S1, E103: Every year, 40 million Brazilian free-tailed bats stream out of their summer homes, the 'guano caves' near San Antonio in Texas. It is the biggest congregation of mammals on the planet. Half are babies precariously taking wing on their maiden flight, running a deadly gauntlet of hawks, falcons, snakes and racoons, who have been eagerly awaiting their debut. How do all these predators know just the right time to arrive, and the best place to be for easy pickings – and how did all these creatures come to meet on this one fateful day? And what can the bats do to stay alive? This massive flock is a blessing and a curse – it gives them safety in numbers, but many will be lost through mid-air collisions. Those that can, use the flock dynamics to their advantage. How does an individual navigate in the vast pulsating flock, and find its way out to safety? Also: once a year millions of salmon fight their way up rivers across Alaska. They are on an unstoppable collision course – a course that began 6 months ago in the depths of the Alaskan winter and will culminate in carnage on an Alaskan waterfall. Here, grizzly bears and bald eagles congregate to intercept the salmon. For one mother bear it's a matter of life and death. Her young need food if they are to survive. The salmon have an imperative to get through the gauntlet. If they don't, the future of the shoal is in jeopardy. They must return to their spawning grounds to lay their eggs. It's a frantic dash with each salmon relying on skills and tactics to keep them on track back home. But at the end of the day does life just come down to one lucky leap to avoid the jaws of death?