The idea that a small bird can change the fate of a forest might sound far-fetched, but science is beginning to show that it's not so far-fetched. Every autumn and every spring, Millions of migratory birds cross Europe carrying inside them seeds from the fruits they have eaten. a few hours earlier, and that silent journey is helping to redraw the map of many plant species. This dynamic even affects the biodiversity of pine forests in different parts of the continent.
An international study with strong Spanish participation has shown that Migratory fruit-eating birds can disperse seeds over distances exceeding 500 kilometersThis process, largely invisible to the general public, is crucial for understanding how ecosystems regenerate, how isolated plant populations connect, and how plants can shift their distribution area in response to climate change.
A European project to track seeds on a continental scale
The investigation has been led by the Cadiz University, with the collaboration of the University of Cordoba, Glasgow University (United Kingdom), the University of Marburg (Germany) and Migres FoundationThe study, published in the scientific journal Proceedings of the Royal Society BIt focuses on a species well known in European landscapes, including the deciduous forests: The common thrush (Turdus philomelos).
The researchers point out that the Fruit-eating birds consume fleshy fruits to obtain energy from the pulp.But in doing so, they also ingest the seeds. Later, these are expelled intact, either through feces or by regurgitation. For birds, it's a food source; for plants, a free transportation service that allows them to place their offspring far from the parent plant.
This work has shown that, during migration, The dispersion distances increase dramatically. Regarding daily movements in breeding or wintering areas, in the latter, seeds usually fall within a few hundred meters, and rarely exceed one kilometer. However, when thrushes begin their migratory flights, some seeds may end up more than 500 kilometers from the point where they were ingested.
According to the team, this process is repeated every year with billions of birds worldwide and It helps connect regions separated by hundreds of kilometersovercoming barriers such as large agricultural areas, cities, mountains or bodies of water and even between biomes such as the taiga.
GPS in thrushes: this is how the journey of a seed is reconstructed
To quantify how far the seeds can travel, the scientific team He tagged common thrushes with satellite GPS devices at various points in EuropeIn the countryside of Cádiz, the spring movements continued northward, while in Heligoland (Germany), Falsterbo (Sweden) and the Cape Ventės (Lithuania) Autumn migration southward was monitored.
GPS data reveals that These small birds are capable of flying several hundred kilometers in a single night.most of the journeys began shortly after sunset. From these trajectories, the researchers were able to estimate the possible routes the seeds would follow as they traveled inside the bird.
In parallel, the following were carried out controlled experiments at the Jerez Zoo and Botanical Gardens to measure the time seeds remain in the digestive tract after ingesting different types of fruit. These retention times are essential: the longer the "internal journey," the greater the potential distance the seed can travel before being deposited.
With the combination of GPS-recorded migration trajectories and intestinal retention timesThe team developed mathematical models capable of simulating seed dispersal on a continental scale. These models allow researchers to estimate how many long-distance dispersal events occur each year and the probability that a particular seed will travel tens or hundreds of kilometers.
The results indicate that, for the common thrush only, migration can generate hundreds of thousands of seed dispersal events over 100 kilometers annuallyOutside of migration, that capacity plummets and the seeds usually remain very close to the plant where they were consumed.
A silent engine of regeneration and colonization
The authors of the study emphasize that the migration of fruit-eating birds acts as a powerful engine for large-scale mass seed movementsThis dynamic helps to explain phenomena that would otherwise be difficult to understand: the presence of certain plants in oceanic islands far from the mainland, the survival of plant populations in landscapes highly fragmented by roads, crops or urbanizations, or the rapid colonization of areas that have suffered fires or other disturbances.
Compared, other frugivorous animals such as foxes, martens, or non-human primates They also contribute to seed dispersal, but typically over much shorter distances, less than 10 kilometers. Only some migratory waterfowl, such as ducks and geeseThey reach similar dispersal ranges to those observed in thrushes during their seasonal migrations.
Claudio A. Bracho Estévanez, a researcher affiliated with the University of Cádiz and the lead author of the study, summarizes that “The potential of fruit-eating birds to disperse seeds over long distances during their migration is enormous.”This is a process that is repeated twice a year and connects regions separated by hundreds or even thousands of kilometers.
This eco-friendly paper translates into a direct contribution to the natural regeneration of ecosystemsbecause birds deposit seeds in new habitats or in areas where plant populations have declined. Furthermore, by moving seeds between distant areas, they help maintain the ecosystem. genetic exchange between populations that might otherwise remain isolated by human activity.
Other co-authors of the study, such as researcher Pablo González Moreno, highlight that bird migration is thus revealed as a key mechanism for connecting distant ecosystemsoften separated by what are veritable insurmountable walls for the plants.
Not all seeds have the same opportunities
One of the most striking findings of the study is that Not all plant species benefit equally from this “air transport service”The size of the seed is confirmed as a decisive variable in determining how far it can travel.
The Small seeds tend to remain longer in the digestive system of birds.This increases the likelihood of coinciding with the start of a long-distance migratory flight. In contrast, larger seeds are usually expelled earlier, when the bird is still moving locally.
The study illustrates this difference with well-known examples from European flora. In species with relatively large seeds, such as the yew, only a tiny fraction, around 0,1%, would reach over 50 kilometers carried by migrating thrushes.
At the opposite extreme, fleshy fruit species with much smaller seeds, like the strawberry treeThey have a much higher probability of reaching great distances. For this type of plant, that same 0,1% of seeds could easily exceed 100 or even 130 kilometers.
In this way, the size of the seed acts as an ecological filter: Plants with small seeds have a better chance of taking advantage of bird migration. to colonize new territories or move following the environmental conditions that are favorable to them.
Climate change, habitat fragmentation and the need for protection
The current context of Climate change and habitat fragmentation This gives special importance to this process. As temperatures rise, many plant species need to shift their distribution areas towards colder latitudes or altitudes to maintain adequate temperature and humidity conditions; many might require moving towards the tundra or areas with similar climate.
Without help, the seeds of most plants would barely advance a few meters or tens of meters per generation. However, thanks to the migration of fruit-eating birds like the common thrush, That displacement can be on the order of hundreds of kilometers in a single seasonThis increases the chances of survival for plant species in a changing climate.
In this sense, the project MIGRANTSEEDS (Migratory birds as long-distance seed dispersers of plant communities under climate change), funded by the Ministry of Science, Innovation and Universities, studies precisely How migratory fruit-eating birds help redistribute plant communities in the context of global warming. The study on the common thrush is part of this line of research.
Juan P. González Varo, professor in the Department of Biology at the University of Cádiz and principal investigator of MIGRANTSEEDS, points out that The ecological role of birds such as thrushes, blackbirds, robins, warblers, redstarts or flycatchers remains largely unknown to society and it is hardly taken into account in public policies aimed at managing, restoring or conserving ecosystems.
The authors insist that the results offer a solid scientific basis for incorporating the role of migratory birds in conservation and ecological restoration plans. Protect both these species and the migration routes on which they depend It is not only a matter of faunal biodiversity, but also of ensuring that a key process for the health and resilience of plant ecosystems continues.
The evidence gathered by this European study shows that behind the nocturnal flight of a seemingly anonymous thrush, lies something else entirely. a discreet but fundamental mechanism that connects distant forests, maintains gene flow between populations, and helps plants keep pace with an increasingly rapid changing climate, placing migratory fruit-eating birds at the center of adapting our landscapes to the future.
