July252007

Birds in Flight: Migration is Here!

Bird migration refers to the regular seasonal journeys of varying distances undertaken by many species of birds. The movements of birds includes those made in response to changes in food availability, habitat or weather. These however are usually irregular and are termed variously as nomadism, invasions or irruptions. Migration is marked by its annual seasonality.

Many land birds migrate long distances. The most common pattern involves flying north to breed in the temperate or arctic summer and returning to wintering grounds in warmer regions to the south.

The primary advantage of migration is energetic. The long days of the northern summer provide greater opportunities for breeding birds to feed their young. The extended hours allow diurnal birds to produce larger clutches than related non-migratory species that remain in the tropics year-round. As the days shorten in autumn, the birds return to warmer regions where the available food supply varies little with the season.

Within a species not all populations may be migratory and this is termed as partial migration. Partial migration is very common in the southern continents; in Australia, 44% of non-passerine birds and 32% of passerine species were partially migratory. In some species the population at higher latitudes tend to be migratory and will often winter at lower latitude past the latitudes where other populations may be sedentary. This is termed leap-frog migration. Within a population, there can also be different patterns of timing and migration routes based on the age groups and sex. Male and female Northern Harriers migrate along different routes, adult males primarily down the middle of the country with few being seen along the East Coast.