The Research Council of Finland webpage tietysti.fi reader writes: I work in early childhood education and we were talking with the children about migratory birds. One child asked a question I didn't know the answer to. How do migratory birds that spend the winter for example in Africa know that spring has begun in Finland and that it's time to fly the long way back?
Katja Rönkä, postdoctoral researcher at HiLIFE funded buy the Research Council of Finland, answers:
Among the breeding birds of Finland, mainly insectivores but also many piscivores escape freezing temperatures to tropical Africa. Of our most common birds, willow warblers and tree pipits migrate the furthest, while chaffinches and robins stay closer to Europe or even in Finland – facilitated by bird feeding.
Returning migrants arrive roughly in the order of the old saying "a full moon from a skylark until summer (or rather until the beginning of preparing fields for crops), half a moon from a chaffinch, just a little from a wagtail, not even a day from a swallow". Skylarks and chaffinches overwintering in Europe have the shortest migration distances, wagtails overwintering in the Middle East and north-east Africa need to fly longer, and swallows overwintering in tropical Africa cover the longest distance.
Short-distance migrants can monitor the progress of spring by many signs, such as changes in day length, temperature or prevailing wind, or the movements of other migrating birds.
Global warming may advance the timing or speed of migration
The reader's question about how wintering species in Africa know when to start their spring migration is currently the subject of active research. Bird migration behaviour is internally regulated and heritable, and long-distance migrants prepare for migration at roughly the right time each year. Since wrong timing of a long-distance migration can affect nesting success and even be life-threatening, birds also use changes in their environment as cues to fine-tune their migration behaviour.
The annual variation in the rainy and dry seasons in tropical Africa is one such cue for the right time to migrate. Climate and weather also influence vegetation and abundance of insect prey, which birds could use as secondary cues to seasonal change.
Unfortunately, global warming has already altered local weather patterns, making it difficult to both predict changes in weather and to estimate the appropriate time for migration. A particularly severe drought in East Africa in 2011 delayed the arrival of many long-distance migrants to their breeding grounds.
The main known trigger for migration is change in day length, although in tropical Africa the changes are small. Since climate warming does not affect the Earth's orbit, axis or rotation speed, it does not affect day length. Changes in day length in the tropics, which act as a cue for the onset of spring, are therefore no longer linked to the onset of spring in northern breeding areas.
As the climate warms, northern habitats are freed from snow and ice earlier, and insects become active and reproduce at faster rates. This means that early migrating birds are more likely to take over the best territories and catch the worms for their young. Long-distance migrants may be particularly vulnerable to global warming if they can not adjust the timing or speed of return migration accordingly.
Studies have shown that long-distance migrants have advanced their spring migration, albeit less so than short-distance migrants (Lehikoinen et al. 2019). Earlier arrival may be the result of earlier departure from the tropics or faster movement and less stop-over time during the long return.
Inheritance affects the willingness to leave
During migration birds consume a lot of energy, so they need to prepare well in advance. “Migratory restlessness” prepares them for the long journey and, in the case of spring migration, for the breeding season. Birds build up fat stores and their hormonal activity increases. Studies have shown that depending on the condition of the bird, low food availability in particular can delay departure.
The willingness to migrate is also influenced by genetics, and species and individuals differ in their flexibility in timing their departure. If some individuals respond more quickly to changes in day length, they may be more successful in timing their preparation for migration, departure and ultimately breeding success. If early migrants pass this sensitivity on to their offspring, the timing of spring migration may advance adaptively from generation to generation.
All factors influencing migration in birds wintering Africa are not yet well understood. For most species, there is less research on what they do in wintering areas than in breeding areas, let alone what cues and environmental factors they pay attention to when deciding when to migrate.
When will we get to see the long-distance migrants?
As strong fliers, lesser black-backed gulls return as early as mid-March and ospreys in early April, followed by Caspian and common terns. Wheatears and the earliest swallows also arrive in early April.
In their wake, many familiar small birds return from Africa in abundance, such as the willow warbler, tree pipit, redstart, pied flycatcher and house marten, as well as waders such as the wood sandpiper and falcons such as the Eurasian hobby.
Next in line to return in the second half of April are the cuckoo, yellow wagtail, sand martin, winchat, lesser whitethroat, wood warbler, thrush nightingale, sedge warbler, nightjar, common whitethroat, spotted flycatcher, the unquestionable messenger of summer’s arrival common swift, and my long-time study species, the reed warbler.
Still at the end of May return e.g. the corn crake, garden warbler, icterine warbler and golden oriole, apparently at the top of their voices. Spring migration continues for many species until early June.
Bird sounds can be recorded and identified for researcher use e.g. with the “Muuttolintujen kevät” app, which already has around 270 000 users in Finland.
This article is an English translation from the original Finnish article published in tietysti.fi https://www.aka.fi/tietysti/kysy-tieteesta/mista-muuttolinnut-tietavat-suomen-kevaan-alkaneen/ (25.3.2025).