(261) THE INCUBATION PERIOD OF THE OYSTER-CATCHER

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1 (261) THE INCUBATION PERIOD OF THE OYSTER-CATCHER BY J. KEIGHLEY AND E. J. M. BUXTON. IN 1939 one of us studied a number of pairs of Oyster-catchers (Hmmatopus ostralegus occidentalis) breeding on Skokholm, Pembrokeshire, and as was stated in the paper then published (Buxton, 1939) it had been hoped to make a fuller study of the species when circumstances should allow. In 1946 and 1947 tn ' s work was resumed, and among' other things special attention was given to the length of the period of incubation, about which some uncertainty seemed still to exist. In the paper referred to the incubation periods for five nests were given as 25, 25, 26, 26, and 26J days. Though these figures are roughly correct they were not determined with sufficient precision, and were too few, to be of much value. Some figures were obtained for about fifty nests on Skokholm and Skomer in 1946, but it was decided that these also were not precise enough. In 1947, therefore, all the 51 nests on Skokholm were visited every morning, and during the periods of laying and hatching visits were also made in the evening. By this means the error was reduced to 12 hours or less, and the figures here given are perhaps as accurate as can be hoped for in studying so many nests in one season. The birds (which are unusually tame on Skokholm) seemed not to be unduly disturbed by these visits and no nests were deserted, though in a few disturbance may have prolonged incubation. In 1947 the first chicks hatched on May 30th, the last on July 15th. (In 1939 the first had hatched about June 1st, and the last on July 5th and in 1946 the first hatched on May 27th and the last on July 6th, while on Skomer, the first clutch, already complete, was found on April 28th, so that the first chick hatched probably not later than May 24th, and the last chick hatched about July 19th). Of the three seasons the finest and mildest in April and early May was 1946, and the least favourable was Late clutches are probably often replacements for clutches taken by gulls. Owing to these depredations it is impossible to give an accurate account of the size of clutches unless the nests are visited daily, since clutches of two eggs may represent clutches of three from which one egg has been taken. It is true that if one egg it taken the birds often desert, and then the others are also taken, but this does not always happen. The figures given for Skokholm in r c /3 an d 12 c/2 are therefore probably misleading, and it is unlikely that in all nests recorded as c/2 only two eggs were laid. Other published records are probably open to the same objection, as are also our own records from Skokholm and Skomer in (These are on Skokholm 1 c/4, 20 c/2, 16 c/2 ; and on

2 262 BRITISH BIRDS. [VOL. xu. Skomer 13 c/3 and 9 c/2). In 1947, 141 eggs were laid in the 51 nests, as follows: 2 c/4, 35 c/3, and 14 c/2. This is probably typical of the normal distribution of clutch-sizes in this part of the country. Campbell (1947) compares the clutch-sizes of 65 nests in the Ardnamurchan district with those of 70 nests recorded by Haverschmidt in Holland. It may be worth tabulating these together with Dircksen's records from Norderoog and Melluni, Germany (Dircksen, 1932). c/4 cli C/2 r/. Pembrokeshire 1939 and & 2.59 Pembrokeshire H 2.76 Argyllshire Holland Norderoog ' Mellum Campbell points out that some of the records of Haverschmidt and himself may not refer to full clutches, and it seems likely that the proportion of c/2 and c/i is too high. In general it may be said that in Germany as also probably in Holland (cf. jourdain: MS note-books) and in Norway (Collett 1921) clutches of 4 seem to be rather more frequent than clutches of 2, but, as in Britain, the normal clutch contains 3 eggs. In all but two of the 1947 nests incubation began with the last egg, but in nest No. 8 (c/4) it began with the 3rd egg, and in No. 49 (c/3) it began with the second. Moreau (1946) suggested that "the incubation period should as a rule be counted from the laying of the last egg", except where incubation is known to have started earlier. We followed this method, but calculated the period up to the hatching of each egg, not of the whole clutch. For that reason the number of records in Table I (38) is much smaller than the number of eggs under observation at hatching (83), but the results are, obviously, much more accurate than if a figure had been given from the last egg laid to the last chick hatched. In those nests where it was possible to mark the eggs as laid there was little to suggest that eggs laid first will normally hatch first. In some nests this did happen, in others all the eggs hatched almost simultaneously, and in others again the 2nd or 3rd egg hatched before the first (see Table I). For example: nest No. 8 contained 4 eggs, laid at the usual intervals of hours; incubation began with the laying of the 3rd egg, and the eggs hatched in the order 2, 3, 1, 4. It is of interest to note that here the first three eggs hatched at intervals of about 3 hours while the 4th egg did not hatch for another 24 hours. This seems to suggest that the incubation which took place before the completion of the clutch was nevertheless quite normal for the first day. This is confirmed by the other 'nest in which incubation began before the completion of the clutch. Here only 3 eggs were laid, but the 3rd egg hatched at least 16 hours after the first two.

