Migratory birds undertake epic journeys connecting oceans and continents, awing people across the globe. Sadly, as spectacular as these seasonal movements are, many populations of migratory birds are in decline due to human threats, but tackling their conservation is challenging, as for most species many mysteries of their migrations remain unknown.
Scientists believe birds migrate to increase their energetic efficiency: put simply, birds migrate in pursuit of better access to food, warmer climates and less competition with birds of their own and other species. On the other side of the coin, though, by spending long periods in disparate areas during the year (like barn swallows breeding in Lithuania and ‘wintering’ in Southern Africa) these amazing birds become exposed to very different threats that can affect their survival. Understanding these connections between the different stages of the annual cycles, or the migratory connectivity, is thus key for informing successful conservation action for migratory species.
What is migratory connectivity and why is it important for conservation?
Think of your favorite migratory bird that you can observe in your backyard. Now think of all individuals of the same species that you can find across your town, or even across your country. If you live in the Southern hemisphere, have you wondered if they all came from the same breeding areas in the North? Conversely, if you live in the Northern Hemisphere, have you wondered if, after raising their offspring, they all migrate to the same places in the South for spending the nonbreeding season?
Migratory connectivity reflects exactly that! The degree to which birds from one nonbreeding area migrate to the same breeding grounds and vice versa. Now if you think of all the populations of that species, migratory connectivity represents the degree to which different breeding populations mix in the (non)breeding areas.
Migratory connectivity has important implications for the conservation of migratory birds, because threats may affect populations differently depending on their migratory routes, distances covered and timings used. For example, a recent study has shown that populations of Ortolan Bunting migrating across France are in greater decline than populations migrating eastwards, due to illegal hunting in that country for gastronomy. Understanding connectivity is key for planning and manage important sites for migratory birds; in the case of the Ortolan Bunting, stopping illegal hunting in France would help conserving the species’ populations breeding across western and northern Europe!
A mathematical model to predict connectivity
Insights into the migratory connectivity of various species can be obtained through ringing birds, using advanced laboratory techniques based on geochemical markers (isotopes) and genetics, and by fitting biologgers that allow recording individual movements throughout the year. In the last decade or so, a remarkable number of studies were published contributing immensely to increasing our knowledge on the connectivity of several species of migratory birds.
Capitalizing on this wealth of data, Marius Somveille and colleagues developed a mathematical model to predict where different populations of migratory birds in the Americas go during the nonbreeding season. The authors predicted the flow of individuals from a breeding area to a nonbreeding area by assuming birds migrate between these areas to maximize their energetic efficiency (birds are ideal, that is, individuals have a complete knowledge of the resources available in a given area and compete for them). According to the authors’ idea, after breeding, birds migrate to areas with a higher abundance of food, and disperse across the landscapes in order to reduce competition with other individuals.
Although based in simple ecological assumptions, to develop the model the authors had to use complex mathematical formulas from graph theory and economics to theoretical ecology. To make the model predictions realistic, they used maps of species distributions and abundances generated with data from the eBird, a citizen-science platform where everyone can contribute with bird observations. By integrating all this information, they managed to calculate how much energy was available for each species in their breeding and nonbreeding areas, how much energy birds had to spend to move between these places and how much energy they need to survive each step!
Then, the authors tested their model predictions with data from 25 bird species for which migratory connectivity patterns have been revealed with field studies, and found that the model captures very well the known migratory connectivity patterns of these species − Great job!
Does the model explain everything? No, but certainly it is useful for conservation!
“The most that can be expected from any model is that it can supply a useful approximation to reality: All models are wrong; some models are useful” (Box, Hunter & Hunter 2005: Statistics for Experimenters)
The model by Somveille and colleagues is necessarily a simplistic representation of the complex phenomena of bird migration and does not take into account many important aspects inherent to the migratory process. For example, the model only explores the links between breeding and ‘final’ nonbreeding areas, not capturing the immense and intricate network of sites in between, which are key for migratory animals to rest and refuel.
Nevertheless, this model is a very nice step towards our comprehension of migration and the connectivity of birds, providing a novel tool with important applications for their conservation. For example, the model can shed light on the migration patterns of populations for which movement (empirical) data are not yet available. The paper by Marius Somveille and colleagues that motivated this blog was recently published in Ecology Letters and can be found here: Somveille, M., Bay, R.A., Smith, T.B., Marra, P.P. & Ruegg, K.C. (2021): A general theory of avian migratory connectivity. Ecology Letters