A healthy baby is good news, but when the baby is a rare or endangered animal, it's both a triumph for the species and a boon for spring zoo visitors. The biggest news comes from the Denver Zoo ($11, $7 ages 3-11; www.denverzoo.org or 303/376-4800), where twin polar bears Koda and Nuka are finally set to go on view in April. Born November 25, they're as awkward as toddlers now. The Feline Conservation Center at the San Francisco Zoo (from $8, from $3.50 ages 12-17; www.sfzoo.org or 415/753-7080) is the country's largest breeding facility for small cats. This spring, enjoy the sound of success―the low purr of a rare ocelot kitten. It's difficult to spot a bison calf, bighorn lamb, or mountain-goat kid in the wild, but they're easy to find at Eatonville, Washington's Northwest Trek Wildlife Park ($9.50, $7 ages 5-17; www.nwtrek.org or 360/832-6117). New youngsters are born here every spring and love meeting their fans.