Migrants: the bird world's entrepreneurs

Why swallows migrate and sparrows stay put

April 2010: Why do swallows migrate, while sparrows don't? Why do pigeons stay put, while warblers wander? Newly published research suggests that variation in migratory behaviour among birds may be a side effect of where they get their food.

Birds are more likely to migrate if they live in highly seasonal environments, such as the Arctic or parts of the tropics with pronounced wet and dry seasons. However, bird communities in most parts of the globe display the full range of migratory behaviour, from strictly resident species to long-distance trans-continental migrants.  So why do birds experiencing the same level of seasonality respond in such different ways?

ON THE LOOK OUT: Greater spotted  woodpecker are 'Office workers'; resident foraging birds. Photo credit Wildlife Extra.

It may be because their perception of the seasonality of an environment varies according to their mode of foraging. Some species rely on food that is out in the open and easily located, such as flying insects, fruits and berries or flowers, while others seek out cryptic or hidden food sources, such as wood-boring insects, soil invertebrates or scattered seeds. Exposed food resources are easy to find, but are spatially and temporally variable, while hidden food sources tend to be ‘buffered' both from environmental fluctuation and overexploitation, so are much more reliable.

In search of food
An analysis of North American songbirds just published in the journal Evolutionary Ecology reveals a strong correlation between foraging behaviour and migration. Species that rely on exposed food types need to move about to track the availability of their variable resources, but also evolve a more flexible and opportunistic approach to foraging, so that they can exploit temporarily abundant food sources, whatever they might be. This in turn facilitates migration, since it enables food resources to be located quickly in unfamiliar environments.