3 VOL. xu.] INCUBATION OF OYSTER-CATCHER. 263 TABLE I. INCUBATION PERIOD, Nest No Eggs i 4 (2) (I) (3) (1) (2) 28.5 (3) (3) (1) (2) (4) (i) May 17 May 17 May 19 May 19 Nest No. 10 Days, 11 J 5 22 Eggs 1 (1) lost (2) (3) (i) (1) (1) 25,75-5 (3) (1) 26.0, , May 17 May 16 May 19 May 23 Nest No. 23 Days ± Days. +! Eggs 1 (1) (2) May 20 (1) j dead in shell ' lost (2) i j infertile 28 [ j 28 May 23 May 28 j May 31 Nest No, [ Eggs 1 (1) (2} 26.0,25 (3) (1) 25,0.5 W (i) (1) (3) May 30 1 June 2 June 15 Incubation periods of each egg laid, showing in parenthesis the order in which these were hatched. NOTE. Incubation began with the last egg except in clutches numbers 8 and 49, in which it began with the penultimate.

4 264 BRITISH BIRDS. [VOL. XLI. The incubation period varied between 25 days hr., ± ujhrs., and 28 days 6fhrs. + gjhrs., and the average incubation period was 26.5 days, or days. A similar period is also given by Heinroth (1928), Turner (1930) for Norfolk, and Dircksen for Germany. Table I shows the incubation-periods for 38 eggs. From this it will be seen that the hatching of 11 eggs occurred after a considerably longer period than the normal, and it seems likely that this was due to disturbance. These delayed hatchings took place in 5 nests: at No. 49 a hide was erected for photography, with the consequent excess of disturbance; No. 37 was between the house and the much visited north pond, and No. 5 was beside this pond; No. 30 was in another much frequented place; and No. 23 belonged to a very nervous pair which left their nest if anyone passed along the lighthouse railway, and so were also constantly disturbed. It is probably better to ignore these 11 eggs in estimating normal length of incubation, which will therefore be slightly under 26 days. This confirms the records from Skokholm in 1939 (quoted above) and may be taken to correct other records which vary from 21 to 29 days or more. Jourdain (1939) considered that the record of an egg hatched in an incubator in 21 days (Paynter 1907) referred to a partly incubated egg; and commenting on Cowin and Ladds (1939) record of 29 days he remarks that "28 days is a fairly common period, but when a bird has been frequently disturbed it may be exceeded considerably". Our experience on Skokholm suggests that any period over 27 days is due to disturbance and that such disturbance will seldom prolong incubation much beyond the 28th day. Presumably if there is very frequent disturbance the birds would desert, but Dircksen records an incubation period of 34 days i5hrs. at one nest on Norderoog, where the birds were continually disturbed (ununterbrochen gestort) owing to their proximity to the observers' hut. At Skokholm in 1946 one pair were still incubating their three eggs after 41 days. Presumably these were infertile. Occasionally, as Ryves (1946) has pointed out, incubation may not start immediately the clutch is, and if this is overlooked the period will be overestimated. Jourdain further states that 24 days is the minimum period, but the very few records of so short a period may be due to a false assumption that incubation always begins with the last egg. Days No. of eggs TABLE II. HATCHING PERIOD. if if ~3i * Table II shows the distribution of the hatching periods for 83 eggs, that is to say of the length of time between the first chipping of the shell and the emergence of the chick. This period varied between 1 day 14 hrs., + iohrs., and 4 days 12 hrs., + 12 hrs., 4

