Do grazing waterbirds surf the green wave?

This widely held idea about waterbird migration might not be so universal.

When you read “waterbirds surf the green wave”, you might think of ducks gently bobbing on the waves. In this case, however, the green wave does not refer to the sea but to a particular hypothesis that attempts to explain the migration patterns of grazing waterfowl. The migration routes of these birds tends to follow the emergence of green plants during spring. As the green wave of food resources rolls to the north, the birds catch up. This ensures plenty of food along the way to their breeding grounds. You can check the animation below to see how the world turns green over time.

 

Correlation is not causation!

The correlation between spring migration and vegetation indices has been confirmed for several grazing waterbirds, such as geese, ducks and swans. However, correlation does not necessarily mean causation. Other environmental factors might play a role. Perhaps birds are just tracking temperature or day length (variables that happen to be associated with the green wave). To disentangle these factors, a group of international waterfowl researchers teamed up and analyzed the migration strategies of 10 species. They compared the timing of migration with several stochastic models. Can the green wave hypothesis explain the patterns better than random movements?

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The ten waterbird species and their migration routes. From: Wang et al. (2019) Nature Communications. Click here for the full picture.

 

Mixed results

The simulations showed that only two species consistently follow the green wave: Barnacle Goose (Branta leucopsis) and Greater White-fronted Goose (Anser albifrons). These species are the most effective grazers – based on their beak morphology – and rely on fresh grass during their journey north. It thus makes sense that they track the green wave.

In East Asia, however, the Greater White-fronted Goose does not follow the green wave. The researchers suggest that this mismatch is due to human disturbance. Hunting pressure, land use changes and poisoning are complicating the migration of waterbirds in this part of the world. They might take a safer route and deviate from the expected pattern.

Anser_albifrons_sponsa_Colusa_NWR.jpg

A pair of Greater White-fronted Geese © Bill Bouton | Wikimedia Commons

 

Other Explanations

Human disturbance is not the only possible reason that certain species do not follow the green wave. Some species can eat other food types and do not solely depend on the fresh supply of grass. For example, Taiga Bean Goose (Anser fabalis), Tundra Bean Goose (Anser serrirostris) and Pink-footed Goose (Anser brachyrhynchus) often rely on grains during their migration.

Other species might just be unable to predict the progress of the green wave, especially when their migration route passes over the ocean. Birds can also deviate from their traditional migration routes due to unpredictable weather or climate change. Perhaps the latter resulted in a mismatch between migration timing and the start of spring in particular species. Finally, some birds might actively overtake the green wave in order to produce their eggs before the peak of food availability. This will ensure that their goslings have plenty of food when they grow up. Clearly, there is still a lot of work to be done here. I will be waiting for the next wave of waterbird migration papers.

Anser_brachyrhynchus_Nairnshire_2.jpg

A group of Pink-footed Geese in Scotland © Duncan Brown | Flickr

 

References

Wang, X., Cao, L., Fox, A.D., Fuller, R., Griffin, L., Mitchell, C., Zhao, Y., Moon, O.-K., Cabot, D., Xu, Z., Batbayar, N., Kölzch, A., van der Jeugd, H.P., Madsen, J., Chen, L. & Nathan, R. (2019). Stochastic simulations reveal few green wave surfing populations among spring migrating herbivorous waterfowl. Nature communications, 10(1), 2187.

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