The Status of the Blackbird (Turdus Merula) at the Copeland Bird Observatory, Co Down


by C W Acheson

(first published in the CBO Annual Report for 1975)

The Blackbird is both a resident and passage migrant at the Observatory. There is a small breeding population of less than a dozen pairs, but visitors swell numbers in March – April and October – November.

Records of Blackbirds ringed, or controlled, at the Observatory will be examined. Recoveries abroad, which are examined in the second section, include details of 3 birds ringed in winter on the mainland close to the Observatory.

For convenience, May to September (inclusive) are termed as “summer months”, while October to April are regarded as “winter months”.

The Blackbird as Resident

Numbers present between late April and the end of September rarely exceed 12 birds, though in a successful breeding season the total number may be temporarily increased by several broods before dispersal of the young. Approximately 6 nests are found on John's Island each year, with an additional 2 nests on Mew Island, and undoubtedly some broods escape detection during the season. Possibly 10 pairs nest on the two islands. The success rate is generally high, and on average the young from 5 nests are ringed as pulli or as juveniles.

Some of the young restock the resident population. This is shown by the retrap histories of island-bred birds in Table I.

Table I        Retrap details of selected residents
Ring number Age & Sex Date of ringing Date of last retrapping Number of times retrapped
S81764 3JM 13.07.57 15.10.60 31
V90673 3JM 04.08.58 31.05.63 13
CA12728 5M 29.02.64 10.10.65 21
CA12911 5M 07.03.64 10.10.65 13
CB14017 3JM 03.07.64 02.06.74 19
CX89687 3F 09.10.65 08.06.68 9
CX89948 3JF 02.07.66 18.07.70 9
CX89955 3JM 09.07.66 12.06.67 7
CV55630* 5M 25.03.67 29.03.70 9
CR56863* 3JM 08.09.69 29.04.72 9
CP90086 3JF 25.10.69 14.05.73 14
CP90285* 1 (M) 15.05.70 09.04.74 10
CJ54636 3JF 05.08.72 30.03.74 7

Those birds marked with an asterisk were found dead, or were killed by predators.

Study of the tables show that life expectancy of Blackbirds on the island is extremely varied. One bird was retrapped regularly for 10 years. The life expectancy of the majority is usually 2 years, though several residents have reached five.

The dispersal of young from the island appears to be complex, and somewhat difficult to interpret.

There are two types of evidence – the direct and the implied. The direct evidence is limited, 4 recoveries on the mainland and one on Great Copeland of birds ringed either as pulli or juveniles (see Map A). Four of these birds were recovered during the summer, while one was killed in February 1963 at Magheralin, Co Armagh (30 miles W.S.W.) 19 months after ringing (A). This movement may have been associated with the severe weather of the 1962/63 winter. More rapid dispersal in indicated by the bird shot at Ballymacormick Point only one month after ringing (B), whereas another bird remained on the island for a year and was then found dead at Clandeboye (8 miles W.S.W), 2 years after its last retrapping on the island (C).
Map A
Other examples of birds being retrapped in the year after ringing but not again are shown in Table II. Obviously some birds die without notice, but it is likely that others move to the mainland permanently, as the Clandeboye bird appears to have done.

Table II        Details of selected young birds retrapped in the year after ringing, but not again.

Ring number Age & Sex Date of ringing Date of last retrapping Number of times retrapped
V90773 3M 22.10.58 22.02.59 1
R76284 3JM 19.07.59 25.06.60 9
R97802 3JF 08.08.60 27.02.61 6
49461R 3JM 22.06.63 05.10.63 6
CB28233 3M 28.08.64 25.04.65 4
CX89904 3JM 08.05.66 29.03.67 2
CR56892 3J 05.07.69 29.03.70 2

The third recovery in summer was at Island Magee, Co Antrim (12 miles N.W.) a year after ringing (D). The fourth recovery on Great Copeland 5 years after ringing may be an instance of inter-island movement during the breeding season, rather than genuine dispersal, although this bird was retrapped only once at the Observatory, a month after ringing.

Other evidence may shed light on the dispersal of birds from the island. Some ringed birds, almost certainly island-bred, seem to leave the Observatory for long spells soon after ringing, but are subsequently retrapped there. This would tend to support the idea that some island-bred birds move to the mainland, or possibly to Great Copeland, returning to John's Island only occasionally (Table III).

It is just possible that a bird could evade recapture, particularly if it was breeding in a remote part of the island before mist-nets were in common use there. Since their introduction, however, it would seem most unlikely that birds could evade recapture over a long period, yet there are recent examples of similar absences.

Table IIIA - Details of birds retrapped only once after a long absence from the island.

