Millions of birds follow a 9,000-kilometre invisible highway every year, crossing mountains, deserts and oceans without a map

<iframe
  src="https://hindustansphere.com/embed/cluster/7584d762d5e1b38fa866865d5bf36aedac797804"
  width="100%" height="320"
  style="border:0;max-width:680px"
  loading="lazy"
  title="Millions of birds follow a 9,000-kilometre invisible highway every year, crossing mountains, deserts and oceans without a map"></iframe>