The only bird guide that I know of in Guatemala is Knut Eisermann, who has a lot of information on his excellent website (
http://www.cayaya-birding.com/). I have never met him in person, but I have been in contact by e-mail and he is certainly one of the top birders (if not the top birder) in the country. The only other guides I know of are at Tikal. They are fairly good on the local birds, but I don't think they do tours elsewhere. My website (see link below) has a lot of information on birding and travelling in Guatemala, as well as plenty on safety, which should not be taken lightly. I have spent a lot of time in Guatemala and I have only ever had one incident, which was a mugging at a notorious location (Agua Volcano). I knew it was risky before I went, so took an old pair of binoculars and nothing of value. In most parts of the country you will be fine as long as you take the proper precautions, like never driving at night.
Don't hesitate to get in touch if you decide to do it on your own and want any information about specific sites. Its a really interesting country to visit, and the birding is top notch. I'm sure you will have a great time.
Tom