BRIXISH BIRDS NUMBER 4, VOL. XLVII, APRIL, 1954.
WILD FRUITS IN THE DIET OF BRITISH THRUSHES. A STUDY IN THE ECOLOGY OF CLOSELY ALLIED SPECIES. BY P. H. T. HARTLEY. (Edward Grey Institute, Oxford.) INTRODUCTION. THE accounts of the food of wild birds which appear in such standard text books as The Handbook of British Birds (Witherby, Jourdain, Ticehurst and Tucker, 1938-41) are largely qualitative. Within such a group as the Turdidae these accounts suggest the similarity of the diets of the different species and, in this sugges tion, produce an instance of the generalization made by Darwin (1859) that "the struggle will be generally more severe" between the members of one genus "than between the species of distinct genera". For example, in the genus Turdus, The Handbook (op. cit.) lists rowan and hawthorn berries as the foods of all the six "common" thrushes—Fieldfare (Turdus pilaris), Mistle Thrush (T. viscivorus), Song Thrush (T. ericetorum), Redwing (T. musicus), Ring Ouzel (T. torquatus), and Blackbird (T. merula)— holly and yew berries as the foods of five species, and the fruits of rose and ivy as the foods of three. Of the five sorts of wild fruits eaten by Song Thrushes, four are also mentioned as the food of Blackbirds: and seven of the nine wild fruits listed as the food of Mistle Thrushes are also recorded as eaten by Blackbirds. The important compilation of Schuster (1930) gives much qualitative information on berry-eating. Early in 1947 an appeal was made in the Bulletin of the British Trust for Ornithology for information on the quantitative aspects of the consumption of wild fruits and seeds by birds. It was asked that records should be made through one year, or a longer period, of the occasions when any species of bird was seen to eat any kind of wild fruit or seed. Emphasis was laid in this request on the importance of repeated observation of common occurrences and on the need to avoid selection of records. Numerical data on the consumption of wild fruits by members of the genus Turdus were sent to me by some twenty correspondents: to all of them I am grateful, and especially to Mr. Edwin Cohen for many records from Hampshire and to Mr. E. C. Rees for many records from South Wales. The names of those who contributed information are shown in Appendix II. To information from correspondents I have added the results of my own observations up to the end of March, 1953. A word may be said about the obviously scant use 98 BRITISH BIRDS. [VOL. XLVII.
TABLE I. RECORDS OF THRUSHES EATING WILD FRUITS : COMBINED RESULTS AND RECORDS FROM THREE LOCALITIES.
COMBINED SWAY, SOUTH BERKSHIRE- RESULTS HAMPSHIRE WALES OXFORDSHIRE BORDER. Mistl e Thrus h Fieldfar e Son g Thrus h Redwin g Blackbir d Son g Thrus h Blackbir d Son g Thrus h Blackbir d Blackbir d Mistl e Thrus h Mistl e Thrus h Son g Thrus h Mistl e Thrus h
Yew in — 404 1 64 58 163 17 — — — 41 232 39
Holly 63 3 27 12 101 1 2 3 48 11 60 12 14 27
Spindle — — — — 4 — — — — — — — — 3
Blackberry — — 6 — 39 — 1 11 — 2 — — — —
Wild Strawberry... 1
Hip 5 5
Sloe 1 Wild Cherry 1
Haw 44 47 3i 143 306 6 9 65 28 10 24 6 4 109
Rowan 3 — — 1 12 5
Crab-apple — 4 — — 26 — — — — — 1 — — 17
Ivy 25 — 29 7 76 — — — 20 6 13 1 15 24 White Bryony — — — — 1 — — — — — — — — 1 Walnut — — — — 1 — — — — — _ — — 1 Whortleberry 2 — 1 — 60 — — — 2 1 60 — — — Privet 4 Goose-grass 1
Elder 2 — 33 2 37 — — — 2 3° 34 — 2 1 Guelder Rose
Honeysuckle
Gladdon ... — — — 2 — — — VOL. XLVII.] DIET OF BRITISH THRUSHES. 99
TABLE II. RECORDS OF FEEDING ON CRAB-APPLES AND ORCHARD APPLES, TO SHOW THE SPECIALIZATION OF FIELDFARE AND BLACKBIRD IN APPLE-BATING.
Crab-apples Orchard Waste Heaps (October to March)
Mistle Thrash 2 Fieldfare ... 4 34 Song Thrush 2 Redwing ... Blackbird 26 138
TABLE III. RECORDS OF FEEDING ON HOLLY-BERRIES BY THE THREE RESIDENT THRUSHES, TO SHOW THE TWO SEASONS OF HOLLY- BERRY-EATING.
Months i ii iii iv V vi vii viii ix X xi xii
Mistle Thrush . 5 — 12 21 25 Song Thrush 2 7 2 4 6 6 Blackbird 9 2 E E 8 5 E 37 26 14 Total . 9 2 — — 10 17 — — 2 53 53 45
TABLE IV. MONTHLY RECORDS OF FEEDING ON WILD FRUITS BY THRUSHES, TO SHOW THE CONCENTRATION OF RECORDS FOR SONG THRUSHES IN THE AUTUMN MONTHS. Months i ii iii iv V vi vii viii ix x xi xii
Mistle Thrush 44 8 4 3 8 4 39 28 37 44 3i Song Thrush 10 9 10 4 3 7 1 5° 187 183 5i 16 Blackbird... 92 62 27 17 n 6 60 36 74 "6 112 132
of journal literature in this paper. My colleague Dr. J. A. Gibb (private communication) has shown that, in the case of "common" birds, the more intensive the search of periodicals, the more mis leading becomes the quantitative account. They are foods which have never been recorded before that are communicated and not (nor is this surprising) the well-known items of the dietaries. The quantitative data on the food of the Goldfinch (Carduelis carduelis) in the London area obtained by Fitter (1945) chiefly from records in The London Naturalist are an interesting example of what may be done in the study of less abundant species. By the collection of numerical data it was hoped to find out how far the apparent similarities in the wild fruit diets of the different thrushes were true similarities, quantitative as well as qualitative, or how far there were ecologically significant differences between species and species. In feeding ecology, two species inhabiting the same area may stand in three different relationships to one another. They may 100 BRITISH BIRDS. [VOL. XLVII. have entirely different diets: in this case there will be no inter specific competition for food. They may have some or many foods in common, but each may specialize upon different items of this common stock, and if the items of special preference bulk suffi ciently largely in the diets of the two species, interspecific com petition may be negligible. Or, they may have many foods in common, but one may be a specialist in a food little used by the other: in this case interspecific competition may press more heavily upon the species without the specialization, while the species with one special diet has a "reserve" of ecological advantage. Gause (1934) has produced theoretical evidence that two species of ident ical ecological requirements cannot long persist side by side in one area, and Park, Gregg and Lutherman (1941) have produced ex perimental evidence of the elimination of one of two species from originally mixed populations. The bases of presentation and discussion are the numbers of records of birds eating the different fruits. This method has been preferred to the numbers of fruits eaten; these can be obtained only when birds are watched into the feeding trees at the beginning of the meal and remain in full view until they depart undisturbed. Such data are hard to obtain, and are not available in numbers sufficient for comparison or discussion. Table I shows the com bined results of all records (no information on Ring Ouzels was obtained). The records of the plucking of the more frequently taken wild fruits are shown graphically, as percentages, in Figs. 1 to 5. In a survey of this kind it is impossible to guarantee that the sampling shall be quantitatively adjusted to the different ecological fades wherein Turdidae are found. The results obtained can present only a general picture, a composite of many local variations. As an instance of this local variation the fruits eaten by Blackbirds in the month of August may be cited. In the midland counties of Berkshire and Oxfordshire, in this month, Blackbirds have been seen to eat berries of yew, rowan, bryony and elder: in South Wales in the same month they have been recorded as taking haws, whortleberries and crab-apples, and in the west of Cornwall feed ing on blackberries and the fruits of honeysuckle. It may, perhaps be added that the mobility, and often vagrant habits, of the thrushes makes the massing of the results from several areas to give a general picture a legitimate proceeding, whereas the massing of data from several rigidly and permanently isolated populations would be wholly improper.
DISCUSSION. In Table I three species of thrush are seen to be specialists upon one fruit. In the Song Thrush 75.6% of the 531 records are of the plucking of yew berries. In the Fieldfare 87% of 54 records VOL. XLVII] DIET OF BRITISH THRUSHES. 101 and in the Redwing 86% of 166 records are of the plucking of haws. In contrast to this specialization, in the Mistle Thrush the berries of yew, the most often recorded food, are mentioned in only 44% of the records, and three of the other seven fruits seen to be eaten —holly berries 25%, haws 18% and ivy berries 10%—also bulk largely in the diet. The Blackbird also shows a catholic choice of fruit foods (21 different fruits appearing in 745 records, as com pared with only 7 fruits in 531 records in the Song Thrush). The eating of haws made 41% of the records, with holly berries 13.6% and ivy berries 10.2% next in importance. Of the three resident thrushes it may be said that the Song Thrush is the yew-berry thrush, the Mistle Thrush the yew-and- holly thrush and the Blackbird the haw thrush. When chi-squared tests are made of the relative frequencies of the consumption of yew-berries by Song Thrushes and Mistle Thrushes, of haws by Blackbirds and Mistle Thrushes and of holly berries by Mistle Thrushes and Blackbirds, the first bird of each pair exceeds the second with a probability of very much less than .01 in every case- that is, with a high degree of statistical significance. The two wintering thrushes, Fieldfare and Redwing, differ from the Mistle Thrush and Song Thrush in their small consumption of yew-berries (The Handbook (op. cit.) mentions yew as a food plant of the Fieldfare) and from Blackbirds, with probabilities of considerably less than .01, in their greater dependence upon haws. But the wild fruit diet of these two species is remarkably similar. Let it be admitted that the full winter diet of these two species is not here discussed; it may yet be suggested that the delicacy of Redwings in hard weather—"the first that fail and die are the Redwings" (White, 1789)—is due, in part at least, to the rigour of interspecific competition. This has, indeed, been advanced by Coward (1925):-
"The food of the Redwing is mainly worms, insects, molluscs and other small animals, and when the supply of these is cut off by hard frost or a heavy fall of snow the bird suffers: it will then take to berries, eating those of the whitethorn, yew, ivy and holly, but if these have been already stripped by the Blackbirds and Mistle Thrushes it is forced to leave or perishes".
Attention may be drawn to the consumption of crab-apples by Fieldfares. The number of records is small but sufficient to show a highly significant difference (p <.oi) from Redwings. Only Fieldfares and Blackbirds were recorded as eating crab-apples; these two species are also the most frequent feeders at the waste- heaps in commercial orchards in winter (Table II). Crab-apples are a "hard weather" food. All four Fieldfare records, and 19 of the 26 Blackbird records were made during cold spells in the month ot February. 102 BRITISH BIRDS. [VOL. XLVII.
Fig. i. MISTLK THRUSH Fig. 2. FIELDFARE (Turdus viscivorus) {Turdus pilaris)
Fig. 3. SONG THRUSH Fig. 4. REDWING (Turdus ericetorum) (Turdus musicus)
Fig. 5. BLACKBIRD (Turdus merula)
FIGS. 1 TO 5: COMPARATIVE FREQUENCIES OF RECORDS OF THE PLUCKING OF CERTAIN WILD FRUITS BY THRUSHES. For each species the relative frequencies are shown as percentages of all records for that species. j J* ^ • J ^ Fig. 6. MISTLE THRUSH (Turdus viscivorus) I a. Sway Hampshire. b. South Wales. c. Oxford- Berkshire border. Fig. 7. SONG THRUSH (Turdus ericetorutn) a. Sway b. South c. Oxford- Hampshire. Wales. Berkshire border. Fig. 8. BLACKBIRD (Turdus merula) a. Sway b. South Hampshire. Wales. c. Oxford- Berkshire border. FIGS. 6 TO 8: WILD FRUITS TAKEN IN THREE LOCALITIES. The relative frequencies are shown as percentages of all the records for each species in each locality. 104 BRITISH BIRDS. [VOL. XLVII. Blackberries are not a favoured food with thrushes. Collett (1927) wrote: - ". . . blackberries, on the other hand, are much less appreciated by birds than man. When we see a Redbreast or a Blackbird picking a black berry from the September hedges it is a sign of dry weather and of thirst . . . Every bird that we see blackberrying is worth noting: we shall not make a long list in ten years". Only on the western cliffs of Cornwall, where there are few other wild fruits have I seen blackberries eaten with any frequency. There in the early mornings, on wet days as well as dry, Song- Thrushes and Blackbirds, with Robins (Erithacus rubecula) and White-throats (Sylvia communis) feed regularly on blackberries. A juvenile Blackbird was seen to eat five at one sitting. Most wild fruits are eaten at, and immediately after their ripen ing, but holly berries are remarkable in that there are two seasons of the year when they are eaten (Table III). Different holly trees seem to vary in their season of ripening or in their edibility. At Cuddesdon in Oxfordshire, in the autumn of 1952, it was observed that certain holly trees were stripped before others were touched, and that the fruit of some trees was not plucked at all. Tun er (*935) wrote:" "It seems to me that of all fruits holly berries are the least sought after by birds. In exceptionally hard winters, Fieldfares and Redwings will soon strip a tree, but normally the berries are left . . .". It is at least possible that the massing of the results from several areas might conceal a number of intense, local interspecific competitions for wild fruits. It is therefore necessary to consider how far the conclusions obtained for the country as a whole apply to the ecological relations of the bird's in a single area. Three samples are sufficient in size to warrant separate discussion—Mr, Edwin Cohen's from Hampshire, Mr. E. C. Rees's from South Wales and the writer's from the Berkshire-Oxfordshire border. Sway, Hampshire. (Table I and Figs. 6a, 7a, 8a). Blackbirds differ markedly from Mistle Thrushes and Song Thrushes in the large proportion of haws and the relatively small proportion of yew-berries in their diet, and in the fairly frequent taking of blackberries. But between Mistle Thrush and Song Thrush there is a marked similarity in wild fruit diet. Both species feed largely upon yew- berries, which form 89% of the recorded foods of Mistle Thrushes and 93% of the fruits taken by Song Thrushes. There is one difference between the two species: 26 of the 58 records of Mistle Thrushes eating yew-berries are in the months of July and August, while only 33 of the 163 cases of yew feeding by Song Thrushes fall in these two months. The difference is highly significant statistically, but its ecological significance is less certain. In a year of small berry crop the Mistle Thrushes might strip the yews VOL. XLVII] DIET OF BRITISH THRUSHES. 105 before the time of the main feeding of Song Thrushes had come, so that it is upon the latter species that the weight of interspecific competition would press. It may be added that no similar seasonal differences could be found between the yew-berry eating habits of Mistle Thrushes and Song Thrushes on the Berkshire-Oxfordshire border. It is at least possible that these Hampshire records represent the effects of a temporarily super-abundant food supply, wherein the united efforts of the members of the two species would not suffice to strip the crop before the fruit began to fall. In such a case it would be incorrect to speak of "interspecific competition", since neither species could limit the supply available for the other. South Wales. (Table I and Figs. 6b, 7b, 8b). The picture presented by Mr. E. C. Rees's results is very different from that for southern or midland England. There are no records of the eating of yew-berries, so that the diets of Mistle Thrush and Song Thrush are particularly affected. The Mistle Thrush shows the highest proportions of holly-berries (48%), haws (28%) and ivy-berries (20%) in the diet. (In both Hampshire and the Berkshire-Oxfordshire border Mistle Thrushes show a slightly larger proportion of haws in the food than do Song Thrushes). The Song Thrush takes the largest proportion of elder- berries-30 out of 60 records-and the Blackbird of whortieberries- 31% of 192 records. Mistle Thrushes show a statistically significant (p <.oi) excess over Song Thrushes in the proportions of records of feeding on holly berries, and over Blackbirds in the records of eating haws, holly- and ivy-berries. The specialization of Song Thrushes on elder-berries is illustrated by figures of the same high statistical significance. In Blackbirds, in addition to the relative monopoly of whortleberries, the proportion of records of feeding on holly berries is probably (p = .o5) in significant excess over the proportion of these berries in the feeding of Song Thrushes. The comparisons above discussed are for the full year. Since proportions of different foods are the basis of discussion, the large consumption of whortleberries by Blackbirds in the late summer months markedly reduce the relative importances of all other fruits in the diet. If comparisons be made of the wild fruit diets of the three species in autumn and winter only, the difference in feeding ecology between Mistle Thrush and Blackbird is sensibly reduced, the proportions of haws and holly berries in the diets of the two species ceasing to show any statistically significant difference, and the significance of the larger consumption of ivy-berries by the Mistle Thrush being reduced to (p < .05). On this seasonally limited basis of comparison the Blackbird appears slightly more in the role of a "haw-thrush", a role which it fulfils in other areas. 