Hughes B, J. Bruce, G. R. Ekins and S. Newson - 2000
Movements and distribution of inland breeding Cormorants in England.
English Nature Research Report No 360 published August 2000 ISBN 0967-876X
Summary - Numbers of cormorants Phalacrocorax carbo breeding inland in England have increased markedly over the past 18 years. Since first breeding inland at Abberton Reservoir, Essex, in 1981, numbers have increased at a mean of 35% per year to reach 1,437 pairs at 23 sites in 1998. The availability of suitable breeding sites suggests that numbers of cormorants breeding inland in England will continue to increase for the foreseeable future. Ringing studies and DNA analysis have proven that inland breeding cormorants in Great Britain are a mixture of Atlantic race P. carbo carbo and the continental race P. c. sinensis. DNA studies and field observations suggest that sinensis predominate, especially during initial colonisation. European sinensis populations show different migratory patterns to carbo, migrating over large distances to winter inland in Europe (especially France) and along the Mediterranean and North African coasts. Coastal breeding carbo populations however, tend to winter much closer to their breeding colonies on the Atlantic and North Sea coasts. This report describes the distribution and movements of inland breeding cormorants in England in relation to season, age, and colony of origin.
Of 1,645 cormorants ringed at four inland breeding colonies (1,336 at Abberton, 191 at Besthorpe Gravel Pits, Nottinghamshire, 81 at Rutland Water, Leicestershire, and 37 at Paxton Pits, Cambridgeshire), 53% of birds were resighted totalling some 4,078 sightings, mostly from southern and eastern England. Some 2-20% of birds were resighted abroad depending on the time of year. Most were resighted in winter, mainly from The Netherlands and France, but three birds were recovered from Spain and one from Tunisia. Only three birds from Besthorpe (2.8% of the total number of birds resighted) were resighted abroad compared with 61 birds from Abberton (8.6%). A much higher percentage (26%) of Abberton recoveries were overseas. This may more accurately reflect the true proportion of cormorants wintering overseas as resighting data may be biased by a higher ring reading effort in England. Cormorants resighted overseas were predominantly young birds, with 90% of recoveries relating to birds of three years and younger. Cormorants were resighted at 269 different sites: 237 sites for Abberton birds, 48 for Besthorpe birds, 20 for Paxton birds and 11 for Rutland birds. However, individual birds were generally seen at only a few sites: 56% of birds were resighted at only one site, and 84% at one or two sites. Only 11 birds (all from Abberton) were seen at five or more sites with a maximum of seven sites used by one individual.
General linear modelling was used to investigate trends in latitude of resighting, distance from natal colony and proportion of resightings abroad. Besthorpe birds were resighted significantly further north than Abberton birds, even after taking into account the difference in latitude between the two colonies, and birds from both Abberton and Besthorpe were on average resighted to the north of their breeding colonies. First year birds were resighted further north than second year birds and adults. This combined with the greater tendency of young birds to be recovered abroad probably reflects the greater tendency for dispersal in immature birds. Cormorants of all ages were resighted further from their natal colonies during autumn and winter, on average some 90-120km away. Adults were found closer to their natal colonies than first or second year birds throughout the year, and first year birds were resighted further away than second year birds except during the post-fledging period. Juvenile cormorants dispersed rapidly from Abberton - within two months of fledging juveniles were resighted on average over 100km from the breeding colony. Mean resighting distance then remained at 100-150km until February from when birds moved back towards Abberton. However, most birds apparently did not return to the colony in their first breeding season with a mean resighting distance of over 50km in May. Second year birds and adults did not show the rapid post-breeding dispersal characteristic of first years. Second year birds and adults returned to Abberton between January and March, although adults returned earlier than second year birds.
As DNA analysis, ringing, and observational studies have all suggested that English inland cormorant colonies hold significant numbers of birds of sinensis origin and that appreciable numbers of these birds winter abroad, the UK should be involved in any further development and implementation of the African-Eurasian Management Plan for sinensis being produced under the African-Eurasian Waterbird Agreement (AEWA). In keeping with the recommendation of the AEWA that action plans be produced for species causing conflict with human interests, the UK should now produce a national action plan for cormorants, including both coastal breeding and inland breeding populations. Building on the extensive, government-funded research programme on the impact of cormorants on fish stocks in Great Britain due to be completed in 1999, this action plan should determine appropriate management measures for cormorants in the UK. Although this study was not designed to investigate site fidelity, movement data suggested that cormorants breeding in inland colonies in England dispersed widely during the non-breeding season, but individual birds appeared to be site faithful. This suggests that limited winter control of cormorants in England is unlikely to have an appreciable impact on inland breeding colonies. However, to predict the effect of winter control more detailed studies of dispersal and site fidelity in individual birds of known age and sex both within and between winters would be required. This study highlighted the need for ongoing and substantial colour-ringing effort at inland cormorant colonies in England if the development of inland breeding is to be monitored adequately. Trends in movements of inland breeding cormorants can only be explained if: a) sufficient individuals at sufficient colonies are ringed annually and on an ongoing basis; and b) adequate data on factors explaining trends in movements are collected.