British

Vol. 55 No. 3

MARCH 1962

Weights from five hundred birds found dead on Skomer Island in January 1962* By M. P. Harris Department of Zoology, University College of Swansea

SKOMER, 722 ACRES, is the largest of the islands off the Pembroke­ shire coast and one of the most westerly points in Wales. A visit was made there from 2nd to 5 th January 1962, during one of the coldest spells of weather this country has experienced for many years. Large numbers of birds, which had apparently been forced out from the mainland by snow and ice, were trying to exist on the island, being either unwilling or unable to attempt the crossing to . The species concerned, and some indication of the numbers involved, are given in Table 1. It was virtually impossible for these birds to obtain food or water, because the whole island was frozen hard, and many of them were

*Mr. Harris is to be congratulated on making the most of an interesting, if some­ what sad, opportunity. This paper was received soon after the event and we have published it as quickly as possible because, although complete in itself, it records an integral part of the impressive hard weather movements that took place in the cold spell at the turn of the year. We hope, therefore, that it will serve as a reminder of the proposed analysis already mentioned in the February issue (page 96). Apart from many sight records of large-scale migrations to the west and SSW (with a few to the east along the south coast), and records of colossal casualties in Ireland and on some coasts of England and Wales, there was also an interesting series of ringing recoveries from south-west England, Wales, Ireland, northern and western France and north at this time. Anyone who has any relevant observations for the week or ten days beginning 28th December 1961, and has not yet submitted a summary, is asked to write to Kenneth Williamson, British Trust for Ornithology, 2 King Edward Street, Oxford, AS SOON AS POSSIBLE. Information required includes records (however fragmentary) of visible migration, with dates, times, species involved, numbers and directions; details of large-scale deaths; and any available weights of birds caught or found dead, with dates, times and localities.—EDS,

97 BRITISH BIRDS

TABLE I—NUMBERS OF BIRDS SEEN AND DEAD ON SKOMER ISLAND, PEMBROKESHIRE, 2ND-5TH JANUARY 1962 All figures in the "Maximum seen" column are estimates and not counts

Maximum Found seen dead Remarks

Lapwing (Vanellus vanellus) 100 10 Many others very weak Golden Plover (Charadrius apricarius) 2 1 Snipe (Gallinago gallinago) 500 - No sign of ill effects Woodcock (Scolopax rusticola) 50 + - No sign of ill effects Curlew (Numenius arqmta) 100 - Some weak Skylark (Alauda arvensis) ? - Many passing through Wren (Troglodytes troglodytes) ? 10 Many had apparently left Mistle (Tardus viscivorus) 15 6 (T'urdus pilaris) 1,000+ 36 Many too weak to fly (Turdus philomelos) 500 + 18 Redwing (Tardus musicus) thousands 245 Many too weak to fly Blackbird (1'urdus merula) 1,000 5 Stonechat (Saxicola torquata) 1 Large decrease from usual Meadow Pipit (Antbus pratensis) ? 23 Pied Wagtail (Motacilla alba) 1 Starling (Sturnus vulgaris) 200 187 Movement made maximum Greenfinch (Chloris chloris) 4 inaccurate Goldfinch (Cardaelis. carduelis) 10 - Linnet (Carduelis cannabina) - 2 Chaffinch (Fringilla coelebs) 20 15 Brambling (Fringilla montifringilla) 10 2

