How can an entry-level developer (CS degree) get into the UX field? Is a HCI degree required to get into the field?

No, an HCI degree isn't required to get into the field, at least not at startups who generally value stuff you've done over certifications (some pedigrees are correlated highly with success, but that discussion is out of scope).  In any event, here's what you should do: Using whatever resources are helpful to you (books, mentorship, online stuff, what the other answers here suggest, etc), start learning as much as you can. As you learn, start building up a portfolio, even if all the projects in it are self-induced.  For instance, you could find sites with shitty UX and redo them. Attend UX meetups. Start a blog where you catalog your experiences, what you've built, and what you've learned.  You don't have to talk about groundbreaking stuff here (I think this is a mistake that eng bloggers make a lot, and it paralyzes them into inaction).  From a career advancement standpoint, the point of blogging isn't to contribute something no one has ever seen to the corpus of human knowledge (though of course it's awesome if you can do that).  The point is to show that you are serious about some subject and are a thinking human being worth knowing. After all this stuff, you should be able to find an internship, and from there things will get easier, in that you won't have to drive every little aspect of your transformation.  Huzzah.

Without wishing to self-promote too strongly, my book Undercover User Experience Design http://undercoverux.com is written for precisely this sort of situation. My summary would be that no, you don't need an HCI degree but the field is becoming very competitive at the junior levels, so you'd need to find other ways to stand out against the graduates.

Learn as much as you can both by reading and doing.  Hang out with other designers who are better at it then you are and learn from them. Here are a few books which to get you started: * Subject to Change - Adaptive Path * About Face 3 - Alan Cooper * Don't Make Me Think - Steve Krug Remember, when learning to design something, we all get it wrong the first few times.  The secret to UX is learning how to gather meaningful feedback and process it into product recommendations.

One book that I'm reading and so far I think it's wonderful to get started: A Project Guide to UX Design: For user experience designers in the field or in the making by Russ Unger, Carolyn Chandler Amazon link: http://amzn.to/fSDWOc

As not to repeat myself, see this answer: