NOTES ON THE SPRING TERRITORY OF THE BLACKBIRD

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1 (47) NOTES ON THE SPRING TERRITORY OF THE BLACKBIRD BY DAVID LACK AND WILLIAM LIGHT. INTRODUCTION. THIS study was made on the Dartington Hall estate, South Devon, in 1940, when the abnormal cold weather in late January and early February made possible the trapping and colour-ringing of an unusually large number of Blackbirds (Turdus m. tnerula). Unfortunately in the following autumn Blackbirds were extremely scarce, and all the colour-ringed birds had disappeared, for which the late summer drought was possibly responsible, while the departure of the senior author on national service in December abruptly terminated the investigation. This is apparently the first time that the spring territorial behaviour and threat posturing of the Blackbird have been described. In other respects the study is extremely incomplete, and courtship was not studied, but it seems worth pointing out that the breeding behaviour presents several unusual features which would repay detailed investigation. The Blackbird is a difficult species to study in the country owing to its wildness. One male with a territory in the middle of a wood was so shy that it was rarely observed at all, but those individuals whose territories included areas regularly frequented by human beings were tamer, especially one pair by a well-used road. Visits to Kew Gardens in January, 1941 confirmed that this normally shy species is far easier to study in places where it regularly sees large numbers of unmolesting human beings. NUMBERS TRAPPED. Between mid-january and the end of February, over an area of some seven acres, 25 male and 13 female Blackbirds were trapped. The figures do not necessarily mean that males are twice as numerous as females in the wintering population, as it is probable that, as with Robins (Erithacus r. melophilus), males enter the traps more freely than females. {Lack, 1940). Males are also seen more often than females, but are more conspicuous in behaviour. Of the 25 ringed males, nine were resident. Indeed all the resident males with territories bordering on the trapping area were ringed. One other male was probably resident farther off, as it was seen occasionally. Another male, ringed

2 48 BRITISH BIRDS. [VOL.XXXV. on January 31st, was found dead at Newton Abbott, seven miles to the east, on March 3rd, suggesting that it may have been a winter or hard-weather visitor on return migration. The other 14 males were not seen again, and doubtless included both residents from farther away and winter visitors. Of the 13 ringed females only one resided in the trapping area, and another had a territory a little way outside. The four other females whose territories bordered on the trapping area escaped ringing, which supports the view that females are less readily trapped than males. None of the other 11 ringed females were seen again. This suggests that a higher proportion of females than males were winter visitors. Four of the males and one female trapped in 1940 had been ringed in the area in previous winters, but only two of these, both males, were resident in the trapping area. One of these males was first ringed in November 1935, hence was at least in its fifth year. SIZE OF TERRITORIES. Blackbirds' territories are much harder to map accurately than those of Robins, as the birds are so much shyer and sing so much less. One breeding pair owned at most i acres of woodland, another pair some 2 acres, a third pair 2-2^ acres, while.one unmated male held 1J-2 acres, and another an area of less, probably much less, than one acre, but was too wild to permit accurate determination. A woodland area which in the springs of 1935 to 1938 held 9, 6, 8 and 7 Robins respectively held 8 resident Blackbirds in late March 1940 (three mated pairs and two unmated males). But whereas the Robins fed almost exclusively in the wood and on the wood edge, much of the Blackbirds' food was derived from the surrounding open fields. Further, unlike the Robins, the Blackbirds owning territories at the wood edge defended a not inconsiderable area of open ground bordering the wood. However, the central parts of the open fields seemed to be neutral ground, since neighbouring males fed here without conflict. When coming out to feed, each Blackbird would at first keep close to the cover of its own territory, only gradually moving out into the open, and at once retreating if alarmed. TERRITORIAL DEFENCE. Colquhoun's statement (1940) that Blackbirds lack a strict territorial sense is certainly not true at Dartington. Territory is as definite as it is in the Robin. If one attempts to drive a

