Birds' long journeys and climate change

To "augur" means to hope or predict that a good thing will happen. You hope for success of a project, an oral test at school, a holiday. The verb to augur has a long and fascinating history. It derives from the Latin words avis, meaning 'bird' and specĕrĕ, meaning 'to watch, to observe' because good omens were once sought by observing the flight of birds. That is, it was believed that by observing how birds fly in the sky you could predict the future—a very romantic idea. Today, we are still enchanted by the acrobatic evolutions of large flocks of birds in the skies over our cities, or by the incredible distances travelled by small birds, like swallows or robins, animals that weigh only a few grams and cover distances of thousands of kilometres.

The record for the longest journey is made by Arctic terns (Sterna paradisaea), when migrating from one polar circle to the other flying a total of 90,000 km every year. It is estimated that in the course of their lifetime, terns fly 2.4 million km. Some migratory birds reach impressive altitudes: bar-headed geese (Anser indicus), for example, fly up to 8,800 metres and common cranes (Grus grus) reach as much as 10,000 metres, like an airplane. In terms of speed, the fastest birds can fly at an average of 80 km per hour.

But why do these animals make such long and tiring journeys? They do this for various reasons but the main one is to find places with the necessary resources for reproduction. The greatest mystery, however, is: how do they orient themselves? How do they find their way to the places to breed and then back again? Scientists have discovered that birds use various systems. Some, like robins, have the ability to perceive the earth's magnetic field thanks to small molecular 'compasses' in their eyes. Others, like geese, orient themselves by observing the position of the sun, moon and stars. Other migratory birds learn the route by following their parents: a veritable flying school. Some birds have an exceptional memory and can remember details of the landscape, such as the course of a river, the coastline and the position of hills and mountains. Unfortunately, when human activities change the landscape, these birds sometimes get confused and lose their "sense of direction".

The construction of roads and cities is not the only cause of disturbance to bird migration. A group of scientists from the University of Milan has conducted research to assess whether climate change has an impact on the long journeys of birds. Analysing data collected since the early 1800s, the researchers found that many species start their spring migration up to three days earlier each decade. Global warming due to human activities is also altering migration routes because the characteristics of ecosystems and the distribution of resources change.

Thousands of years after the divinitory observations of our ancestors, watching what birds are doing in the sky can give us information in our present age on how the climate of our planet is changing.

By Andrea Bellati