Crow pose is part of Second Series in Ashtanga, known as Nadi Shodina, which means nerve purification. It involves a lot of hip openers, backbends, and arm balancing poses.
Just as in Primary Series, you'll do Sun Salutations A and B, standing poses like Triangle, and then move onto the poses in Second Series. It takes 1 1/2 hours to complete and by the end, you'll feel amazing.
Depending on who you talk to, some Ashtanga yoga instructors think you need to master Primary Series before moving onto Second Series. I don't agree. Primary Series is all about forward bending, and Second Series has a lot to do with back bending. I am a firm believer that your body can benefit from all yoga postures, and you shouldn't be restricted from doing any - you should be encouraged to try them all.
Also, if you are constantly bending forward and stretching the back of your body, you'll need to strengthen those muscles too. You need the poses in Second Series to counterbalance the poses in First Series and vice versa. The two Series' complement each other brilliantly.
Second Series is so much fun. You get a chance to try out some more advanced poses like Crow, Forearm Stand, Bow, and Leg Behind the Head. You never know what your body is capable of doing unless you try.
After you finish a Second Series class, your body will feel different. Since it involves a ton of backbending poses, you'll feel a sense of lightness and love from stretching through and opening your chest.
Fit's Tips: I recommend trying Second Series 100%. It's challenging and you'll strengthen and increase flexibility in the muscles of your entire body. Locate an Ashtanga Yoga studio near you and find out if they offer Second Series classes or workshops. You can also check out David Swenson's Ashtanga Yoga Second and Third Series DVD for $24.95.
Check out some pictures of poses from Second Series. Just so you know, these aren't in order.