3 VOL. xxxv] SPRING TERRITORY OF BLACKBIRD. 49 Blackbird out of its territory, it usually behaves like a Robin under similar circumstances, flying in front of the observer till it reaches the boundary, then refusing to go farther, and eventually flying back past the observer into the centre of the territory. The Blackbird not infrequently leaves its territory to feed or to mob an owl, and occasionally when suddenly alarmed, also, but not apparently at Dartington, for communal roosting and for the communal display described by Morley (1937) and Lack (1941). But these habits are not inconsistent with well-defined territories. In February and March, males regularly patrolled their territories, taking short flights with intervals for feeding and perching quietly, and usually the observer was not long in the territory before the male came by on his round, the female often being in attendance. The owning male at once attacks any other male Blackbird trespassing in the territory, and does not desist till the trespasser leaves. Probably the male also drives out trespassing females, as some violent male-female chases were seen. For instance, the unmated male of one territory pursued the trespassing female of the next territory back into her own territory, at which he was himself chased back by her mate. (The possibility that some of such male-female chases are sexual is not excluded.) But females were not attacked nearly so often as trespassing males, and sometimes were apparently ignored. We only once saw a female attack a male, though on several occasions a trespassing male fed in full view of the owning female. We also saw only one fight between two females. Morley (1937) notes that the owning female chases out trespassing females ; Coward (1939) also describes fights between females. But, contrary to Morley's view, which is also stated by other observers, we found that the females certainly take much less part than males in the defence of the spring territory. (It is usually a female, not a male, which starts the attack on an owl, but this is a quite different pattern of behaviour from territorial fighting). THREAT-DISPLAY. As with other territorial species, most encounters between males are settled by threat-display. We saw this only between males, not with females. On seeing an intruding male, the owning male flies towards it and, if the intruder flies off, pursues it out of the territory. But if the intruder stays put, the male does not usually attack at once, but perches some E

4 50 BRITISH BIRDS. [VOL.XXXV. feet away and, with lowered and retracted head, approaches gradually and indirectly in a series of hops, runs or very short flights. This occurs both on the ground and in the trees. In the latter case the attacker often approaches in a succession of hops and short flights in spirals round the trunk. By the time that the attacker is within a few inches, the intruder usually departs. On three occasions when the intruder did not retreat, the attacking male repeatedly snapped its beak open and closed. The bright orange-yellow of the beak and the inside of the mouth, and also of the eyelid, are then in contrast with the black plumage, and can perhaps be regarded as threat-colours. Except for this, the attacking male does not usually posture. But on two occasions, the wings were flicked open and closed and the body jerked. Nearly always the bird keeps silent, but occasionally gives a sibilant " seep " note and on one occasion faint " chucking" accompanied the beak-opening. Comparatively rarely does the intruder wait to be attacked, but this was seen occasionally. Threat-display is also common between two resident males along the common boundary of their territories, and is usually remarkably formalised and unexcited. When one male sees the neighbouring male by the boundary, it flies up, settles a few feet away and then hops or runs towards it with lowered head. As it approaches, the other male usually turns and hops or runs unhurriedly back into its own territory, followed by the first male. After a few yards, the retreating male turns and hops or runs towards the approaching male, which now turns and retreats leisurely back into its own territory, followed by the other. This procedure may be repeated a number of times. Usually there is no posturing save for the lowered head, but there is greater excitement if one male penetrates too far into the other's territory. The performance occurs both in the trees, the males hopping from branch to branch, and also on the ground, where the measured hopping or running of the two birds looks particularly pointless. Of course territorial encounters are occasionally more serious, and there are records of one male killing another. The late Eliot Howard informed us that he found one male Blackbird dying after a fight with another. But, as with the Robin, serious fights are rare, and probably occur chiefly when one male is trying to dispossess another of its territory. BEHAVIOUR TOWARDS STUFFED SPECIMENS. A stuffed male and stuffed female Blackbird were twice placed in prominent positions in one territory in March, and

