It is surprising how many birds migrate; it is estimated worldwide about 4,000 do so, which is 40% of all species, in the search of better feeding and breeding grounds. However, in the UK we are way below that percentage which, I guess, speaks to our relatively benign climate with only around 70 migrants amongst the 574 that live here for all or part of the year. I’ve struggled to find any British bird that matches the godwit for travelling but the Willow Warbler makes a fine effort for such a small bird arriving as it does from Southern Africa in late spring having flown 5,000 miles over mountains, deserts and seas.
How do the Willow Warblers and their like find their way on a route that they are often charting both alone and for the first time? Scientists think that birds use their sense of smell to follow odours, their remarkable eyesight to follow the Sun, the stars, the Earth’s magnetic field, and landmarks, and wind directions to achieve navigation. And a bit like the European eel, just to mix things up, the return trip can often follow an entirely different route.
And why do they travel at all? Well, obviously avoiding the extremes of climate is a motivator but the length of the day, scarcity of nesting sites, easy availability of food and predators have all aggregated over time so birds seek the best way of preserving their particular tribe, even if it involves massive relocation with all the risks that that entails. But then again migration is not always a baked in evolutionary strategy. Sometimes it is driven by necessity, irruptions as they are called, when birds flee their native land when a primary source of food fails. For the recipient land these are more invasions than migrations, such as when Waxwings appear along the east coast of the British Isles, in the wake of the occasional failure of the Scandinavian rowan berry crop.