NATURE NOTES

Evening Post, Volume CXXX, Issue 90, 12 October 1940, Page 9

NATURE NOTES

CUCKOO ARRIVES

A MIGRATION RIDDLE

(By R. H. D. Stidolph.)

The first shining cuckoo of the season heard by the writer was recorded at the Carter's Bush Reserve, near Carterton, on September 29, when it appeared that more than one bird was (tailing. There were no signs of the bird in the same area a week earlier. A correspondent at Melling records the arrival of this bird in that district on October 1. "You may be interested to know that this morning at 4.45 o'clock the shining cuckoo was heard by my wife and myself in the bush at Melling," Mr. E. M. Cimino wrote on j that day. "It was about half an hour before daybreak and at dawn its song was practically continuous," he added. On October 6, in the Mount Bruce Forest Reserve, which occupies the hilly country between MauriceviUe and the main Tararua Range, the shining cuckoo was heard calling on several occasions, so that it would appear that this bird is now well distributed in the Wellington district and elsewhere in, New Zealand. Last year, the first shining cuckoo was heard by the writer on October 8, though no doubt it was present a week or two earlier than that, as it usually reaches this country from its winter home towards the end of September. HAZARDOUS FLIGHT. In war or peace, the shining cucktjpjs flight over the ocean from the Solomon Islands to New Zealand proceeds as usual and never fails to capture the imagination of the public. When it is remembered that this bird is not much bigger than a sparrow, a flight of two thousand miles or so across ocean with few resting, places, and these many hundreds of miles apart, is certainly an achievement that gives just cause for thought. Actually, the islands of New Zealand, which are the objective of the cuckoo in its southern movement, are really a small area in the midst of the expanse of the southern ocean, and it can be readily appreciated that a slight error in the line of flight would mean missing this country altogether. How many, if any, of these birds meet such a fate? No one can know. Even more remarkable is the return flight in which young birds hatched and reared in New Zear land set out across unknown oceans and islands to reach their winter home. It is asserted, moreover, that the young do not leave with the adult birds, but independently, and therefore have no guides to keep them on the correct route. What is the answer to this riddle of migration? INHERITED INSTINCT. One of the world's foremost authorities on bird migration, Dr. A. Landsborough Thomson, discusses the flight of the shining cuckoo in a volume on that subject. "If its southward flight in spring were only vaguely orientated, or were easily liable to diversion by the wind," he writes, "only a small proportion would hit the mark at all; the majority would pass wide and head for inevitable destruction in Antarctic seas. It is not credible that any species could survive wastage on that scale—a rate of reproduction much higher- than. is> possible^in birds, would surely"'be necessary.' Even welldirected movements involve a heavy enough loss, through the inevitable toll of the journey, through imperfectly developed instinct in some individuals, and occasionally through wholesale disasters due to stormy weather." One has therefore to assume," he states, "that a tendency to travel in a particular direction or by a particular route, for a certain distance or until a certain destination is reached, forms part of the inherited instinct. That is not a very satisfactory answer, as it leaves too much unexplained, but we have at least the consolation that the r difficulty is of a kind which is encountered, in all study of animal behaviour." FLIGHTS WITHOUT LANDMARKS. Referring to flights without landmarks such as that undertaken by the shining cuckoo, Dr. Thomson states that the birds frequently fly only a few feet above the waves and are then out of sight of land except when quite close to the shore. Even if they flew at great altitudes, he points out, their vision could not span many of the stretches of water that are commonly crossed. These crossings are constantly made and somehow or other the birds seem able to maintain- the direction.of their flight without the aid of landmarks: whether this is done in relation to the sun and stars or in some other mariner, this authority states, it is at present impossible to say. The question, he observes, is the more difficult because it is necessary to allow for the effect of wind upon flight, though sometimes a strong wind will defeat all efforts and the birds will lose their direction. Flying without landmarks, Dr. Thomson states, seems to leave the birds without any means of estimating their lateral drift. They cannot feel the wind, because once they leave the ground they form part of the air cur--rent in which they fly; they move in an apparent calm, feeling no wind except the head-on resistance to their own passage. • - • * • ANSWER UNKNOWN. , In spite of what has been learnt about the migratory movements of birds, an answer to the question as to what guides migration is not possible, at least that is the conclusion of Dr. Thomson, who is well qualified to express an opinion on that subject. "How do migrants find their way?" he asks. "The problem really goes deeper than that, as we must also ask how they *know' what way to find. What is it that decides the path and goal of any particular migration and how do the birds succeed in following that path to that goal? There are some things of which we can be certain in negative ; fashion," he continues. "Migration, at i least in its more highly developed forms, is not haphazard in direction; nor is it merely directed in a • vague? general way—towards the south, say, or whatever the favoured quarter may ( be. Equally is it untrue that migrants, , once they respond to the urge and rise j In the air, are the sport of whatever winds may happen to blow. We have ( seen that migration is too successful in bringing birds to appropriate destina- -| tions and too regular in its manifesta- y tibns from year to year to be capable of. explanation in any such manner." Perhaps some day fur/her light may be . thrown on this arresting subject.