Although only 51,000 square kilometers, smaller than West Virginia, Costa Rica has 894 recorded bird species - more than Canada and the US combined. Around 600 species are residents, and the rest migrate in and out. Three are endemic to the mainland (not including one extinct), and three to Cocos Island. It has the greatest density of birds in any American country. The diversity is partly because of the land bridge Central America formed, causing a flood of animals to come to Costa Rica, and the variety of habitat offered in such a small space: mangrove swamps by the Pacific, coastal plains by the Caribbean, dry northern Pacific lowlands, and many mountain chains offer a diverse environment to support many types of wildlife and avian species. The Costa Rican National Bird is the Clay-covered thresh.