South Dublin Branch Migration

Migration

Why do birds migrate? While the fact that certain bird species migrate has long been known, the reasons have not always been easy to understand. Long-distance bird migration is a phenomenon that is mostly confined to birds that breed in the Northern Hemisphere, though that is not to say that all bird species in the Northern Hemisphere migrate. Birds breeding in the tropics tend not to move very far, if at all, and Southern Hemisphere breeders tend not to migrate to anything approaching the same degree.

Redstarts .. by Robert Vaughan

Why in the main, only Northern Hemisphere birds migrate There are several reasons for this, though they all boil down to availability of food and the desire to raise chicks in as secure an environment as possible. The climate and environmental conditions in the tropics are generally more-or-less constant year-round, hence a steady supply of birdfood (be it insects, fruits, other birds, or whatever) can thrive year-round and so the birds that breed there have no need to move away. If you look at a globe or a map of the earth there is a lot more land in the Northern Hemisphere than in the Southern Hemisphere, and the land in the former extends much closer to the polar regions than in the latter. In fact, the only place in the Southern Hemisphere (apart of course from Antarctica, where very few birds occur anyway) where the climatic conditions at all approach those of the northern boreal and tundra zones of Europe, Asia and North America is in Patagonia, in the very south of Argentina and Chile. In Patagonia certain species do undertake a migration to warmer climes during the southern winter, just like their northern counterparts.

Why breed in the Arctic tundra The main reason that certain birds like to breed in sub-polar regions is that this is an ideal place for them to raise chicks. Although freezing and barren in the winter, the Arctic tundra in Europe, Russia and Canada and the boreal forests covering Siberia and parts of North America are full of insect life in the summer months, and this ensures a rich source of protein that their offspring require to grow and develop. During summer months, the increased amount of daylight in these regions also allows more time for the birds to locate their food. These factors, coupled with the fact that there are fewer mammalian predators in these regions to kill their vulnerable chicks (since these predators cannot fly south to escape the bitter winters and so would have much greater trouble surviving), explains why they go there to breed. The same logic can be used to explain why migrants also travel to the more temperate areas of Europe and the U.S., though obviously the situation with food, daylight, warmth and predators is not quite so black and white there and hence the proportion of migrant species is fewer.

Why our summer migrants go South in winter In the winter, of course, it is a different story, as the food supply dries up, the temperature plummets and the number of daylight hours is reduced dramatically. The northern breeders then must move to warmer winter quarters such as central and South America, India or, as our Irish summer visitors do, to Africa and/or the Mediterranean to find food. Their young are by now able to fly, so their vulnerability to predators is not such an issue, and some species that predominantly eat insects during the summer switch to a mainly fruit-based diet in winter, as fruit is more readily available at that time of year; Waxwings are a good example of birds that adopt this strategy. Also, by avoiding these regions during the breeding season migrants can avoid competition with resident species that are busy feeding their own young and are better adapted to the local environment and so can exploit the available food more effectively. Presumably for these resident species the benefit they get from their feeding specialisations outweighs the threat from local predators, and so it would be pointless for them to put themselves through the physical stress of migration.

The benefits of migration to the species Birds are, of course, not aware of the reasons behind why they migrate, nor do they make a conscious decision to do so, but rather millions of years of evolution have 'hard-wired' it into their brains. In nature, nothing happens without a reason, and almost everything that wild creatures do is done to increase their chances of survival. There is no doubt that such long-distance migrations are a risky strategy, and the effort it requires does exact a heavy toll on many unfortunate individuals. In overall population terms, however, the benefits to the species as a whole outweigh the risks, as it means that more of their offspring survive to carry on their parents’ genes, and at the end of the day that is what biology is all about.

- Niall Hatch from FlightLines, the monthly newsletter of the South Dublin Branch, April 2004 See BTO's Migration Watch www.bto.org/migwatch See BirdTrack Projects