Migratory birds are arriving earlier each spring
When migratory birds return to Sweden after a long winter it signals the arrival of spring for many people. However, climate change is affecting how and when birds migrate. Research shows that many migratory species are now arriving earlier than they did just a few decades ago, which can have consequences for both the birds themselves and the ecosystems they depend on.
Migratory birds have recently arrived in Sweden, and for researchers at Tovetorp Zoological Research Station, the birds' breeding season marks an intense period of fieldwork.
“It's a hectic but fantastic time. We follow the arrival of the birds day by day and see clear signs that the climate is affecting their behaviour“, says Cilla Kullberg, associate professor in ethology at the Department of Zoology at Stockholm University.
In an ongoing research project, scientists are studying how climate change and habitat alterations affect behaviour, breeding success, and survival of migratory birds. One particular focus is on the willow warbler, one of Sweden's most common bird species with around 13 million breeding pairs (estimated in 2008). Sven Jakobsson, professor emeritus was involved in initiating studies of the breeding biology of willow warblers around Tovetorp in 1979, and the population around the research station has since been studied periodically by Sven and colleagues to this day.
“The willow warblers that nests around Tovetorp spends the winter in West Africa, and we see that both males and females arrive earlier in the spring now than in the 1980s. An interesting thing is that the males always arrive before the females in the spring, but the difference in arrival time between the sexes has increased over the years from about 9 days in the 1980s to just over two weeks today, says Cilla Kullberg.
A willow warbler's journey
Thanks to ringing, a way of marking birds, and extensive international cooperation, researchers have gained valuable knowledge about bird migration patterns over the years. This year, they were also able to follow the journey of a particular willow warbler in an unusual way.
“In early April, we received a report from the Bird Ringing Centre at the Swedish Museum of Natural History. One of the male willow warblers we ringed at Tovetorp last year had been caught by a ringer in Malta on 9 April this year,” says Cilla Kullberg.
When weighed, the male turned out to weigh a full eleven grams, which means he had enough fat reserves to survive a longer flight. A male willow warbler without fat reserves weighs around nine grams.
“So, we just had to wait and see if he would also show up in our research area. And he did! On 21 April, he appeared, with his colour markings Red/Aluminium Pink/Yellow, in exactly the same territory as last year”, says Cilla Kullberg.
During these twelve days, he has flown over 200 kilometres per day. Along the way, he has used up his fat reserves and probably weighs around 9 grams upon arrival at Tovetorp.
“Since his arrival, he has been sitting in his territory, singing and fighting with other males. The first females arrived on 5 May, and both this and the arrival of the first males on 21 April are earlier than we have ever recorded before. However, the male Red/Aluminium Pink/Yellow did not have a female today, so we will see if he has found a partner tomorrow!”
Climate change affects migratory birds
Scientists believe that earlier arrival patterns are linked to global warming. Springs are arriving sooner and temperatures are rising, making it less risky for males to arrive early. In the past, early arrivals risked encountering cold weather and food shortages.
“There are evolutionary advantages to arriving first. Males that arrive early can secure better territories and increase their chances of attracting a mate. Females, however, are more dependent on the availability of food resources like insects, which are critical for egg-laying and incubation,” explains Cilla Kullberg.
If males and females are affected differently by climate change, it could ultimately impact reproductive success and the stability of bird populations. There is worrying data indicating that the Swedish willow warbler population is currently declining, but researchers do not yet know why.
“By combining data from bird ringing, climate statistics, and long-term field studies, we can better understand how the entire life cycle of migratory birds is changing. The willow warbler serves as a valuable indicator species because it is common, easy to study, and highly sensitive to environmental changes,” says Cilla Kullberg.
World Migratory Bird Day
Migratory birds depend on being able to move between different parts of the world to find suitable conditions for survival. Most birds that nest in the Nordic region migrate south during the winter, often to Africa. When conditions improve, they return to their breeding areas, navigating their long journeys with the help of the sun, stars, and Earth's magnetic field.
World Migratory Bird Day is celebrated globally on May 10. The aim is to raise awareness of the importance of migratory birds and the challenges they face. This year's theme focuses on creating bird-friendly cities and communities.
Further reading
Tovetorp Zoological Research Station
More research projects on migratory birds at Tovetorp
Wild birds possess genetic predispositions to learn population-specific songs
Last updated: May 9, 2025
Source: Communications Office