Animal and Plant Health Inspection Service (APHIS). This is the main site for APHIS which is part of the US Department of Agriculture (USDA). Within this site APHIS gives information on importing and exporting birds including pet birds. This agency is responsible for preventing unwanted diseases from gaining a foothold in the US. This is one of the two agencies you will need to get permits to import or export birds.
Fish and Wildlife Service (FWS). The FWS is an agency within the US Department of Interior. This is one of the two US Government agencies regulating the importation of birds. It issues permits required for the import and export of birds and other animals related to CITES.
Centers of Disease Control (CDC). This is the site to search for information on diseases you can contract from birds. Provides recommendations on how to manage Chlamydia psittici infections.
Individual State Agencies. Each state has additional regulations related to birds and other animals. At this APHIS site you can find links to most of these state agencies.
National Agricultural Library (AGRICOLA). This is your access to the USDA's searchable data base of the agricultural literature.
National Library of Medicine (PubMed). This is your access to a very large searchable data base of medical literature. Its primarily focus is human medicine, but it also includes many veterinary journals. It is a United States government site.
Convention on International Trade in Endangered Species (CITES). The US subscribes to CITES. This organization maintains lists species in need of protection to prevent their extinction in the wild.