The Motivation Behind Migration

Species like this red-crowned crane perform yearly migrations, but how do they weigh up the costs and benefits? (Image Credit: Alistair Rae, CC BY-SA 2.0)

Where the wild birds go: explaining the differences in migratory destinations across terrestrial bird species (2018) Somveille, Manica & Rodrigues. Ecography, 42, p. 225-236.

The Crux

Migratory birds make up a huge chunk of the world’s bird life, yet there are still a lot of gaps in our knowledge concerning why they migrate to the areas they do. There’s a variety of potential benefits to migration, from remaining within a comfortable temperature range or a preferred habitat, to gaining access to areas that have a surplus in resources, to escaping competition with resident species. However, migration also results in increased mortality due to the amount of energy it takes. This week’s study tried to analyse the drivers of migration, and what trade-offs were made between migration’s potential benefits and costs.

What They Did

The researchers analysed the habitats of 621 species of birds, defining migratory birds as those whose wintering (non-breeding) and breeding habitats showed minimal overlap. They measured the access to resources in both habitats, geographical distance between the habitats, as well as the differences in average temperature and habitat type between them.

The researchers then simulated a series of alternative habitats for the species, and tested how likely the species were to use these habitats. This allowed them to estimate which drivers had the most impact on species migration.

Did You Know: Release From Competition

Suddenly finding yourself in an area where your regular competitors are absent can do all sorts of things for a species. It’s one of the reasons invasive species are often able to establish: the species that normally take a share of their resources are gone. It can lead to population explosions, with some species able to expand well beyond their normal limits, and even into areas previously thought inhospitable for them. It’s one reason many species migrate, to get away from competing species at times when resources become scarce.

What They Found

Results indicated that the choice of migratory habitats was largely determined by the access to resources and the ability to remain within a confortable temperature interval. Species which traveled further often had larger bonuses in terms of available resources, but experienced larger fluctuations in temperature and greater energetic costs.

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Migratory birds are often thought of as large flocks of waterfowl or Arctic travellers, but small birds like this Chestnut-sided warbler also shift habitat on a seasonal basis (Image Credit: Matt Tillett, CC BY 2.0)

Problems?

Large-scale studies of species like this are generally unable to account for the myriad differences in behaviour that different species’ biologies produce, rather equating all species as the same. Additionally, the proxy used to analyse resources (the normalised difference vegetation index or NDVI), whilst widely used, is a VERY vague method for measuring any habitat-based variable.

So What?

Whilst non-migratory species can be analysed year round in one habitat, measuring fluctuations in migratory species can be difficult. Estimating the effects of climate change and reductions in land use are therefore also difficult. Studies like this allow us to predict which species are going to be most heavily affected by the aforementioned phenomena.

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