Human life, according to the pessimistic 17th-century philosopher Thomas Hobbes, "is solitary, poor, nasty, brutish and short". So it is, too, for many of the songbirds that seem to hop happily from tree to tree in Irish gardens. The Irish robin, for example, was calculated in the early years of this century to have an average life span of two years and 10 months, and any later estimates suggest that this may have been a month or two too long.
Winter contributes to the high mortality of birds. It is not especially the cold that poses such a danger, but rather its consequences for the food supply. Even the smallest birds are hardy creatures, provided they have an adequate supply of food. During the winter season, however, plants stop growing, many insects disappear, and other potential sources of essential nourishment may be rendered inaccessible for extended periods by a layer of ice or snow.
One solution for the birds is to move to a better climate for the winter. Every autumn, tens of thousands of young Irish swallows migrate to a winter home in the southern half of Africa. Indeed, it has been calculated that as many as five million birds from more than 200 species leave the northern regions of Europe, travelling south to avoid the European winter.
But migration has its dangers, too. In the case of the swallows, of the five or six offspring of any adult pair, on average only one is likely to survive to return to Ireland in the following spring.
But on balance, migration seems to be a safer strategy than staying at home. That it is less risky can be gleaned from the relative breeding strategies of different species: resident songbirds, apparently, produce several broods each year, each containing as many as a dozen eggs; migratory species, by contrast, in general lay fewer eggs and only breed once.
The reason for the difference is assumed to be that the resident species have a higher mortality rate in winter than that experienced by their travelling cousins on their lengthy journeys.
Autumn migration may begin in August for some impatient species, but November may well have arrived before the last tardy stragglers leave for the south. Southbound flights often begin in the aftermath of a deep depression, so that the birds can take advantage of the north-westerly flow of air behind the low. For obvious reasons, the travellers prefer a following wind, and favour cloud-free skies to help them with their navigation.