You will want to find the area of the whole refuge first. A=b x h. A=650x450. A=292,500. The area of each refuge region is A=25x25. A=625. To find out how many birdhouses you would need you would set up the equation 292,500=625 times x. x=number of bird houses. You would divide 625 from 292,500 getting 468 and that equals x. They will install 468 birdhouses.