dying from starvation or cold. A search of the boulders in North Haven, South Haven and The Wick, the three accessible beaches on the island, revealed 5 64 recently dead corpses, which are also recorded in Table 1; most of these had apparently died while roosting among the boulders. A few live birds were caught and ringed, but some of these were found dead within 24 hours, the actual numbers being 12 out of 24 Starlings (Sturnus vulgaris) ringed, five out of 22 Redwings (Turdus musicus) and three out of nine Song Thrushes (T. philomelos). Many other birds were killed by such predators as Buzzards (Buteo buteo), a Hen Harrier (Circus cyaneus), a Peregrine (Falco peregrinus), a Tawny Owl (Strix aluco), Ravens (Corpus corax) and Carrion Crows (C. corone). Kestrels (F. tinnunculus), which two weeks earlier had very frequently been seen on the island, had entirely deserted it, however, as had all the Stonechats (Saxicola torquata) (except for one found dead) and the majority of the Dunnocks (Prunella modularis) and Wrens {Troglodytes troglodytes). The birds most commonly found dead were Redwings, Starlings, {Turdus pilaris), Meadow Pipits (Anthus pratensis), Song Thrushes, Chaffinches {Fringilla coelebs), Lapwings (Vanellus vanellus) 98 '['ABLE 2— SEXES AND WEIGHTS OF BIRDS FOUND DEAD ON SKOMKR ISLAND, PEMBROKESHIRE, 2ND-5TH JANUARY 1962 All -weights are in grams. Weights of "normal" birds have been taken mainly from those quoted by Ash (1957), Browne and Browne (1956) and Williamson (1958), with the addition of some unpublished records. The figures in brackets show the numbers of birds on which the "normal" averages and ranges are based "Normal " Se x Tota l weighe d "Normal " averag e rang e Tota l examine d Weigh t rang e weigh t Averag e

Lapwing (Vanellus vamllus) 6" 3 2 136-145 140.5 ? 4 3 124-131 128 210 (1) ? 1 I 140 Total 8 6 124-145 134.2 Golden Plover (Cbaradrius apricarius) ? 1 1 114 Wren {^Troglodytes troglodytes) IO 5 5-8 6.4 9.2 (70) 7.4-11.7 {Turdus viscivorus) d* 3 3 7^-75 73 $ 2 2 62-68 65 Total 5 5 62-75 69.8 146 (1) Fieldfare (J'Urdus pilaris) c? 8 8 53-60 57.6 9 20 17 53-66 58.5

Total 29 25 53-66 58.4 81-124 Song Thrush (Turdus philomelos) 0 3 2 41-43 42.0 v 5 5 40-49 43.2 p 1 1 34 Total 9 8 34-49 41.8 73-5 (29) 63.0-82.7 Redwing (Turdus rnusicus) c? 7° 55 33-44 37.8 ? 89 73 31-46 37.1 8 ? 12 9 34-43 3 -9 Total 171 IJ7 31-46 37.8 65.5 (68) Blackbird (Turdus merula) $ 3 3 54-58 56.6 94-7 (34) 78.6-109.6 Meadow Pipit (Anthus pratensis) 16 12 8-16 12.7 18.2 (800) 13.9-23.4 Pied Wagtail {Motacilla alba) 6* 1 1 14 21.5 (2) 20.6-22.4 Starling (Sturnus vulgaris) 6* 54 52 36-61 52 0 59 57 41-62 53 ? 16 n 46-56 50 71.8 (38) 56.5-79.7 Total 129 120 36-62 51.4 Greenfinch (Chloris Moris) c? 4 4 16-22 20.2 3°-4 (5) 29-32 Linnet (Carduelis cannabina)

TABLE 3—WING AND TAIL MEASUREMENTS OF MALE AND FEMALE REDWINGS (T«r& mra'na) FOUND DEAD ON SKOMER ISLAND, PEMBROKESHIRE, 2ND-;TH JANUARY 1962

Wing length Tail length mm. mm.

