For many people, a welcome sign of spring is the return of those migratory bird species that come here to breed, but spend their ‘winters’ further south. Generations of people have looked forward each year to the reappearance of swallows and cuckoos, redstarts and warblers, and all the other familiar birds that signal the return of warmer days. Over the years, naturalists have noted their first sightings, as different species arrive one after another in well-established sequence. Annual records of bird arrivals in different parts of Europe now extend back over three centuries. They confirm that the arrival dates of many species vary from year to year, according to prevailing temperatures, with earlier sightings in warmer springs. But underlying these annual fluctuations is a longer term trend towards increasing earliness, coinciding with progressive climate warming. Among 983 European bird populations whose spring arrival dates have been monitored over periods of years, 59% showed no significant long-term trend, but 39% had become significantly earlier and only 2% significantly later. The average advance in first arrival dates over all species and sites was 3.7 days per decade. Most of this change has occurred in the last 50 years, as the effects of global warming have become more marked. In general, the earliest species to arrive each year showed the biggest advance in arrival dates. This fits the findings that, in western Europe, temperatures in early spring have shown greater fluctuation, and greater long-term change, than temperatures in later spring.
Some migrants are also now departing later than in the past, spending more of the year in their breeding areas. This trend has been detected mainly in multiple-brooded species, some individuals of which might squeeze in an extra brood in a lengthened season. In contrast, normally single-brooded species now tend to leave earlier than in the past. They arrive earlier and breed earlier, so the whole cycle is shifted forward. In addition, many bird species have become less migratory over the years, with increasing proportions of individuals remaining in their breeding areas year round, or at least migrating shorter distances. For example, Chiffchaffs which were formerly seen here only in summer are now appearing increasingly as winter visitors to gardens across southern Britain. On the continent, White Storks and other species, which formerly wintered entirely in Africa south of the Sahara, are now remaining in increasing numbers in southern Spain. Such changes in the schedules and distributions of migratory birds make this an exciting time for bird-watchers intent on recording the outcome. Worryingly, however, population declines are now more frequent and more marked among migrants than among sedentary birds; and the question is how much these negative trends are due to climate change and how much to other human impacts. Understanding these changes is essential if we are to conserve these species into the future, and if the spectacular phenomenon of bird migration is to remain as a source of wonder and inspiration to subsequent human generations.