For years, Fritz trained his ibis to migrate from Bavaria in the south-east of Germany to Tuscany in central Italy, but climate change means they are now migrating later in the year – which means crossing the Alps in colder, more dangerous weather without the help of warm currents of air.

Now he is teaching these birds to avoid these mountains, and end up more than a 1,600km (1,000 miles) away in southern Spain. It’s a longer route but it’s safer. “It shows how close climate change and biodiversity loss are related,” Fritz told me. Around 20% of birds migrate. It made me question, what happens to those who don’t have an eccentric Austrian conservationist to show them the best route to go?

Migration is already a dangerous time – birds can get lost, blown off course by bad weather, shot, or arrive at their destination to find the habitats they normally rely on have been destroyed. But the additional pressure of the climate crisis is pushing a finely balanced system into unknown territory.

Dr Chris Hewson, who leads the British Trust for Ornithology’s cuckoo tracking project, says: “One of the big questions is the degree of flexibility birds have in changing their migration routes in response to changing conditions.”

As summer draws to a close here in the UK, temperatures are dropping and the days are getting shorter – it’s back to school time. Birds are also sensitive to these cues, which result in hormonal changes, causing them to feel restless, start to flock together and eat food to stock up for the journey ahead. You may start to notice swallows lining up on telegraph wires – they’ll soon be off to southern Africa.

How birds migrate remains one of the mysteries of the natural world. It is believed that they navigate using landmarks like rivers, coastlines and mountains, taking direction from the sun, the stars, the Earth’s magnetic field and their sense of smell, as well as following their friends. Once a bird has successfully completed its first annual migration it is likely to continue to try to do the same route again and again.

Nigel Collar from BirdLife International says: “If the conditions on a particular route become too extreme then the population using it will either die out or in some way adjust and adapt. Some species have rather strongly programmed migration routes … Other species will not be so tied to a particular route, and will have the facultative ability to get round obstacles and make use of unexpected opportunities (like ephemeral wetlands as stopovers).”

Some birds have truly incredible migrations. The bar-tailed godwit, which can fly 12,000km (7,500 miles) from Alaska to New Zealand, absorbs up to 25% of its liver, kidneys, and digestive tract in anticipation of its epic journey. The Arctic tern travels even further from pole to pole, clocking more like 90,000km(56,000 miles) in one year – the longest known migration in the animal kingdom.

“The longer distant migrants are the ones most likely to struggle with climate change, says Hewson. “The number of stopovers which are used by the birds increase the chance of there being a mismatch in timings.”

Under the more extreme climate projections, these huge journeys are likely to become extremely challenging. “What we’re looking at now is the tip of the iceberg in terms of what will happen – things will get a lot worse in the future,” says Hewson. “Just because they’ve done it in the past doesn’t mean they will be able to do it in the future.”