106 BRITISH BIRDS. [VOL. XLVII. The Eerkshire-Opcfordshire border. (Table I and Figs. 6c, 7c, 8c). The ecological relations of the thrushes in this area do not differ greatly from those shown by the massed figures. The proportion of yew-berries in the diet of Mistle Thrushes is less than in that of Song Thrushes, and the proportion of holly-berries is greater (p < .01 in each case). The Blackbird is the "haw-thrush" of the three resident species and shows a less specialist diet than the other two. In the month of March ivy-berries were the chief wild fruit food of both Song Thrushes and Blackbirds. Redwings fed almost entirely on haws. CONCLUSION. It must be emphasized again that this account deals with only a part of the dietary of thrushes in Britain. The thesis implicit in Darwin's (op. cit.) generalization that there must be ecological differences between two or more closely allied species which are able to maintain themselves in one area, is, in general, well supported by the differences in the wild fruit diets of the thrushes. Half a score of berry-bearing plants supply the main needs of five different species of thrush: and no one fruit is a diet of only one species of bird. The ecological differences between the thrushes are differences in selection from, or utili zation of, the constituents of a common stock. Competition between the species will depend, therefore, on three varying factors—the sizes of the wild fruit crops, the population levels of the birds, and the extent to which other foods may be available. A general account can do no more than indicate the probabilities of interspecific competition or freedom therefrom, in times of dearth. In the Song Thrush the records of wild fruit feeding are notably concentrated in the autumn months (Table IV), two early ripening fruits—elder-berries and yew-berries—bulking largely in the diet. This limited period of fruit feeding at least suggests that in this species there is such a seasonal change in preferences as Noll and Tobler (1934) found in the Curlew (Numenius arquata). SUMMARY. 1. The wild fruit foods of five species of thrush were investigated, on a basis of numbers of records of birds seen plucking the fruits of different plants. 2. The Song Thrush is a specialist on yew-berries: when these are not available it feeds much on elder-berries. 3. The Fieldfare and the Redwing are specialists on haws. The Fieldfare has an "ecological reserve" in the habit of feeding on crab-apples in hard weather; the smaller species, with no such "reserve", is the first to succumb in times of dearth. VOL. XLVII.] DIET OF BRITISH THRUSHES. 107 4. The Mistle Thrush and the Blackbird show a fairly even choice between several fruits. The Mistle Thrush is a "yew- and holly-berry-thrush"; the Blackbird is something of a "haw- thrush". REFERENCES. COLLETT, A. (1927). The Heart of a Bird. London. COWARD, T. A. (1920). The Birds of the British Isles and their Eggs. 3 Vols. London. DARWIN, C. (1859). The Origin of Species. London. FITTER, R. S. R. (1945). London's Natural History. London. GAUSE, G. F. (1934). The Struggle for Existence. Baltimore. NOLL, H. AND TOBLER, E. (1924). Sumpfvogelleben. Leipzig, (seen in reference). PARK, T., GREGG, E.V. AND LUTIIERMAN, C. Z. (1941). "Studies in popu lation physiology. X. Interspecific competition in populations of granary beetles." Physiol. Zool., 14: 395-430. SCHUSTER, L. (1930). "Uber die Beerennahrung der Vogel." /. fur Orn., 78: 274-301. TURNER, E. L. (1935). Every Garden a Bird Sanctuary. London. WHITE, G. (1789). The Natural History of Selborne. London. WITHERBV, H. F., JOURDAIN, F. C. R, TICEHURST, N. F. AND TUCKER, B.W. (1938-41). The Handbook of British Birds. London. APPENDIX I. SCIENTIFIC NAMES OF PLANTS MENTIONED IN THE TEXT. Yew Taxus baccata L.. Holly Ilex aquifolium L.. Spindle Euonytnus europaeus L. Blackberry Rubus spp.. Wild Strawberry Fragaria vesca L. Hip Rosa canina agg.. Sloe Prunus spinosa L.. Wild Cherry Prunus avium L.. Haw Crataegus monogyna Jacq.. Rowan Sorbus aucuparia L. Crab-apple Malus sylvestris (L.). Ivy Hedera helix L.. White Bryony Bryonia dioica L.. Walnut Juglans regia L.. Whortleberry Vaccinium myrtillus L.. Privet Ligustrum vulgare L.. Goosegrass Galium aparine L.. Elder Sambucus nigra L.. Guelder Rose Viburnum opulus L.. Honeysuckle Lonicera periclymenum L.. Gladdon Iris foetidissima L., APPENDIX II. LIST OF CONTRIBUTORS OF INFORMATION. W. B. Alexander, Dr. M. M. Betts, Dr. G. Beven, Dr. Bruce Campbell, E. Cohen, R. S. R. Fitter, Dr. J. A. Gibb, P. Gladstone, G. Gush, Dr. R. A. Hinde, Dr. and Mrs. David Lack, D. J. May, H. J. R. Pease, E. C. Rees, G. N. Slyfield, J. F. Thomas.