"Every summer, U.S. fighter pilots in Southern Texas prepare for an invasion." "A dusk falls, something huge enters their air space." "A force capable of destroying even the most sophisticated jet fighter." "The threat is not a storm, or even an enemy barrage." "It's a freak of nature." "Bats. 40 million Mexican free-tail bats." "This is the biggest flock of bats in the world." "Today, all 40 million leave this cave, in one mass exodus." "Half of them are babies, on their very first flight." "It's the biggest day of their lives, but for some, it will be their last." "Because outside lurk deadly predators." "So, who will live and who will die ?" "At the moment of impact, what are the odds for each little bat in the flock ?" "Unlucky bats can become fast-moving snake fodder." "Every bat is a potential target." "So, is there anywhere in the crowd that's safe from attack ?" "In a flock of millions, is life and death simply a loterry ?" "Or can a bat do anything to increase its chance of survival ?" "To find out, we're going to travel back 5 months." "To when this cave stood empty." "Our story begins hundreds of miles south of Texas." "In Mexico." "Deep inside another cave the colony has spent the winter fattening up on insects." "Now, with the longer spring days, their biological clocks start ticking." "Staying here to breed would be catastrophic." "There's not enough food left." "They must move on." "A huge migration begins." "They follow the warm weather, as it sweeps northwards." "Flying at 10000 feet, with winds behind them they're heading for Texas, a journey of 1500 miles." "They make this incredible journey in just one week." "But their instinct of survive draws them closer to a place where killers are waiting." "In Texas, a long winter draws to an end." "In this semi-desert scrub, it's hard to find food." "Life here is a challenge for the red-tail hawks." "And the snakes emerging from their winter hibernation." "They had nothing to eat for months, and they're starving." "It's too cold to hunt on the cover of darkness but out in the open, they're vulnerable." "The red tails patrol the skies." "Their eye sight is so sharp, they can spot the slightest movement." "And with so little on the menu, they're willing to take big risks for any meal." "Even venomous snakes." "For now, snake and hawk are sworn enemies." "But one day soon, they'll be partners in crime." "When the walls of this cave echo to the sound of millions and millions of bats." "At last, the colony has reached Texas." "But they're still 20 miles from their final destination." "Exhausted, some roost under bridges, for the rest." "Now, as the live, a silent separation takes place." "While the males head off to bachelor pads, millions of females continue their journey alone." "They've made it this far, but there's still one more challenge ahead." "And the future of the colony depends on it." "Incredibly, after 5 months away, and a 1500 mile journey their memory guides them back to the exact cave where they were born." "20 million are soon crammed into the cave." "Every square inch is taken." "And each of these females is pregnant." "They'll have just 5 months to give birth and raise their young." "Before this safe cave turns from heaven to hell." "The return of the bats is good news for their predators." "As the spring turns to summer, there's an explosion of life." "But, many new hungry mouths, mean more pressure to find food." "Soon the predators will be drawn to the cave, but in doing so, they'll be risking their lives." "Forced into dangerous proximity with their own mortal enemies." "Still, if they play it right, the rewards could be high." "Because deep inside the cave a huge transformation is taking place." "Each female now has a baby." "It's synchronized breeding on massive scale." "With 40 million bats this is now the biggest colony in the world." "Yet, at the moment of birth, each mother and baby are on their own." "Like a safety rope, the umbilical cord connects them, but not for long." "They've got just one hour to learn each other's unique smell and call before the cord breaks." "It's a lesson no baby can afford to miss." "Because then, the mother must leave it." "To survive, each baby must stay put, clinging to the ceiling for dear life." "But down below them, triggered by the urine from the bats a mass of flesh eating bugs are hatching." "In an ironic twist of fate, the colony creates its own killers." "As dusk falls, the mothers begin the night hunt." "They've got just a few hours, before they must return to their babies." "But with 20 million new mouths to feed" "Where on earth can they find enough food to sustain them ?" "The answer lies a mile above the Texan hills." "It will be a race against time, but the colony has a very clear and specific target." "Now, over 100 million bats invade the sky, as other colonies join forces." "They're all on a collision course." "At 10000 feet, the bats fan out to face another airborne battalion flying towards them." "It's billions of moths, heading to their summer feeding grounds." "Incredibly, just when they need a massive food supply 250 tons of flying insects arrive on the scene." "The tiny babies are crammed together for warmth." "But with so much jostling, it's hard to keep a firm foothold." "Some grow weak from lack of food." "For those that fall, their only chance of survival it's to crawl to the nearest wall and climb, fast." "If they don't, they'll be eaten alive." "The flesh eating bugs on the cave floor can strip a bat to its bones in just 45 minutes." "For this