Ring number Age & Sex Date of Ringing Date of retrap
S76991 1 (M) 08.06.57 21.05.60
43465X 3F 22.10.60 18.03.65
CB28481 6F 23.10.65 26.10.68
CP90220 6M 14.03.70 02.04.72

Table IIIB - Details of birds with recorded absences of over 2 years in their retrap history

Ring number Age & Sex Date of ringing Absent between Date of last retrap Number of times retrapped
V90788 3M 23.10.58 05.04.59 and 25.04.63 25.04.63 2
R97637 3F 24.10.59 24.10.59 and 03.03.62 10.03.62 2
CA12512 3JM 31.08.63 26.04.64 and 10.06.67
12.05.68 and 10.05.70		 06.06.71 8
CA12788 1 (F) 30.05.64 20.09.64 and 16.10.66 28.04.67 4
CX89749 3M 26.10.65 19.05.68 and 02.06.70 22.04.72 8
CX89850 6F 05.03.66 05.03.66 and 26.02.72 17.03.73 2
CX89862 6M 26.03.66 01.10.66 and 22.06.69 22.06.69 5
CV55639 5M 27.03.67 10.04.70 and 28.06.72 28.06.72 6
CV55645 6F 29.03.67 29.10.67 and 14.03.70 07.04.74 12
CS48143 3JF 12.07.68 24.09.70 and 14.10.72 21.04.73 5

There is another group; birds ringed later in life, as full grown, and with subsequent retrap histories. Clearly not all island-bred birds are ringed as pulli or juveniles – possibly as many as 4 broods escape our attention each year. Whether they have evaded capture by simply avoiding the nets or traps, or by leaving the island soon after fledging and spending some time on the mainland before returning, is not known.

Table IV - Details of birds ringed as adults which have subsequent retrap histories.

Ring number Sex Date of ringing Date of last retrapping Number of times retrapped
54165S M 29.07.61 23.09.64 6
54081S* F 11.03.62 20.04.69 16
49403R F 23.04.63 15.05.66 10
CB14005* F 06.06.64 09.06.68 6
CS48151 F 31.08.68 13.03.71 12
* Found dead at CBO

It is relevant to consider the significance of a young male ringed at Tubber, near Kircubbin, in November 1973 and controlled twice at the Observatory during the following breeding season – in April and September 1974. it is tempting to regard this as evidence of a young bird leaving the island soon after fledging, and spending the winter months on the mainland before returning to the Observatory at the beginning of the breeding season. Probably a number of residents spend the winter, or part of it, on the mainland. Five birds, ringed at the Observatory as full grown have been recovered on the mainland during later winters (Map A).

Eight birds ringed on the island as full grown, but not necessarily adult, have been recovered on the mainland during the summer months (map A). Thus, some CBO-ringed birds are present during the breeding season as far away as Carrickfergus (12 miles), Holywood (13 miles), and Portavogie (16 miles). There is no way of knowing whether these were island-bred, but it does seem likely that some may have moved away from their natal territory to breed. The following example would also seem to be evidence of such a move:

54008S   28.10.61 Copeland Bird Observatory, Co Down
3M V 03.02.70 Ballyree, Bangor, Co Down
  X 02.11.71 Ballycrochan, Bangor, Co Down

An alternative explanation of the summer recoveries may be considered. During some mornings in July and August, especially if conditions are hazy, small influxes of Blackbirds occur, though dispersal of these birdsis usually quite rapid. These birds may come from the adjacent mainland, or in some cases from further away, and are wandering in the post-breeding period. Both adults and juveniles are involved, and of the 8 birds subsequently recovered during summer away from the Observatory, only 3 were ringed as juveniles and only 1 was ringed in July and August. Thus both explanations must be considered for a fuller understanding of local Blackbird movement.

Finally, the steady level of the island's resident population over 20 years is a further indication of dispersal. Clearly, the island can only support a small population.

The Blackbird as a passage migrant

It is virtually impossible to estimate the number of Blackbirds passing through the Observatory during the migration seasons, as most of the migration is nocturnal.

Daily observations are misleading. Manning is usually confined to weekends during the appropriate months and it would be of little value to use the figures recorded then to estimate an annual total, especially as numbers fluctuate, depending on weather conditions. The recorded spring maximum of birds per day varies from 10+ to 50+, while that of autumn tends to be a little higher.

Numbers of birds ringed are even less representative of the total. A very small proportion of migrants are ringed and not all Blackbirds ringed in March – April and October – November are migrants, as we have already seen.

It is more appropriate to examine, through recoveries, the origin of the birds involved, and the pattern of migration through the British Isles and Europe.

Blackbirds move into Ireland in some years from north-east Europe to winter. In addition birds from Scotland and northern England are, in certain years, forced westwards by hard weather, and these winter in Ireland before returning to their breeding grounds the following spring. Most recoveries of Copeland-ringed birds can be assigned to one of these groups, and thus a relatively clear picture emerges.

Recoveries of Blackbirds within the British Isles are shown in Map B. Three birds recovered in Ireland were wintering there in years subsequent to ringing, having presumably returned to their breeding grounds elsewhere during the intervening period. The two birds recovered in Co Meath, however, are difficult to explain. The one recovered in Navan in early April 1964 was ringed on the island as a young bird in August 1961, and was thus hardly of European origin. There are two possible explanations – it could evidence of dispersal of a Copeland-bred bird, though there is no proof of this, or it could have been a young wanderer, of Irish or Scottish provenance, ringed at the Observatory and later forced south by the winter of 1962/63, and then remaining there.
Map B
The other bird, recovered in Duleek in August 1962 had been ringed at CBO in March 1959. this is probably further evidence of a bird, whatever its origin, staying to breed in that part of Ireland.