108 1 72 I 109 73 no 2 74 in 2 75 2 112 2 14 76 1 3 113 7 77 3 6 114 3 13 78 2 7 "5 12 10 79 4 10 116 6 9 80 5 8 "7 15 16 81 8 6 118 8 4 82 6 12 119 12 5 83 13 6 120 3 4 84 10 5 121 2 2 85 4 3 122 6 86 8 3 123 87 2 t 124 1 88 1 89 3

Total 7° 89 Total 67 76 Mean 117.3 115.1 Mean 82 78.5 and Wrens. Other species which were present in large numbers but seemed to suffer little were Snipe (Gallinago gallinago), Woodcock (Scolopax rusticola), Curlew (Numenius arquata), Skylark (Alanda arvensis) and Blackbird (T. merula). As mentioned by Ash (1957), Blackbirds seem able to survive in hard weather and can presumably exploit food sources not available to the other thrushes. All suitable corpses were weighed, measured, sexed by dissection and examined for helminth parasites. Most had small numbers of one or more trematodes, cestodes, nematodes or Acanthocephala, but the infestations did not seem high enough to cause any appreciable mor­ tality. Sexes and weights are summarised in Table 2 and the paragraphs which now follow discuss and amplify the data presented there.

Redwing A total of 245 Redwings were found dead, but only 171 of these were in good enough condition to be examined in the laboratory. All but two were first-year birds and all but one were of the Continental form, T. m. musicus. The measurements of wing, tail, tarsus and bill were taken in most cases. Williamson (1958) has stated that there is

IOO DEATH WEIGHTS ON SKOMER IN JANUARY I962 no evidence that the sexes differ in size, but Table 3 shows that the Skomer males had, on average, slightly longer wings and tails than the females. On the other hand, there were no appreciable differences in the bill and tarsus measurements. Similarly, the average death weights for 55 males (57.8 gm.) and 73 females (37.1 gm.) were very close. These weights agree well with those of Ash who gave 38.1 gm. for a single male and 37.2 gm. for the average of two females found dead in cold spells in 1954 and 1956. Williamson gave the average living weight of 68 migrants of this race on Fair Isle—birds which had, therefore, undertaken a long sea crossing and so lost much weight—• as 65.5 gm. Two Redwings, race unknown, trapped on Skokholm (Browne and Browne 1956) weighed 77.5 gm. and 88.2 gm. Even taking Williamson's rather lower figure as the normal for birds on migration, it would appear that some of the Skomer Redwings had lost 50% of their weight.

Fieldfare Of 36 Fieldfare corpses, 29 were examined, 28 dissected and 25 weighed. The sexing criteria put forward by Cornwallis and Smith (i960)—namely, that the crown and nape are grey in the male and grey suffused with olive-brown in the female—were critically applied and indicated 12 males to 16 females (with one indeterminate as it had been decapitated!). However, on dissection, six of the supposed males proved to be females and two of the females to be males, so that the true figures were eight males and 20 females. On the other hand, it is interesting to note that this 2:5 ratio is exactly the same as that given by Phillips (1961), who sexed 47 living Fieldfares by the characters given by Cornwallis and Smith. Ash found two males and nine females in a much smaller sample. Taken together, these counts begin to suggest that there may be a predominance of female Fieldfares in the British winter population. The average weights of the eight males (57.6 gm.) and 17 of the females (58.5 gm.) on Skomer agree with Ash's figures and show a drop of nearly 50% when compared with those of Fair Isle birds (quoted by Ash), which ranged from 81 to 124 gm.

Starling The Starling had the second highest total of individuals found dead and in proportion to the numbers present it suffered to a greater degree than did the Redwing. Of 129 corpses examined, 37 were adults and 92 were first-winter birds. Browne and Browne gave the average weight of eight adult Starlings as 80.2 gm. and of 32 first-winter birds as 70.3 gm. The corresponding death weights on Skomer were 54.8 gm. and 51.0 gm., indicating a loss of approximately 30% in each

101 BRITISH BIRDS case. The average weight of dead males was 5 2 gm., and of females 53 gm- Song Thrush Only nine out of 18 Song Thrush casualties were examined and the average death weight of eight was 41.8 gm. This is slightly over half the average weight of 29 Song Thrushes trapped on Skokholm (Browne and Browne). A Song Thrush killed by a car on the Pem­ brokeshire mainland on 7th January 1962 weighed 82.5 gm., so had apparently not suffered in the hard weather.

