The Bird Island Sanctuary was created in 2000 with the enactment of Public Law #12-46 (the same law that created the Forbidden Island Sanctuary). It stretches from right outside the grotto to just below the Bird Island Lookout (see map below, but it is pretty much everything you see in the above photo).
Marine Protected Areas (MPA) are marine areas where the harvesting of natural resources is limited. The reasons for creating MPAs are many, but on Saipan there are primarily two strong arguments. First of all, on Saipan, the MPAs are areas that are important for the local tourist economy. Many tourists identify Saipan's beautiful coral reefs and sandy beaches as the main reason for their visit. The MPAs ensure that an important source of revenue for the islands (i.e. tourist dollars) are protected in the present and in the future. Secondly, the MPAs serve as sanctuaries for fish, trochus, octopus, and other species that are culturally important for the local people. Protecting these species will ensure that Chamorros and Carolinians, the indigenous people of the CNMI, will be able to continue their cultural practices, such as talaya and spear fishing, for generations to come.
The following map, from the CNMI Division of Fish and Wildlife website, illustrates the location of the MPAs on Saipan:
If you want to hike down to Bird Island Beach, here are the rules for visiting:
And this is what you might see: