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African Birdlife|November/December 2021
Migration as an evolutionary driver
PETER RYAN & MICHELLE VRETTOS

Birds are the most mobile organisms on the planet. Their ability to fly vast distances enables them to exploit predictable, short-term peaks in food availability. Almost one-fifth of all birds undertakes regular movements, usually tied to seasonal cycles. However, such large-scale movements also incur risks from commuting across the landscape. The balance between these costs and benefits determines who stays and who migrates. Recent studies suggest that migration promotes speciation, principally through the formation of sedentary daughter species. Here we highlight some examples and speculate how rapid global change might affect this process.

Migration has long fascinated people and considerable effort has been devoted to learning how birds migrate. We marvel at the physiological adaptations that enable Bar-tailed Godwits to fly nonstop for eight days from Alaska to New Zealand without eating or drinking. We have determined the range of cues that birds use to navigate to return to the same breeding and wintering sites year after year. We have demonstrated that the migratory urge is inherited, with genes coding for the direction and duration of migration. However, we know that the risk of being blown off course during a young bird’s initial migration can be greatly reduced by travelling with more experienced individuals and we have even used small planes to establish new migration routes for threatened species. But why do birds migrate in the first place?

Eurasian Blackcaps exhibit very rapid shifts in migration behaviour. Over the past few decades, the arrival dates of migrants on their breeding grounds have advanced by several weeks and departure dates have been delayed in many populations, linked to ongoing climate change. Birds breeding in central Europe traditionally wintered in Africa or southern Europe, but increasing numbers now move to the UK in winter, where they are supported in part by bird feeders. Interestingly, when they return to their breeding grounds, they almost always breed with a partner from the same wintering area, resulting in the potential for further differences between the two populations to accumulate. At the same time, the migratory urge of blackcaps in southern Germany has almost halved over the past few decades and experiments with captive birds suggest that residency might soon evolve in this entirely migratory population if there is ongoing selection for shorter migration distances.

Why migrate?

The proportion of migratory species varies predictably with latitude, ranging from more than 70 per cent of species in polar regions to barely 10 per cent in the tropics. Migration is particularly important at high latitudes because of the marked difference in day length between summer and winter. Primary production is driven by photosynthesis, which requires sunlight, and longer days increase temperatures, which promote faster growth rates of plants and invertebrates on which birds depend. Thus high latitudes typically experience a large peak in food availability in summer, followed by a dearth of food in winter, when low temperatures also greatly increase the cost of thermoregulation. Birds are well suited to exploit such seasonal peaks in food availability because they can migrate to more benign areas in winter.

The seasonal signal is more extreme in the northern hemisphere due to the greater amount of land in the north. This results in more continental climates than in the southern hemisphere, where the extensive oceans buffer seasonality. As a result, at least at lower latitudes, the proportion of migrants tends to be less in the south than in the north. Also, in the south, rainfall is often more important than temperature in driving migration, although the underlying cause is the same; rainfall triggers a flush of food, which migratory birds exploit. However, rainfall is more erratic than seasonal changes in temperature and so migration among many southern hemisphere species is less predictable than among their northern counterparts.

Another consequence of the much greater amount of land in the northern hemisphere is that most migrant birds breed in the north and migrate south in winter. There are plenty of short distance migrants within the southern hemisphere, but only a few land birds move beyond the tropics. Even among seabirds, which are more diverse in the south, more trans-equatorial migrants breed in the northern hemisphere. It is only among Pterodroma petrels that there is a strong southern hemisphere bias, with nine species breeding in the south and wintering in the north, compared to only two that breed in the north and winter in the south.

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