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

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1 British Birds Vol. No. MARCH Weights from five hundred birds found dead on Skomer sland in January * By M. P. Harris Department of Zoology, University College of Swansea SKOMER, 7 ACRES, is the largest of the islands off the Pembrokeshire coast and one of the most westerly points in Wales. A visit was made there from nd to th January, 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 reland. The species concerned, and some indication of the numbers involved, are given in Table. t 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 somewhat 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 ). Apart from many sight records of largescale migrations to the west and SSW (with a few to the east along the south coast), and records of colossal casualties in reland and on some coasts of England and Wales, there was also an interesting series of ringing recoveries from southwest England, Wales, reland, northern and western France and north Spain at this time. Anyone who has any relevant observations for the week or ten days beginning th December, and has not yet submitted a summary, is asked to write to Kenneth Williamson, British Trust for Ornithology, King Edward Street, Oxford, AS SOON AS POSSBLE. nformation required includes records (however fragmentary) of visible migration, with dates, times, species involved, numbers and directions; details of largescale deaths; and any available weights of birds caught or found dead, with dates, times and localities. EDS, 7

2 BRTSH BRDS TABLE NUMBERS OF BRDS SEEN AND DEAD ON SKOMER SLAND, PEMBROKESHRE, NDTH JANUARY All figures in the "Maximum seen" column are estimates and not counts Maximum seen Lapwing (Vanellus vanellus) 00 Golden Plover (Charadrius apricarius) Snipe (Gallinago gallinago) 00 Woodcock (Scolopax rusticola) 0 + Curlew (Numenius arqmta) 00 Skylark (Alauda arvensis) Wren (Troglodytes troglodytes) Mistle Thrush (Tardus viscivorus) Fieldfare (T'urdus pilaris),000+ Song Thrush (Turdus philomelos) 00 + Redwing (Tardus musicus) thousands Blackbird ('urdus merula),000 Stonechat (Saxicola torquata) Meadow Pipit (Antbus pratensis) Pied Wagtail (Motacilla alba) Starling (Sturnus vulgaris) 00 Greenfinch (Chloris chloris) Goldfinch (Cardaelis. carduelis) 0 Linnet (Carduelis cannabina) Chaffinch (Fringilla coelebs) 0 Brambling (Fringilla montifringilla) 0 Found dead Remarks Many others very weak No sign of ill effects No sign of ill effects Some weak Many passing through Many had apparently left Many too weak to fly Many too weak to fly Large decrease from usual Movement made maximum inaccurate 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 recently dead corpses, which are also recorded in Table ; 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 hours, the actual numbers being out of Starlings (Sturnus vulgaris) ringed, five out of 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, Fieldfares {Turdus pilaris), Meadow Pipits (Anthus pratensis), Song Thrushes, Chaffinches {Fringilla coelebs), Lapwings (Vanellus vanellus)

3 '['ABLE SEXES AND WEGHTS OF BRDS FOUND DEAD ON SKOMKR SLAND, PEMBROKESHRE, NDTH JANUARY All weights are in grams. Weights of "normal" birds have been taken mainly from those quoted by Ash (7), Browne and Browne () and Williamson (), 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 Sex examined weighed Weight range Average weight "Normal" average "Normal" range Lapwing (Vanellus vamllus) Golden Plover (Cbaradrius apricarius) Wren {^Troglodytes troglodytes) Mistle Thrush {Turdus viscivorus) Fieldfare (J'Urdus pilaris) " d* $ c O ^ (70) 0 () () 7..7 Song Thrush (Turdus philomelos) Redwing (Turdus rnusicus) Blackbird (Turdus merula) Meadow Pipit (Anthus pratensis) Pied Wagtail {Motacilla alba) Starling (Sturnus vulgaris) Greenfinch (Chloris Moris) Linnet (Carduelis cannabina) Chaffinch (Fringi/la coelebs) 0 v p c $ * * 0 c <J < ' J7 7 n (). () 7 (). (00). () 7. () () i. {). ()

4 BRTSH BRDS TABLE WNG AND TAL MEASUREMENTS OF MALE AND FEMALE REDWNGS (T«r& mra'na) FOUND DEAD ON SKOMER SLAND, PEMBROKESHRE, ND;TH JANUARY mm. 0 0 no in " "7 0 Wing length $ < 7 0 mm Tail length 0* t Mean Mean 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 (7), 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 mortality. Sexes and weights are summarised in Table and the paragraphs which now follow discuss and amplify the data presented there. Redwing A total of Redwings were found dead, but only 7 of these were in good enough condition to be examined in the laboratory. All but two were firstyear 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 () has stated that there is OO

