GREAT CORMORANTS PHALACROCORAX CARBO ON MOORS POOL, UPTON WARREN, WORCESTERSHIRE: A PRELIMINARY STUDY

Tessa Carrick

A major portion of my study was an evaluation of different methods for recording behaviour patterns of cormorants. Although this produced a large number of tables of data, I will merely summarise the findings.

An important part of the study was to survey the extensive literature on Great Cormorants. Much of what is known is summarized by Cramp (1977), Johnsgard (1993) and Kirby. Since completing this study there have been two major publications on Great Cormorants (Feltham et al, 1999; McKay et al, 1999).

Population
Cormorants visited Moors Pool for part of the day only. From various sources and with the help of John Belsey, I found the maximum number of cormorants recorded at Moors Pool for the winters between 1989 and 1994.


Year Maximum number recorded
1989 1
1990 6
1991 11
1992 9
1993 At least 9
1994 30

During the winter of 1994-1995, numbers declined as the winter progressed. Fluctuations in weather and water level made no obvious difference to the numbers of birds observed.

Habitat utilisation
Birds fished in both open water, mainly in the deeper northern half of Moors Pool, and among the bistort.

Observations on behaviour
The behavioural studies undertaken took account of the work on methods of sampling behaviour by Altmann (1974) and by Martin and Bateson (1993). Four methods of observing behaviour proved useful, namely:

a) a descriptive, timed record of events;
b) a tabulated record of activities for a number of birds, recorded at regular time intervals;
c) a coded record of duration of states of behaviour for a single focal bird;
d) a coded record of frequency of activities for a single focal bird.

Spread-wing behaviour
Spread-wing behaviour, in which birds extend their wings, was observed on a number of occasions. Such behaviour invariably occurred soon after emergence from water but it did not always take place. There was considerable variation between individuals on the same day and with different environmental conditions.

Many articles (e.g. Curry-Lindahl, 1970; Elowson, 1984; Rijke, 1968) have been written about the reason for spread-wing behaviour. It has been suggested it may either help to dry the wing feathers or be thermoregulatory, but research results do not provide conclusive support for either possibility. Cormorants at Moors Pool did not always spread their wings after a period of diving before flying off to some other site. It is not, therefore, essential for the wings to be spread before flying and so, even if such behaviour does fulfill a role in drying the feathers, this is not a necessary precursor to flight.

Wing-spreading was more limited when weather was damp and was absent when raining. On such occasions the value of wing-spreading for the purpose of drying feathers would be limited. Equally, in such weather it would be unsatisfactory as a method of thermoregulation, since there would be little radiation from the sun to provide energy. Detailed observation of wing-spreading behaviour in relation to small-scale changes in weather and with regard to orientation to the sun might help to elucidate the value of wing-spreading.

Preening
Preening occupies prolonged periods and is a major occupation of cormorants when out of water. One bird, which appeared to be sick, did very little preening throughout a whole day.

Agonistic behaviour
Few records were made of agonistic behaviour (conflict behaviour) but there was some competition when fishing and some displacement of one bird by another from resting sites. There was sometimes resistance to displacement but no defence behaviour was observed at Moors Pool.

Diving
Analysis of the data for the three birds observed diving were as follows:

Period of diving session /minutes Max diveduration/seconds Dive duration/ seconds Pause between dives/seconds
25, with interruptions 27 Mean = 19.75 Mean = 14.5
8 (approx.) 34 Mean = 12.23, s = 6.96 Mean = 8.43, s= 6.98
35 (approx.) 25 Mean = 16.5, s = 3.646 Mean = 4.4, s = 1.674

These figures are similar to published figures (Cooper, 1986). Statistical analysis on results from Moors Pool suggest that there may be variation in the pattern of dive/pause during a prolonged bout of diving.

Estimation of fish consumption during the winter of 1994-1995
Cramp (1977) claims that at least larger fish are brought to the surface for swallowing. In this study it was assumed that continuous observation at Moors Pool revealed the bulk of the fish caught. The length of fish caught was estimated by comparison with the available figures for the length of a cormorant's head.

Using these assumptions, it was possible to estimate the number and mass of fish consumed at Moors Pool for a period of three months during the winter. Probably, the main fish taken from Moors Pool are roach, perch and eels. Hartley (1947) found that roach of 30 cm weighed about 250 g. Most birds observed at Moors Pool moved out of the water after bringing a single large fish (probably roach) to the surface, that is a fish of at least 250 g, and were not seen to fish again. An immature cormorant observed for a whole day took a single, small fish, weighing about 125 g.

If each cormorant visiting Moors Pool on 26th November, when 30 birds were seen, caught one large fish from the lake, 30 fish or about 7.5 kg of fish would have been consumed. For the 3 months November to January, 388 cormorant visits were recorded for the 53 days on which observations were made, giving an average number of birds visiting each day of 7.3. This figure is possibly on the low side since the maximum number is usually present for a very short period of the day and could occur when no-one was recording birds. However, assuming this average is applicable for the whole three months (that is 92 days), the total number of daily visits would be 674. If a single 30 cm fish were consumed on each visit, 674 fish would be eaten or 168 kg of fish during the three months November, December and January.

These calculations merely provide a very rough estimate of the fish consumption. They take no account of the variation in foraging ability between birds. Incidentally, I was told that the Management Committee for the Christopher Cadbury Wetland Reserve (personal communication) are not averse to the presence of cormorants on Moors Pool as they hope that the consequent culling of the numerous larger fish will leave food so younger fish stock survive to provide more suitable prey for other species of birds. Since no figures are available for the fish population in Moors Pool, it is impossible to gauge the impact of the cormorants on the fish stock.

In the past, the arguments about whether cormorants depleted fish-stocks on inland waters related to the winter. Now that there are large numbers of birds breeding at a few inland sites, such as Abberton Reservoir, it would be valuable to have some idea of the impact of birds on fish-stocks when feeding their chicks.

Although cormorants may sometimes have a significant impact on the fish population of an inland body of water, caution should be exercised in lessening restrictions on shooting or granting shooting licences. There is no doubt that cormorants can be a nuisance to fish farms, but in some situations cormorants may aid management for conservation, perhaps favourably altering the age structure of the fish population or reducing the number of a coarse fish such as perch. There is also some evidence that selective predation may improve the quality of fish-stocks remaining.

Conclusion
As an exploratory study of cormorants on Moors Pool, no earth-shattering insights into cormorant biology were likely. Because this investigation did not concentrate on one particular aspect of cormorant activity and because there is variation in behaviour between individuals, the results for any one part of the study were limited.

However, these preliminary findings might provide a starting point for more focused, prolonged investigations. For instance, it would be of interest to investigate the following:

What is the age structure of the population of cormorants visiting Moors Pool?
What are the distinctive habitat features of inland sites where cormorants breed, roost or forage?
Does the pattern of dive duration and pauses vary during a period of diving and with the circumstances?
To what extent does the behaviour of individuals when out of water vary?
Is the behaviour of wintering cormorants different at inland and coastal sites?
Do environmental conditions influence spread-wing behaviour?
What is the effect of disturbance on preening behaviour?
What factors affect the decisions cormorants make about choice of alternative behaviours?
To what extent is it possible to model the costs and benefits of alternative behaviours?

Acknowledgements
The following kindly helped with this study: my advisor, Professor Graham Martin; my husband, Michael Carrick, as recorder; John Belsey, Arthur Jacobs and other members of the Christopher Cadbury Wetland Reserve Management Committee, who helped with information. Cormorant drawing by R M Bishop

References
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