1965 SHORT COMMUNICATIONS 533

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DIURNAL RHYTHMS OF FULMARS F U L M A R U S GLACIALIS IN THE ARCTIC AUTUMN

This paper describes changes in the diurnal rhythms of Fulmars Fulmarus glacialis on Jan Mayen (71" N., 8" 40' W.) during the post-breeding season. The object of the study was to determine whether Fulmars changed their daily activity during the rapid shorten- ing in daylength towards winter, a change which would be expected since Fulmars usually occur on Jan Mayen all winter (Bird & Bird 1935, ' Ibis ' (13) 5 : 837-855). They may, however, be absent when pack-ice surrounds the island though this has not occurred in recent years.

Daylength lasted 24 hours until 11 July and had decreased to 12 hours by 22 Septcm- ber. Cullen (1954, ' Ibis ' 96 : 31-46) found that Fulmars on Jan Magen showed a marked diurnal fluctuation in activity in July and August of 1950. The present study covered the period 12 August to 18 September 1962, spanning the transition from a night consisting of a short period of poor light by which a book could be read at midnight, to one of about 12 hours of darkness and twilight.

METHODS

Fulmars were counted during eight watches, the first seven lasting 24 hours or more and the eighth from 16.40 to 06.00 hrs. (local time). The watch point was on the narrow beach below Maria Musch Bay, which faces N.N.E. It was chosen because most of the ledges watched were fully visible and very few birds were out of sight if they retreated to the backs of the ledges. All Fulmars on a certain area of cliff were counted every 20 minutes as were the number flying over a larger area of cliff, which included the first and usually contained about 30 times as many sitting birds.

Weather notes were kept and compared with data from the more exposed weather station 14 miles to the northeast; but weather variations on Jan Mayen are often very local. Light intensity was not measured.

RESULTS Fig. 1 records, as far as light permitted, the number of birds sitting on the first area

of cliff throughout each watch, and Fig. 2 the number in flight over the second area. The total number in flight was usually 5-lO0/b of the total population.

Increase in population. The figures show an approximately two-fold increase in Fulmars on the study area

occurring largely between 27-28 August and 2-3 September (watches IV and V) and reflect a general increase on the whole of this particular cliff. This was not checked by making counts of the entire cliff but by 12-13 September (watch VII) the number on the whole cliff almost certainly exceeded the number of pairs which could have bred there, since not only were all the obvious breeding ledges occupied but also many ledges accessible to Arctic Foxes Alopex Zagopus as well as many ledges on unstable parts of the cliff. (Fulmars on Jan Mayen normally breed successfully only on cliffs inaccessible to Arctic Foxes and stable enough to support vegetation. Marshall (1952, ' Ibis ' 94 : 310- 333) nonetheless suggests that more Fulmars could breed on Jan Mayen than actually do so since many ledges hold fewer pairs than other apparently similar ledges. If this were the case in Maria Musch Bay, breeding density could have been exceeded before watch VII.)

On at least two other cliffs numbers declined over the period studied. A population of 2,0004,000 on a cliff in Tommerbuktu (6 miles southwest) decreased to 600-1,000 by 14 September and numbers on the cliffs by Walrus Gut (4 miles southwest) also decreased markedly, though no counts were made there. Many of the birds observed in the study

534 SHORT COMMUNICATIONS IBIS 107

/50

27-28 Augusi YI

L I --

12.00 2 4 W hT1 12.00

.r-"': vl+

7-0 Septemaer

FIGURE 1. Number of Fulmars Fulmarus glacialis sitting on part of a cliff on Jan Mayen. The gaps about midnight when no counts were made are due to darkness. The break in each watch at another point marks its start (S) and finish (F). The change during watch VI in both Figs. 1 and 2 was associated with the onset of rain and sleet (not uncommon) and also the roughest seas of any watch; wind strength (15-20 knots at weather station) and direction (N) remained unchanged.

\ i

\ ,f 17.18 September

F 7 8 September I

1200 2 4 00 nrs 12 0 0

FIGURE 2. Number of Fulmars Fulmarus glacialis flying over part of a cliff on Jan Mayen (for details see Fig. l).The early morning peak in watch IV was associated with thick mist which lifted to cliff-top level a few minutes before the main influx occurred.

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area were probably full-grown young of the year, since most showed no signs of moult. (Venables (1952, in ' The Fulmar ' by James Fisher)) suggests that during September in Shetland most birds of more than a year old are at sea moulting.) Thus the increase in the study area may have been due to an influx of young (1 962) birds from other cliffs, possibly because the Maria M u s h cliffs were at that time better sheltered from the prevailing winds.

Rest-period. Figs. 1 and 2 show that most birds left the cliffs at night. They usually flew out to

sea, sometimes out of sight, sometimes to visible rafts of birds sitting on the water. Cullen (1954) showed that Fulmars on Jan Mayen have a " rest-period " about midnight, when the number of birds flying along the shore at a point far from any colony dropped very low. Even in the light nights of the early watches more birds were asleep (head along back) at night than at any other time, and the continual gobbling and clucking was always less intense then. As the nights became longer and darker, those birds which did remain on the cliff at night were more quiet, until by the later watches there were several hours of silence. The rest-period became longer as the season advanced, so that most Fulmars spent more time at sea later in the season.

The figures change shape as the season advances. I t may be seen from Figs. 1 and 2 that although the numbers of birds in the study area increased in later watches, a smaller proportion stayed on the cliff at night. I t is also clear that birds departed from the cliffs in the evening more abruptly in the later watches than in the earlier ones. This was not the case with the morning return when, if anything, the birds took longer to accumulate at the cliffs in the later watches.

I t was only after watch V (2-3 September) that the rest-period began to increase fairly regularly from watch to watch. The fact that it became difficult to see the birds at each end of the rest-period at this stage suggests that the birds were being forced to remain at sea by the lengthening period of darkness.

It would seem, then, that in August (watches I-IV) most birds spent about 5 hours at sea, but that in September, as the nights became longer, they left the cliff earlier in the evening and more rapidly and returned later and possibly more slowly in the morning, spending about 10 hours at sea by 17-18 September (watch VIII).

ACKNOWLEDGMENTS

I t is a pleasure to thank Jim Ditchburn, John Hood and Brian Hooton for help and company on the watches, Drs. W. R. P. Bourne and J. M. Cullen for advice and Dr. Adam Watson for comments on the manuscript. Thanks are also due to the Norwegian government for permission to live on and for transport to and from Jan Mayen and to all the personnel there for many kind- nesses. The expedition was aided by a grant from University College London Exploration Society and by a loan of instruments from the Royal Geographical Society.

Nature Conservancy Unit of Grouse and Moorland Ecology, ROBERT Moss. Natural History Department,

Marischal College, Aberdeen.

23 February 1965.

NOTES ON GAMBIAN BIRDS I was in The Gambia from April 1956 until September 1963. During my first

four years I trekked continuously throughout the territory covering an average of 1,100 miles a month, but without much time to observe birds. In the next three years I lived in Bathurst, on the coast, and seldom went up river, but had more time for ornithology and, as a result, found it necessary to modify some of the opinions I was responsible for