Breeders from Scotland and northern England, which wintered in Ireland at least once, are represented by a group of 4 recoveries in summer in their area of origin, one as far north as Invergordon in Ross-shire.

The other 6 recoveries in Scotland and England may include breeders from that area recovered in winter, but one was most likely a bird of north European origin. This was an adult male, ringed on CBO in March 1960 and controlled on the Isle of May, Fife, 19 days later. This averages less than 10 miles a day but is, nevertheless, direct movement.

Map C shows Blackbirds ringed on the island and recovered in subsequent years in Norway (4), Sweden (2), West Germany (1), the Netherlands (2) and Belgium (2). Also plotted are an adult female ringed at Carrowdore, Co Down, in January 1964 and recovered in Latvian SSR in August 1968; and two adults ringed at Portavo and on Mahee Island and recovered in Norway.
Map C
Only three of the recoveries have been in summer, two in Sweden, one in Latvian SSR; and the rest in winter. It is tempting to regard Sweden and European Russia as the area of origin of our birds, and it seems likely that Goodacre (1959) and Spencer (1975) underestimated the importance of Ireland as a wintering area for Swedish Blackbirds. Some of the Norwegian recoveries are likely to be birds on passage. Indeed, south-west Norway is probably a danger area for Blackbirds, as large numbers appear to be killed there by trappers.

Discussion

The Blackbird population of northern Europe is in a dynamic phase, and the colonization of Finland by the species from Sweden is well attested (Kalela 1949). Blackbirds are now common locally on the Faroes, and the species is also likely to become established as a breeder in Iceland, as it has become regular there in winter. If one accepts that Sweden, as well as Norway, is the home area of many of Ireland's winter visitors, and agrees that the migration urges of a particular population are likely to remain basically unchanged, then in time the possibility of a recovery in Finland of a Copeland-ringed bird becomes stronger. Indeed, a recovery from farther afield becomes a remote possibility.

In recent years the numbers of Blackbirds ringed in Britain and subsequently recovered in central France and Iberia has increased. It has been shown elasewhere that West german, Dutch, Czech and Polish populations move roughly in a south-westerly direction into France and Iberia. As a high proportion of birds recovered are ringed in September, October and November, it seems likely that they were passage birds which were drifted across to the east and south coasts of England during autumn movement. Presumably these birds were able to re-orientate after ringing, and proceed to their normal breeding grounds. It is unlikely that birds from these populations would reach the Copeland Islands and consequently the chances are minimal of a Copeland-ringed Blackbird being recovered in this area.

It is clear from the evidence that some Blackbirds winter in Ireland one year, but elsewhere in Europe in subsequent years, perhaps when the weather is less severe. It is more likely that the Copeland-ringed birds recovered in the Netherlands (2) and Belgium (2) were birds from northern Europe and behaving in this manner, rather than birds belonging to a more easterly population.

There is evidence to suggest that more than one migration route may exist between Ireland and north-west Europe. Blackbirds breeding in Sweden tend to follow a more northerly line, through Scotland and the Isle of May and south-west Norway, while birds breeding in western USSR perhaps take a more southerly route through northern England, across the North Sea through Heligoland and into Russia via West Germany, Denmark and the Baltic. One adult female ringed on CBO in March 1957 was controlled in Heligoland 15 days later – and average of 36 miles a day. Possibly the bird recovered on the Isle of Man in April 1970 was on passage along this route. It will be interesting to see if future recoveries confirm this.

There is a theory that female Blackbirds tend to follow a more southerly migration route. The two recoveries from the Netherlands, both of females in October of different years tend to support this theory, but further records are needed.

Summary

From the evidence available Blackbirds born on Copeland Bird Observatory island appear to a) stay as residents, b) leave the island for good, or c) leave the island for a period (usually around 2 years) and return. Migrant Blackbirds ringed on the island have been recovered in the northern half of Great Britain, Scandinavia, Latvia and Ireland. The possible origins and movements of these birds are discussed.

Conclusion

Until we have several more recoveries it is not possible to be certain about many of the points raised. Indeed, future recoveries may alter the picture, but it will be interesting to see whether any of our Blackbirds do breed in Norway and ultimately, in Finland, and whether there is further evidence of males and females using separate migration routes.

References

  1. Goodacre MJ (1959) The origin of winter visitors to the British Isles. Bird Study 6: 37-50
  2. Kalela O (1949) Changes in geographical ranges in the avifauna of northern and central Europe in relation to recent changes in climate. Bird Banding XX: 77-103
  3. Spencer R (1975) Changes in the distribution of recoveries of ringed Blackbirds. Bird Study 22: 177-190


© Copeland Bird Observatory, 2001 – 2004

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