5 VOL. xxxv] SPRING TERRITORY OF BLACKBIRD. 51 later near the nests of three different pairs. In some cases male or female seemed curious, but no attempts were made either to attack or to court the specimens, which were soon ignored. PAIR-FORMATION. One pair had formed before February 13th, another before the end of February, but how long before is not known. The great scarcity of Blackbirds in autumn prevented further work on the time of pair-formation. But a male was seen following after a female on December 3rd. Also, in a garden at Horsham, Sussex, on December 25th, 1940, two Blackbird pairs were already definitely formed, and at Kew on January 13th, 1941, many pairs had already been formed, and territorial encounters were seen. Coward (1939) considers that pair-formation occurs in October and November; he gives no details, but doubtless had more data than he published. The manner of pair-formation and the first staking out of the territories would well repay investigation. Morley (1937) and Lack (1941) describe remarkable communal display among Blackbirds from early spring to April. Coward's mention (loc. cit.) of six males fighting for one female perhaps refers to the same phenomenon. Since Blackbirds pair up before midwinter, the relation of these communal gatherings to pair-formation needs further study. Despite careful watch, no such gatherings were seen at Dartington. D. Lack saw such a communal display near Richmond, Surrey, in February 1941, and this without special watch being kept. A good performance is conspicuous. Perhaps they are a local phenomenon. The shyness of the birds, and the fact that only one resident female was ringed, made it difficult to determine accurately the proportion of mated to unmated males at Dartington. At the end of March, of seven males, five were definitely mated and two definitely unmated. It is not known if the latter obtained mates later in the season. SONG. Like Colquhoun (1940), we found that a few males sang fairly frequently, a few were never heard to sing at all, most sang occasionally and quite sporadically. Song was definitely rather more frequent from unmated than mated males, but even unmated males sang very irregularly. Blackbirds started singing in the fourth week of February, but little song was heard till near the end of March, with an increase in April.

6 52 BRITISH BIRDS. [VOL. XXXV. (1940 was a late spring. In 1941 one male was singing in late January). The two functions usually attributed to song are advertisement (a) of territory, (b) of an unmated male. Round Dartington most male Blackbirds have staked out territories and possessed mates for several weeks, and, if Coward is correct, several months, before they start to sing. Hence, though unmated males sing somewhat more than mated ones, song would seem to have little or no survival value to the species at the present time, which is particularly curious in view of the beauty of the song. But, as compared with typical song-birds, the Blackbird has a relatively small territory for its size, and is visually conspicuous, while pair-formation appears to have been pushed back to the autumn. As Colquhoun points out, the song has relatively poor carrying power compared with that of other Turdus spp. Observations would be of interest on the continent of Europe in places where the bird is, at least partially, a summer visitor and presumably stakes out territory and forms into pairs after arrival in spring. MOBBING BEHAVIOUR. When Blackbirds discover a Tawny Owl (Strix aluco) they gather round with loud calls. In the cases we observed, such gatherings were started by the resident female of the territory giving a characteristic " quick quick " call, which was promptly taken up by neighbouring Blackbirds of both sexes, the call of the males being not so loud as that of the females. If the original female continued to call, neighbouring Blackbirds of both sexes would fly to the spot. Territorial boundaries are forgotten, and males perch close together and if the Owl takes wing, fly on after it together. Similarly the otherwise rigidly territorial males of the Red Bishop-Bird (Euplectes hordeacea) left their territories to join in mobbing a Coucal (Centropus superciliosus (Lack (1935).) At at least half such gatherings, we did not find an Owl. In one case we suspected a rat, but many others were definitely false alarms. OTHER LOCALITIES. Casual observations at Kew and near Richmond, Surrey, in the early spring of 1941 suggest that territorial and other behaviour may be rather different in areas where the bird is densely distributed from that observed at. Dartington. Further study is desirable.

7 VOL. xxxv]spring TERRITORY OF BLACKBIRD. 53 SUMMARY. 1. Breeding pairs of Blackbirds own sharply denned territories some two acres in size. 2. Defence is mainly by the male and threat-display is described. The orange-yellow beak, inside of mouth and eyelid may be used as threat colours. 3. Pair-formation apparently occurs before mid-winter but needs further study. 4. Song is irregular and, apparently, almost functionless. REFERENCES. COLQUHOUN, M. (1940). A note on song and the breeding cycle. Brit. Birds, Vol. xxxiv, pp COWARD, T. A. (1939). The Birds of the British Isles and their Eggs. 6th Edition, Vol. i, p LACK, D. (1935). Territory and polygamy in a Bishop-bird {Euplectes hordeacea hordeacea) (Linn.). Ibis, p LACK, D. (1940). The Behaviour of the Robin. Population changes over four years. Ibis, pp LACK, H. L. (1941). Display in Blackbirds. Brit. Birds, Vol. xxxv, PP MORLEY. A. (1937). Some activities of resident Blackbirds in winter. Brit. Birds. Vol. xxxi, pp