good dance moves demand practice and determination. he rehearses his vertical leaps, hoping to impress. her interest is piqued. his efforts pay off. now an item, they dance together.

their synchronized routine ties the knot. their enthusiasm is infectious. it triggers a mass dance-off. the happy couple leave the party early to start their new life together. and the dance goes on without them. barnacle geese fly continuously for days to reach the arctic ocean.

it's hard work -- for every breath they take their wings will beat three times. once the fat reserves are gone, they will start burning muscle. the last few miles are the hardest, but the destination is in sight. the svalbard archipelago, midway between norway and the north pole. europe's furthest wilderness may look a barren wasteland to us, but it's paradise for the geese. 24 hours of daylight provides all the grazing time they need

as they prepare to mate and lay eggs. back in the stork village, the return of the females hasn't gone to plan. the males are still waiting. when weather conditions are bad, whole flocks can get lost at sea. but things are looking up... the girls are back in town. the males can't contain their excitement. she remembers the location of her nest and drops down to greet him.

together at last, they clatter a greeting. in africa, they spent eight months apart. now, they must get to know each other again. back on svalbard, the barnacle geese are already experiencing a baby boom. but this attracts a rare visitor.

and chicks make a tasty snack. mothers desperately shepherd their young to safety. a lost chick is first to go. and then another. the geese launch a counterattack, but they can't maneuver easily. a hungry polar bear can wipe out an entire colony.

they desperately need reinforcements. nesting arctic terns join the aerial assault, united by a common enemy. their plan is to make the bear's life a misery. the bear's irritation starts to show. skuas join the combat mission. faster and more aggressive, they drive home the attack. working together, the birds have saved the colony. although meeting polar bears is an increasing problem,

by nesting in the arctic, the geese avoid human disturbance altogether. but storks are among the many birds that deliberately seek us out. they raise their young among us, in the very heart of europe. like so many birds, they have managed to prosper in a continent that we have changed more than any other. next time, birds will take us on an extraordinary journey to discover the hidden secrets of the south american continent. on an extraordinary journey a measles outbreak begins. this is new york; we don't see measles in new york. if vaccination rates drop, what are the consequences? man: if you start to decrease immunization rates,

the diseases reemerge. nova sorts facts from fears. woman: there's just so much information. there's no such thing as an unbiased source. man: is it okay to question vaccines? of course it is. that's just the start of the conversation. "vaccines: calling the shots," next, only on pbs. victoria falls. they call it "the smoke that thunders." man: people use many fancy words when they talk about victoria, but words mean nothing when you are near her. she grabs you and shakes you to your bones. it is like you are standing on the edge of the world. now you can watch "nature" online. go to pbs.org to screen complete episodes from this season and seasons past. visit "nature" online for production updates

from the field. well, here we are on the alaska coast. go behind the scenes with our filmmakers. we also used a borescope lens, and that allowed us to put the lens right into a flower. and get connected with "nature's" online community. all at pbs.org. by the corporation for public broadcasting... and by contributions to your pbs station from viewers like you. thank you.

to learn more about what you've seen on this "nature" program, visit pbs.org. a past they never knew... until now.

a. cooper "it's incredible!" in his new season on pbs, henry louis gates jr. helps them uncover the truth about where they come from. s. field "i can't believe that these documents even exist!" c.b. vance "wow!" get to know them, as they discover who they really are. k. alexander "that's pretty amazing." finding your roots with henry louis gates, jr. starts tuesday, september 23rd - only on pbs. the women were breaking the glass ceiling and that just thrilled me. we're traveling back in time... wow. ...to help solve family mysteries. no way that's amazing. fdr: a new deal for the american people. we wouldn't be who we are without them. ♪ ♪ i just got to get get get on back to my home at any cost now now ♪ [laughing]

connected every day in a thousand unseen ways. but sometimes these connections can pose an invisible threat... if the object we touch or the air we share carries a dangerous germ. simon fensterszaub: you don't have to cough, you just have to breathe. it's the worst kind of contagion; it's airborne. narrator: diseases largely unseen for a generation are returning. fensterszaub: i'm, like, "this can't be. how does a kid have measles?" this is new york! we don't see measles in new york. jane zucker: it's astounding to me that you can have people dying of measles in the developed world. narrator: today, children are getting sick and dying from preventable diseases