The 163-acre Orote Peninsula Ecological Reserve was established by the Navy in 1984. It features two biological "units," one marine and one forest. In the latter, you can explore with your binoculars and check some birds off your bucket list.
According to the bird database Avibase, Guam is home to 134 species, three of which are endemic and 11 of which are threatened. One to look for: the Micronesian kingfisher, pictured here.