Racial Hybridisation and its Effect on the movements of Bearded Tits ringed in Suffolk

RACIAL

HYBRIDISATION

AND

ITS E F F E C T S O N

THE

M O V E M E N T S O F B E A R D E D TITS R I N G E D IN S U F F O L K JOHN NEWTON

T h e locations of bearded tits Panarus biarmicus ringed by m e m b e r s of the Dingle Bird Club at Walberswick, Suffolk, and recovered elsewhere were extracted from The British Trust for Ornithology recovery forms (BRC3), and organized into three groups according to the year in which birds were found. These data were then plotted on a map in order to facilitate interpretation.

T h e majority of birds ringed were recovered locally, 77 per cent within a 30 mile radius of Walberswick and a large n u m b e r were found three miles south at Min'smere (65 per cent). A u t u m n and winter recoveries were common indicating a post fledgling dispersal away from the breeding sites.

By examining the distribution of all recoveries (Fig 1) together with the data available, it became clear that a movement in the direction of dispersal of bearded tits ringed at Walberswick, from the south and south west in the late f950's and early 1960's, to the west and north west in the late 1960's and following years, had taken place. Over a similar period, netting activity in Norfolk is thought to have increased, and obviously goes some way in interpreting this pattern of dispersal. However, it does not fully explain the relative absence of recoveries to the south, especially as trapping activity has shown no recent signs of decline in that area, (Pearson, personal comment).

T h e severe winter of f947 decimated the bearded tit population of Britain with reputedly only ten birds surviving (Richards 1975). From the recovery data available it appears that the Suffolk population was instrumental in the recolonisation of habitat left vacant after this period, and two theories have been suggested as to why this was so.

M e a d and Pearson (1974) by examining historical data have suggested that the British and Dutch populations of bearded tits have been able to exploit the seeds of phragmites for well over 100 years, and that plumage characteristics reveal that both populations are in fact of birds which are hybrids between the nominate race biarmicus and the East European race Russicus. Spitzer (1973) states that the latter have the ability to exploit and digest the seeds of phragmites due to a change in the lining of the gizzard in winter months. This

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becomes a valuable asset in hard weather when very often all but the seed heads of reed are covered by snow, and allows the eastern birds and presumably hybrids to survive where birds of the more southerly race would die. However, despite this the populations of Britain and Holland were ill-equipped for winter 1947 and were, as stated above, dramatically reduced in numbers. At the same time Mead and Pearson (1974) report that birds of the pure Russicus race moved west from Poland and North Germany in an attempt to escape the harsh weather and interbred with surviving hybrid birds in Holland. East Anglian birds, it is suggested, received this input of genetic material in the late 1950's, later than the Dutch population, although subsequent increases in numbers ocurred later than was expected due to the slow recovery of suitable habitat after the 1953 floods.

Following their recovery, British birdsdisplayed an enhanced tendency for post breeding dispersal, a behavioural aspect which was again reported in 1965 when bearded tits were also spreading north from Holland to Scandinavia (Olsson 1975). It seems likely that this ability too was conferred on our own hybrid populations by interbreeding with the East European race.

T h e second theory under consideration is put forward by Richards (1975) who suggests that the survivors of the hard winter of 1947 lived due to genetically conferred resistance to the harsh conditions, which was bred into subsequent generations. Conditions which left alive only those birds that had the ability to cope with the hard weather and were able to escape by eruptive movements. An evolutionary process which he has called catastrophic selection.

Owing to insufficient data it is difficult to be objective about the two theories. However, I personally find Richards' theory too instantaneous to be plausible. Natural selection very rarely acts so radically over such a short period of time, and I feel that a race which has recently acquired a new strain of hybrid vigour would be better equipped to take advantage of vacant habitat, than one subjected to Virtual decimation by unfavourable environmental factors.

In conclusion it seems from the literature that the British population of the bearded tit has been a hybrid of the two races biarmicus and Russicus for some time, but received fresh genetic material from the Russicus race via the melting pot of the Dutch colonies after the severe winter of 1947. This new population almost certainly inhabited the nearby reed beds of

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East Anglia before dispersing throughout the rest of the country, a fact illustrated by Walberswick ringing recoveries. It is interesting to note that ringers on the south coast are now reporting movements of up to 150 miles between colonies as was noted ten years ago between Suffolk and the south (Pearson, personal comment).

The only foreign recovery of a bearded tit on the Dingle Bird Club files is that of a bird ringed in October and recovered at Beningerskikken in Holland in July 1973, a recovery which illustrates the autumn movements of Dutch birds to East Anglia and gives credence to the theories drawn from the literature.

In the late 1960's a tendency for Walberswick birds to disperse in a more northerly direction became evident, a fact which coincided with movements in the Dutch colonies. This may have been due either to available habitat to the south becoming occupied and birds having to search further north for breeding sites, or to behavioural changes brought about by further hybridisation. With Russicus being the more northerly of the two races and better able to withstand hard winters, the latter suggestion seems more likely although occurrence of both cannot be ruled out.

The British and Dutch populations appear to share the same gene pool, with Dutch birds initially undergoing genetic change and consequently transferring it to their neighbours in East Anglia. The reed beds of Suffolk and Norfolk obviously play an extremely important part in the reception and dispersal of this new genetic material, with Walberswick and Minsmere being of particular importance. If the bearded tit is to survive in Britain it is essential that everything possible is done to ensure the conservation of these and similar areas and that none are lost through man's ignorance or lack of foresight.

Acknowledgements I would like to thank ringers of the Dingle Bird Club for allowing me to use the data which they have collected over the past years and for the help and encouragement they have given me whilst preparing this paper. In particular I would like to thank David Pearson for his invaluable assistance.

References M e a d , C. V. and Pearson, D. J. 1974. Bearded Reedling.

Populations in England and Holland.

Bird Study, 21, 211.

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Olsson, V. 1975. Bearded Reedling Populations in Scandinavia.

Bird Study, 22, 116.

Pearson, D. J. 1975. Moultand its relation to eruptive activity in the Bearded Reedling.

Bird Study, 22, 2Ăœ5 Richards, A. J. 1975. Bearded Reedling populations in England and Holland.

Bird Study, 22, 118.

John Newton,

M.Sc.,

Holme Next The Sea,

Norfolk