5 VOL. XLI] INCUBATION OF OYSTER-CATCHER. 265 and the average hatching- period was 2.9 days. (Jourdain gives this period as from 1 to 3 days). There is nothing to show that disturbance plays any part in delaying the final emergence of the chick, and the length of time taken presumably depends on the strength of the chick and the thickness of the shell. A few chicks die during this stage, and we have records of the deaths of one at Skokholm in 1939, two at Skokholm and one at Skomer in 1946, and two at Skokholm in In , or perhaps 4, out of 63 eggs failed to hatch, while in 1946 (at Skomer) the number was 7 out of 60, and in 1947 (at Skokholm) 7 out of 141. We have no complete records of the number of eggs taken by gulls, and occasionally by crows, for previous years, but in out of 141, or over 30% were taken. (At Skokholm in 1946 at least 34 out of 101 were taken.) An unknown number of chicks is also taken in the same way before they can fly, and in 1947 the rings of 6 chicks were found in gulls' pellets within a month of the chicks having been ringed. Oystercatchers vary considerably in their aggressiveness, and though of six nests near one colony of Lesser Black-backed Gulls (Larus fuscus grcellsii) five lost all their eggs, in another nest, which was in the middle of such a colony, only one ^gg was taken. The Handbook states that the chicks remain in the nest from 1 to 2 days, presumably basing this on Dewar (1920), who adds that if the chicks do leave earlier they "do not go more than a few feet away". In our experience in three seasons it is quite exceptional for the chicks to remain in or near the nest for more than 12 hours, and normally they run as soon as they are dry, so that one chick may have left the nest before one of the other eggs has hatched. If the chicks hatch in the morning they will normally have run by the evening, but if they hatch late in the day they may stay in the nest overnight. In fine weather chicks often leave the nest in 5 or 6 hours after hatching, and though they obviously cannot run far on the first day it is certainly not true to say that they remain within a few feet for the first two days. Our thanks are due to the West Wales Field Society for the use of their records, and to Messrs. D. Lack, R. E. Moreau and J. A. Gibb for most useful criticism and suggestions. REFERENCES. BUXTON, E. J. M. (1939). The Breeding of the Oyster-catcher. Brit. Birds, Vol. xxxiii, pp CAMPBELL, B. (1947). Clutch Size of Oyster-catcher. Brit. Birds, Vol. xl, p HAVERSCHMIDT, FR. {1946). Notes on the Nest-Site of the Oyster-catcher, etc. Brit. Birds, Vol. xxxix, p DIRCKSEN, R. (1932). Die Biologie des Austernfischers usw. Journ. f. Orn., 1932, pp L COLLETT, R. (1921). Norges Fugle, Vol. ii, p HEINROTH, O.U.M. (1928). Die Vogel Mitteleurofas, Vol. iii, p. 17. MOREAU, R. E. (1946). The Recording of Incubation and Fledging Periods. Brit. Birds, Vol. xxxix, pp

6 266 BRITISH BIRDS. [VOL. XLI. TURNER, E. L. (1930). Country Life, October 25th, JOURDAIN, F. C. R. (1939). Brit. Birds, Vol. xxxiii, p PAYNTER, F. G. {1907). Quoted in Brit. Birds, Vol. ii, p. 65. COWIN, W. S. and LADDS, E. F. (1939). Incubation-Period of the Oystercatcher. Brit. Birds, Vol. xxxiii, pp RVVES, B. H. (1946). Some Criticisms of the Recording of Incubation Periods of Birds. Brit. Birds, Vol. xxxix, pp WITHERBY, H. F. et al. (1940). The Handbook of British Birds, Vol. iv, p DEWAR, J. M. (1920). The Oyster-catcher's Progress towards Maturity. Brit. Birds, Vol. xiii, pp