ONE of the oldest poems in the English language, dating from the middle of the 13th century, goes
Summer is icumen in,
Loude sing cuckoo!
Groweth seed, and bloweth meed,
And springth the wood now
Sing cuckoo!
In their quaint medieval way, the lines express a belief that is still with us today that the arrival of the cuckoo signifies a change of season and an end to all wintry kinds of weather.
The return of other emigrants has a similar significance the prudent admonition that "one swallow does not make a summer" merely reinforces the underlying tenet that when the crowd arrives, summer must be here.
The spring "return migration", as it is called, is the second leg of a round trip for the participants. The previous autumn the travellers will have left their breeding grounds to escape the harshness of the northern winter, heading for the Mediterranean basin or even further south across the Sahara into southern Africa.
It is not the low temperatures they seek to leave behind, but the consequences for their food supplies during the winter months, plants stop growing, many insects disappear, and other potential sources of essential nourishment are rendered unobtainable for long periods by a layer of ice or snow.
The return migration in the spring is a much more hurried journey than the more leisurely southward leg a few months previously.
For one thing, the migrants now are more experienced on the autumn trip many would have been young first timers, but by now they will have become attuned to essential orientating techniques and navigational cues.
Secondly, they are heading for a familiar goal their birth place. And thirdly there is the biological urge to get on with the eternal process of reproduction those migrants who are home first will have had the pick of the best territories and therefore a greater chance of success in breeding.
Meteorologists have amused themselves over the years by trying to relate migratory behaviour in a prescriptive sense to the weather on arrival but without success. There is a homely regularity in these perennial movements, but also a variance from year to year quite independent of the local weather.
An early or a late appearance by the cuckoo or the swallow cannot be construed as any kind of long range forecast for the coming summer, and is much more likely to be related to the weather at their starting point, or to that along their route.