5 DEATH WEGHTS ON SKOMER N JANUARY no evidence that the sexes differ in size, but Table 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 males (7. gm.) and 7 females (7. gm.) were very close. These weights agree well with those of Ash who gave. gm. for a single male and 7. gm. for the average of two females found dead in cold spells in and. Williamson gave the average living weight of migrants of this race on Fair sle birds which had, therefore, undertaken a long sea crossing and so lost much weight as. gm. Two Redwings, race unknown, trapped on Skokholm (Browne and Browne ) weighed 77. gm. and. 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 0% of their weight. Fieldfare Of Fieldfare corpses, were examined, dissected and weighed. The sexing criteria put forward by Cornwallis and Smith (i0) namely, that the crown and nape are grey in the male and grey suffused with olivebrown in the female were critically applied and indicated males to 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 0 females. On the other hand, it is interesting to note that this : ratio is exactly the same as that given by Phillips (), who sexed 7 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 (7. gm.) and 7 of the females (. gm.) on Skomer agree with Ash's figures and show a drop of nearly 0% when compared with those of Fair sle birds (quoted by Ash), which ranged from to 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 corpses examined, 7 were adults and were firstwinter birds. Browne and Browne gave the average weight of eight adult Starlings as 0. gm. and of firstwinter birds as 70. gm. The corresponding death weights on Skomer were. gm. and.0 gm., indicating a loss of approximately 0% in each 0

6 BRTSH BRDS case. The average weight of dead males was gm., and of females gm Song Thrush Only nine out of Song Thrush casualties were examined and the average death weight of eight was. gm. This is slightly over half the average weight of Song Thrushes trapped on Skokholm (Browne and Browne). A Song Thrush killed by a car on the Pembrokeshire mainland on 7th January weighed. gm., so had apparently not suffered in the hard weather. Meadow Pipit The average weight of 00 Skokholm birds was. gm. (Browne and Browne). The average weight at death of Skomer birds was.7 gm., a difference of. gm. or 0%. Loss of weight seems to be more serious in small birds. Browne and Browne gave an instance of a Willow Warbler {Phylloscopus trochilus) losing 7% of its weight in five and a half hours. Obviously, therefore, a small bird can reach its death weight very rapidly. Other species nsufficient numbers or lack of normal weights prevent any definite conclusions being drawn for the other species listed in Table. However, Wrens appear to have lost in the region of 0% of their normal weight, male Blackbirds 0%, Greenfinches {Chloris chloris) %, Linnets {Carduetis cannabina) %, Chaffinches (Fringilla coekbs) % and a Pied Wagtail {Motacilla alba) %. t would seem from the death weights that larger birds, such as thrushes, can lose approximately 0% of their normal weights before dying, while the smaller birds may be able to lose only about %. 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 concerned or a result of their being more susceptible than males to cold weather. t 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 () 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 semilethal recessives on the X chromosome (Haldane ). 0

7 DEATH WEGHTS ON SKOMER N JANUARY ACKNOWLEDGEMENTS 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 () birds of species included in Table were found dead on Skomer sland, Pembrokeshire, during the cold weather of early January and some 0 of them were weighed, measured and sexed by dissection, as far as the state of the remains allowed. The weights of birds are summarised in Table. () The majority of the Redwings (Tun/us musicus) found were firstwinter birds of the race T. m. musicus. Males had slightly longer wings and tails than females. () There were eight male and 0 female Fieldfares (Turduspilaris). The colour of the head and nape is of limited use in sexing this species. () The Starlings (Sturmis vulgaris) included 7 adult and firstwinter birds. () t appears that birds of the size of thrushes (normal weight 0 gm.) can lose more than 0% of their body weight before death, while small Passerines may be able to lose only %. () 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. (7): "Postmortem examinations of birds found dead during the cold spells of and ". Bird Study, :. BROWNE, K. and E. (): "An analysis of the weights of birds trapped on Skokholm". Brit. Birds, : 7. CORNWALLS, R. K., and SMTH, A. E. (i0): The Bird in the Hand. British Trust for Ornithology Field Guide No.. Oxford. HALDANE, J. B. S. (): "Sex ratio and unisexual sterility in hybrid animals". /. Genet., : 00. KENDEGH, S. C. (): "Effect of air temperature on the rate of energy metabolism in the English Sparrow". J. Exp. Zool, :. PHLLPS, J. H. (): "Sex and age counts of wintering thrushes". Brit. Birds, : 77. WLLAMSON, K. (): "Autumn immigration of Redwings Tardus musicus into Fair sle". bis, 00: 0. 0