Meadow Pipit The average weight of 800 Skokholm birds was 18.2 gm. (Browne and Browne). The average weight at death of 12 Skomer birds was 12.7 gm., a difference of 5.5 gm. or 30%. Loss of weight seems to be more serious in small birds. Browne and Browne gave an instance of a Willow Warbler {Phylloscopus trochilus) losing 17% of its weight in five and a half hours. Obviously, therefore, a small can reach its death weight very rapidly.

Other species Insufficient numbers or lack of normal weights prevent any definite conclusions being drawn for the other species listed in Table 2. However, Wrens appear to have lost in the region of 30% of their normal weight, male Blackbirds 40%, Greenfinches {Chloris chloris) 33%, Linnets {Carduetis cannabina) 26%, Chaffinches (Fringilla coekbs) 44% and a Pied Wagtail {Motacilla alba) 35%. It would seem from the death weights that larger birds, such as thrushes, can lose approximately 50% of their normal weights before dying, while the smaller birds may be able to lose only about 35%.

Sex ratio Substantially more female than male Fieldfares and Redwings were found dead, while there were approximately equal numbers of each sex among the Starlings. The predominance of females in the two thrushes could have been a direct reflection of the populations con­ cerned or a result of their being more susceptible than males to cold weather. It seems that the former explanation may well apply to the Fieldfare. On the other hand, in fasting experiments with the House Sparrow {Passer domestkus), Kendeigh (1945) showed that the survival time for females was shorter than the average for the species. The viability of the heterogametic sex (the female in birds) may be lowered by the action of semi-lethal recessives on the X chromosome (Haldane 1922).

102 DEATH WEIGHTS ON SKOMER IN JANUARY I962

ACKNOWLEDGEMENTS 1 should like to thank G. Jones for assistance with the collection of the corpses, and B. L. James and L. C. Llewellyn who gave much help in the dissections.

SUMMARY (1) 564 birds of 16 species included in Table 1 were found dead on Skomer Island, Pembrokeshire, during the cold weather of early January 1962 and some 380 of them were weighed, measured and sexed by dissection, as far as the state of the remains allowed. The weights of 333 birds are summarised in Table 2. (2) The majority of the Redwings (Tun/us musicus) found were first-winter birds of the race T. m. musicus. Males had slightly longer wings and tails than females. (3) There were eight male and 20 female Fieldfares (Turduspilaris). The colour of the head and nape is of limited use in sexing this species. (4) The Starlings (Sturmis vulgaris) included 37 adult and 92 first-winter birds. (5) It appears that birds of the size of thrushes (normal weight 65-150 gm.) can lose more than 50% of their body weight before death, while small may be able to lose only 35%. (6) Substantially more female than male Fieldfares and Redwings were found. This may have been due to a lowering of the viability in the heterogametic sex, but in the case of the Fieldfares is more likely to have been the result of a predominance of females in the population.

REFERENCES ASH, J. S. (1957): "Post-mortem examinations of birds found dead during the cold spells of 1954 and 1956". Bird Study, 4: 159-166. BROWNE, K. and E. (1956): "An analysis of the weights of birds trapped on Skokholm". Brit. Birds, 49: 241-257. CORNWALLIS, R. K., and SMITH, A. E. (i960): The Bird in the Hand. British Trust for Ornithology Field Guide No. 6. Oxford. HALDANE, J. B. S. (1922): "Sex ratio and unisexual sterility in hybrid ". /. Genet., 12: 101-109. KENDEIGH, S. C. (1944): "Effect of air temperature on the rate of energy metabolism in the English Sparrow". J. Exp. Zool, 96: 1-16. PHILLIPS, J. H. (1961): "Sex and age counts of wintering thrushes". Brit. Birds, 54: 277-282. WILLIAMSON, K. (1958): "Autumn immigration of Redwings Tardus musicus into Fair Isle". Ibis, 100: 582-604.

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