Migratory connectivity: high or low?

Our paper on land-bird migratory connectivity is now out in the Journal of Animal Ecology – it can be viewed here (now open access). Here are some words about it:

If you’re interested in understanding how an animal population might be affected by environmental change, a good starting point is knowing where the population is located. Tracking migratory animals throughout their annual cycle is therefore a fundamental step towards uncovering what environmental conditions they’re being exposed to, and how they might respond in the future.

But we can do more than just plotting dots & lines on maps. The movements of individual migrants from different source populations form a network, and the properties of this network (‘migratory connectivity‘) – whether a population spreads out over a narrow or wide non-breeding range, and whether it mixes with other populations or remains separate – probably have important consequences for population dynamics and conservation.

As tracking technology becomes increasingly lightweight and affordable, the necessary data for describing the patterns formed by migratory networks has grown and grown. In our recent paper, we pulled out the dots & lines from all peer-reviewed maps of tracked migrant land-birds from North America (389 individuals of 16 species) and Europe (323 individuals of 29 species).


This is the raw data on which the paper is based – lines connect start (breeding) and end (non-breeding) points of the migration of >700 individual migrant land-birds.

We grouped birds into populations – individuals of the same species, usually tagged at the same study site – and measured the average distance between the non-breeding sites of individuals from each population. On average, 2 birds tracked from the same population used non-breeding sites more than 700 km apart. As a result, individuals from different breeding populations tended to mix during the non-breeding season, rather than occupying discrete areas.

Is 700 km a large or a small amount of spread? It depends. But for the purposes of protecting a population throughout its annual cycle, 700 km is wider than most protected areas. This is what we mean by ‘low migratory connectivity (‘low’ as in ‘low connected-ness between a particular breeding site and a particular non-breeding site’, rather then ‘low connected-ness between individuals from different breeding sites’).


Distribution of population-level migratory spread.

This result suggests something important about how migrants might respond to future environmental change. The loss or degradation of any site in Africa / South America is unlikely to have a pronounced affect on any particular breeding population; its impact will instead be diluted across several breeding populations. From a conservation perspective, this means that it might be impossible to provide a migrant population with strict year-round protection; conversely, protecting a particular patch of land in Africa / South America might (diffusely) benefit individuals from several breeding populations.

Of course, 700 km is an average value; population spread ranged from 140 km to more than 2000 km. Whilst 40% of this variation can be explained simply by geography (see the paper to find out more), 60% remains unexplained; some populations spread out less than average (perhaps they’re habitat specialists, or are able to use social information to aid navigational decisions, or converge at bottlenecks) and some spread out more. An obvious and important next step is to test for ecological drivers of variation in migratory connectivity.

Finch, T et al. (2017) Low migratory connectivity is common in long-distance migrant birds. Journal of Animal ecology, doi: 10.1111/1365-2656.12635


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