Review
Migratory Birds as Global Dispersal Vectors

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Trends

Migratory birds are important vectors of long distance dispersal (LDD).

Reviewed information and tools may be used to make more realistic estimations of bird-mediated LDD.

The vectoring services of migratory birds may lead to rapid range shifts.

Propagule dispersal beyond local scales has been considered rare and unpredictable. However, for many plants, invertebrates, and microbes dispersed by birds, long-distance dispersal (LDD) might be regularly achieved when mediated by migratory movements. Because LDD operates over spatial extents spanning hundreds to thousands of kilometers, it can promote rapid range shifts and determine species distributions. We review evidence supporting this widespread LDD service and propose a conceptual framework for estimating LDD by migratory birds. Although further research and validation efforts are still needed, we show that current knowledge can be used to make more realistic estimations of LDD mediated by regular bird migrations, thus refining current predictions of its ecological and evolutionary consequences.

Section snippets

The Need to Quantify Long-Distance Dispersal

Long-distance dispersal (LDD; see Glossary) allows organisms to cross population boundaries, move among habitat patches, and colonize remote areas, thus having important ecological, biogeographical and evolutionary consequences 1, 2, 3. Its study and quantification have been, however, hindered by the low frequency of LDD events, the difficulty of tracking propagules over large geographic scales, and the unpredictable nature of LDD vectors operating at such scales (such as ocean currents,

Overlooked Vectoring Potential of Migratory Birds

Birds are probably the most abundant and competent vertebrate vectors [23]. They can disperse propagules both internally, following voluntary or involuntary ingestion of propagules (endozoochory), and externally, following attachment of propagules to feathers or legs (epizoochory). Birds also transport entire organisms, including pathogens and parasites, in both ways 24, 25 (Box 1).

Among birds, migratory species can be key LDD vectors because (i) they move seasonally over broad spatial scales

A Framework for the Study of LDD by Migratory Birds

Propagule dispersal comprises three consecutive phases: initiation (propagule uptake by the vector), transport (propagule movement along with the vector), and deposition (propagule retrieval following transport) [4]. To understand the various determinants of each of these three phases, it is particularly useful to consider the movement ecology framework proposed by Nathan et al. (2008) [38], which comprises four basic components: internal state, motion capacity, navigation capacity, and

Effectiveness of LDD

The realization of dispersal depends on its effectiveness, that is, on the combination of successful transportation and deposition of viable propagules, plus their successful establishment and reproduction. Such effectiveness is critically related to the gains and costs involved in reaching distant habitat patches through LDD (e.g., [49]), and ultimately depends on the constraints posed by a combination of abiotic and biotic filtering of arriving propagules. The expected establishment

Ecological Consequences of LDD

Migratory birds can promote the movement and connectivity of many taxa over extremely large spatial scales, with important ecological consequences. They can promote large-scale connectivity in anthropogenic (e.g., forest–pasture mosaics) and naturally isolated (e.g., lakes and wetlands, mountain tops) landscapes 59, 60, as well as the colonization of distant habitat patches, including those in different continents 24, 61 or hemispheres 16, 17 and on oceanic islands 29, 62, and thus contributing

Hypothesis Testing and Model Validation

LDD predictions might be tested using a combination of direct observations and analysis of their ecological consequences. Direct observations of LDD (e.g., [29]) might be achieved by examining birds arriving from long-distance flights, such as those killed while on active migration by predators, human hunters, or collision with man-made structures (e.g., lighthouses or wind turbines). The origin of collected propagules might then be traced using stable isotopes or genetic markers (see [69] for

Concluding Remarks and Future Directions

A wide range of organisms uses the LDD services provided by birds; hence, more accurate LDD estimations might be achieved by incorporating the birds’ vectoring potential, and thus the full dispersal potential of vectored organisms. Studies of diaspore (e.g., seed) dispersal and pathogen dispersal have traditionally been studied in parallel research lines, but studying the common and distinct processes underlying their dispersal might contribute to and cross-fertilize both research lines. The

Acknowledgments

We thank Ran Nathan and one anonymous reviewer for useful discussions. This study was supported by project CGL2015-65055-P from Ministerio de Economía y Competitividad (Spain) and RECUPERA 2020, Hito 1.1.1, cofinanced by the European Regional Development Fund (FEDER).

Glossary

Endozoochory
dispersal of propagules inside an animal vector.
Epizoochory
dispersal of propagules attached to an animal vector.
Disjunct distribution
species showing large discontinuities in their distribution (e.g., transoceanic and bipolar distributions).
Dispersal kernel
a probability distribution of dispersal distances and the associated spatial distribution of dispersal units.
Dispersal vector
any agent transporting propagules (e.g., birds or wind).
Long-distance dispersal
dispersal acting beyond

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