that since you don’t absolutely have to pass them to progress in the game, some of them will be harder than the actual game levels they borrow elements from.
Figuring out your personal limit, when it’s good to call it a loss and save yourself the frustration, versus when you are so close that you can probably get it if you just keep trying, is a really good skill to develop.