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If you’ve ever wondered why cuckoos or swallows always arrive in the British Isles at around the same time in April, then you’re echoing a question that has fascinated biologists for centuries. Timing for any migrant is critical, because the animal not only has to weather the journey, it also needs to find food and complete its breeding cycle. A swallow that arrived in Britain in February wouldn’t find enough flying insects or a mate, and would probably starve.
Just what prompts the urge to migrate is still uncertain, but in the case of birds, some biologists believe that it is triggered by the production of hormones known as gonadotrophic hormones into the bloodstream. These hormones, secreted by the pituitary gland, control the state of reproductive organs and even the build-up of fat which acts as fuel during the journey. An increase in the amount of light will cause the pituitary gland to release more hormones, so our swallow, for example will be “told” to migrate in spring by its own body-clock.
Lack of food is also a powerful stimulus. When cuckoos run out of caterpillars to eat, or painted lady butterflies can’t find flowers, they have no choice but to head south to warmer areas where they can find food. Wildebeests in the Serengeti National park in Tanzania, East Africa are also driven by the need to find food and head for green pastures at times of drought.
Competition with the same species can also force emigration. When large numbers of Painted Lady or Large White butterflies hatch in the same place, their natural tendency is to seek out food over a wider area, and so dispersal will benefit all the individuals by rationing the food supply.
Finding the right habitat is vital. Some species which aren’t strictly migratory respond very quickly when conditions are suitable for breeding. The Australian Pink-eared Duck nests on temporary lakes in very dry areas and spends much of its time prospecting for breeding places made available by seasonal rainfall.
