Abstract from BTO Research Report No 433:

Newson, S.E., Ekins, G.R., Marchant, J.H., Rehfisch, M.M. & Sellers, R.M.

The status of inland and coastal breeding Great Cormorants Phalacrocorax carbo in England
ISBN No. 1-904870-72-4

Executive summary

1. Since the establishment of a tree-nesting colony of Great Cormorants Phalacrocorax carbo at Abberton Reservoir in Essex in 1981, the inland breeding cormorant population in England has developed considerably through colony growth at Abberton and the establishment of new inland colonies. Up to 2005, cormorants have bred successfully in one or more years at 58 English sites that were not coastal cliffs or offshore islands, and so were defined as “inland”. The majority of these colonies were in central and south-east England, with few or no colonies in the north, west or south-west of the country. Whilst cormorants have been actively dissuaded from breeding at a number of these sites, the inland breeding cormorant population in England in 2005 is known to have reached at least 2,059 breeding pairs, although there is some evidence of stabilisation in recent years. This population is thought to have been founded by cormorants of the continental race P. c. sinensis mainly from the Netherlands and Denmark, although an increasing proportion of nominate race carbo originating from coastal colonies in Wales and England are believed to have contributed to its development.

2. Colony counts for inland breeding cormorants were obtained through a number of sources including County Bird Reports and correspondence with County Bird Recorders, the BTO Heronries Census by courtesy of the British Trust for Ornithology, and personal communication with birdwatchers, bird ringers and reserve or site managers. However, there is currently no coordinated means of obtaining inland colony counts from observers, making the process of collating counts very time-consuming. In addition we do not know to what extent counts represent the maximum numbers of nests. Ideally, nests should be counted several times within the same season, and the maximum number used. Several visits should be made because the observer rarely knows at exactly what time of the season nest numbers can be expected to culminate, and this is likely to vary among years and colonies. An improvement to the quality of the counts could be achieved by identifying a network of volunteers able to make several visits to colonies on an annual basis. Counts would feed then directly into a central database.

3. Between 1986 and 2005, cormorants bred at 80 coastal colonies in England. The majority of these were in the south-west of the country, with a smaller number of colonies in south, north-east and north-west England and the Isle of Man. Estimates for the coastal breeding cormorant population in England 1986-2005, suggest that this population has remained stable over this period, at approximately 2000 pairs. This is in agreement with Mitchell et al. (2004), although the absolute level of the population is estimated here to be higher than suggested previously. Particularly large confidence intervals in 1989 and 2003 reflect particularly poor coverage in these years.

4. Colony counts for coastal breeding cormorants have mainly been collated by JNCC’s Seabird Colony Team through the Seabird Colony Register (SCR), the Seabird Monitoring Programme (SMP) and the Seabird 2000 survey. However, only a small proportion of colonies are counted on an annual or near-annual basis. This is mainly a reflection of the difficultly in surveying coastal colonies, which are on coastal cliffs, stacks or offshore islands and in many cases not viewable from land. For many colonies boat-based surveys or aerial surveys would be required. Whilst it would be possible to improve coverage of accessible colonies, the only practicable way to obtain a high level of coverage would be to carry out aerial surveys (for ground-nesting colonies) and boat-based surveys for (cliff-nesting colonies). At the other extreme, it is important to maintain annual monitoring at a sufficient level to avoid the situation in 1989 and 2003, where we have little ability to detect change in the population.

5. The collation of counts for coastal cormorant colonies in England has allowed us to produce revised population estimates for coastal colonies in England and to assess how well we can currently monitor this population through existing schemes. However, because inland waters in England also support cormorants from coastal breeding populations in Wales and Scotland, it should be considered that the collation of counts and production of population trends using the methods described here is performed for these countries.

Back to Research Reports 345 onwards