Ardea
Official journal of the Netherlands Ornithologists' Union

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de Graaf C. (1950) De nachtvluchten van de Gierzwaluw Apus a. apus (L.). ARDEA 38 (3-4): 165-178
After the publication concerning the crepuscular and supposed nocturnal flights of the Swift (De Graaf 1947) this subject has been further studied. A special investigation bas been made of the expected descent of Swifts in the early morning from about half an hour before till half an hour after sunrise. During the years 1947, 1948 and 1949 observations were made on about 20 different mornings, which showed that Swifts, often alone, often in groups (up to 20 birds) appeared as tiny spots in the sky at a great altitude, whereas in the lower regions not a single Swift was to be seen. Several times Swifts appeared high in the sky and later mounted till they became invisible again, even when prism glasses 8 were used. The facts so far established in relation to the nocturnal flights are the following: 1. In spring the mature birds arrive at their nesting places first and both ??and ?? roost in the nest from their arrival till the time the young have left the nest (Weitnauer 1947). 2. The non-breeding 1 year old birds arrive later than the adults (Weitnauer 1947). The number of Swifts increases till the beginning of June (De Graaf 1947). 3. No crepuscular flights occur till about the beginning of the third decade of May (De Graaf 1947and this paper). 4. When longer periods of bad weather occur, no crepuscular flights are to be seen and the number of Swifts present decreases considerably (Weitnauer 1949 and De Graaf, this study). Apparently the year-old birds and most of the breeding birds have temporarily disappeared (Weitnauer 1949, Koskimies 1947, De Graaf, this paper). 5. Swifts cannot enter their nests in the dark (Weitnauer 1947 and 1949) . 6. After the juveniles have flown out the number of ascending birds is much larger than before (De Graaf, this study). 7. Neither the old birds nor the juveniles return to the. nest after the young have fledged (Weitnauer 1947). 8. From about the 21st of May till their autumn migration a number of Swifts appear about sunrise at a great altitude singly or in groups, flying over or soaring round, later on dispersing in different directions. Now and then they mount again till they are out of sight (De Graaf this paper). Combining these facts it seems dear that at first only the one year old non-breeding birds ascend for the crepuscular flights, spending the night in the sky and descending at sunrise. As soon as the young have flown out, either the old birds or the juveniles - but probably both join the crepuscular and nocturnal flights. When a long period of bad weather occurs no ascents are to be seen, probably because most of the Swifts have migrated to regions where weather conditions are better. Since the young birds can go without food much longer than the adults, this habit not only enables the old birds to save their own lives, but to be back again before most of the young ones have died.


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