baby it's a mammoth task" "Scaling the 30 foot wall, it's just like climbing a 10 story building." "It's made it back to safety." "But survival still, isn't guaranteed." "But now, that it's moved, how will its mother find it ?" "At dawn, the mothers return." "After a night of feeding, they're laden with milk." "They won't all make it back but by dive bombing the cave at 80 mph they are difficult targets for the hawks." "Most get through." "But for those who don't their babies will die." "A mother now has a 1 in 20,000,000 chance of finding her baby." "And she's racing against time." "Although she knows the layout of the cave, they can land close to where she left the baby." "She has to search while other desperate youngsters try to steal a drink." "But she'll only feed her own young." "She tries again, listening for its call, amongst millions of other voices." "At last she hears it." "And recognizing its smell, her baby finally gets to feed." "If it can continue to survive the next 6 weeks her baby will make it to the exodus when the whole colony flies out together." "And then each youngster will face a new danger." "Running the gauntlet of bat-hungry predators." "It's the height of summer." "The body heat from 14 million bats pushes the temperature even higher and the air is thick of ammonia from their droppings." "Bats are immune to the deadly fumes, but this place is becoming intolerable." "Midsummer is the prime hunting season for snakes." "With the worm night temperatures they can be out stalking at the time the baby bats will fly." "For now, they hunt other prey, on the cover of darkness." "They use a special heat sensor to track their victims." "Their venom takes just seconds to work." "It's a huge meal, but it'll only last the snake for a few days." "Hot and overcrowded the cave that's been a safe nursery, it's becoming a stench filled death trap." "Vast numbers of adults now roost near the entry, to keep cool." "But this broadcasts their presence to the world outside." "It's an irresistible temptation to their predators." "Despite the terrible smell, a raccoon goes deep inside." "Little does it know that by entering this cave, it's risking it life." "It's looking for fallen babies." "But hunting in the pitch dark is tricky." "It's impossible to see them." "Still, with so many casualties it's only a matter of time before the raccoon blunders into one." "It could feast all night, but the bats have secret weapon." "The air in the cave is now so toxic, that every minute spent feeding here brings their predators closer to death." "The youngsters are now 6 weeks old and their instincts tell them it's time to fly." "They begin taking warm-up flights to flex their wings." "When the time comes, though, how will they find the way out of the cave ?" "Bats use echolocation to navigate through the dark." "But the skills of these novices will be tested to the limits in the next few hours." "For some, their maiden flight, will their last." "Outside predators assemble." "More are attracted to the cave now that any other time of the year." "Somehow they seem to know that today will be a good day for bat catching." "But the snakes run a considerable risk in coming here." "Until the bats fly out, they are target for the hawks." "Once inside, though, they're safe to set up the perfect ambush." "Day turns to dusk." "It's the signal the bats have been waiting for." "The adults go first." "It could be a maternal tactic to lure the predators away and protect their young." "5000 bats emerge every second." "Deep inside, the moment for these babies has finally arrived." "It's time to join the exodus." "With so little experience in echolocation many collide and become disoriented." "Exactly where and when each baby flies now will make the difference between life and death." "Too near the wall, and it could end up as snake bait." "To catch bats in caves, some snakes anchor their tails in crevices and hang out into open space, waiting for victims to blunder into their coils." "With so many streaming past, a catch is inevitable." "So, is there anywhere in this massive flock, that's safe for a baby to fly ?" "Just by the entrance, the bats do something quite extraordinary." "They circle round and round, forming a massive tornado-like vortex that pulses and spins." "What are they waiting for ?" "It could be they're warming up and gaining altitude for the long flight ahead." "Maybe they're building up numbers to create safety in the crowd." "A swirling, dense ball of bats makes it dangerous for a finely tuned hawk to attack." "But the bats can't stay circling for ever, the hawks will get their chance." "At some mysterious signal, the mass unwinds." "This is the cue the hawks have been waiting for." "These are the greyhounds of the bat world flying at 60 mpg but speed alone won't save them from this enemy." "Those sheer numbers can." "Diving into this massive flock would be suicide for a hawk." "A calculated attack strategy is needed." "Gliding slowly along the column, the hawk targets strugglers." "Those on the edge may become victims, while those on the middle will stay safe." "Bats can only echolocate objects in front of them so the hawk closes in from behind." "This is the final exodus." "Soon the babies will begin their long migration south." "The sacrifice of the weak and unlucky is a small price to pay for the survival of the colony." "Their losses are only a tiny fraction of the whole." "Each bat can make a smart move, and increase its chances of survival." "But in this bat world, the colony is king."