WI - Cormorant Research Group The Bulletin - No. 4, June 2000 Original papers

AUTOMATIC RECORDING OF NEST VISITS BY CORMORANTS
Phalacrocorax carbo sinensis BREEDING IN TREES IN
OOSTVAARDERSPLASSEN, THE NETHERLANDS

Stef van Rijn & Mennobart van Eerden

Rijkswaterstaat Riza, P.O. Box 17, NL-8200 AA Lelystad, the Netherlands.

The IJsselmeer area, in the Netherlands, has undergone many changes like the reduction of the level of nutrients and the decline of the effort of fisheries. It’s observed that the birds are foraging alone or in small flocks in the current situation and not in big social groups as was the rule in the eighties and early nineties. So nowadays it’s more difficult to record where the birds forage because they are scattered around in the area. The first step is to understand how the changed situation will affect the Cormorant’s population and why. The second step is then how the birds react on this and why. To understand how conditions of both adult parents and their young and also food delivery is changing throughout the breeding season, the time that the foraging pressure on the lake will be maximal, a pilot study was conducted using platforms under nests to measure the changes in weight in the period of raising young. In spring 1999 we will measure the whole breeding season around, the incubation period included.

On 28 May 1998 a set of five weightplatforms were installed under nests with nestlings of Cormorants. Weight was measured by means of pressure points and 10 m length of cables. All platforms were connected to one (Campbell) datalogger which recorded total mass every ten seconds. Batteries were recharged by solar panel. The nests were chosen carefully in order to achieve a stable situation for the platforms. It was possible to have some variation between the nests chosen. Four nests were situated 3.5-6 m high in old (30 years) willow trees. The fifth was installed in a willow bush. From the four ones in trees, two were placed in more closed parts of the wood with under growth scrub of willow and elder. The other nests were situated on the shore line. In the evening of placing the platforms it turned out that one of the chosen nests in the tree at the water’s edge contained two freshly died young of about 3 weeks old.

Eventually it was possible to measure three nests with young, growing Cormorant nestlings. One of the empty nests was the rest of the time used as a resting place of adult Cormorants. The nests were checked several times till the period of independence of the young. Table 1 shows the fate of the young of the three nests during the breeding season.

Table 1. Contents of the three nests were young were growing and where, in the period 28 May - 14 June, changes of weight were measured.

place

eggs

young

 

14/4

8/5

25/5

28/5

15/6

24/6

30/6

1. willow wood

4

4

4

3

2

2

1

2. willow wood

3

3

3

2

2

2

2

3. willow shore

5

3

2

2

2

1

0

total

12

10

9

7

6

5

3

After the mild soft winter of 1997/98 the first Cormorants started to incubate already in February. May and June were very windy and quite a big amount of rain has been recorded. Mortality rate of young was very high and comparable to that in 1996 (Van Rijn & Platteeuw 1996). In the three nests where platforms were used, out of 12 incubated eggs, 10 resulted in chicks in the first week of May. At the end of May when the chicks reached the age of about 3 weeks old 3 chicks died because of undernourishment. At the end of June, just before the birds should fledge, only three birds were still alive. It was remarkable that the nest with five eggs produced no young at all and the nest with three eggs, initially the lowest number, produced best (Table 1).

The number of visits to the nests by parent birds was measured in a period of 17 days. The age of the young was about 3 weeks in the beginning of the records and about 5 and a half weeks at the end, the time that the birds had to fledge. In this period the number of visits by adult birds was very variable. In the first 5 days of measuring the number of visits was relatively high for the three nests with 5-9 visits each day. This means that on average one adult male or female visited the nest between 2 and 5 times a day. In the period there after only 0-3 visits per day were recorded, at the time that the young reached the age of about 26 days old. This means that another 2 weeks was needed to be spend on the nest to be fed before fledging. Growth rates are high in the period before leaving the nest so the parents needed to supply in a lot of food for raising their young. The high rate of mortality in the first half of June is possibly well explainable by the decrease of the number of visits to the nest by the adult birds. Some differences in the number of visits between the three nests are visible. The two nests with the highest mortality, showed the most fast decrease in the number of nest over time. The most successful nest, in which 2 young fledged, showed a more stable pattern of nest visits and days without any visit at al did not occur (Figure 1).

 
Figure 1. Daily number of visits to the nest by one of the adult birds in the period of 3-5.5 weeks old chicks.

A second parameter recorded is the weight of the adults during the period of study. The weight of adult birds seemed to be very variable because visits with both food supply and not were recorded. There also was observed differences in male, which is known to be bigger and as a result of this heavier, and female birds. The result of all recorded weights was a big scatter of points due to the inaccuracy of the system as yet. Some trends are indicated by taking the mean as a calculated linear. From the end of May till half June the average weights for nest 1 and 3 decreased a lot. The average weight of the adults of nest 1 decreased from 2.5 kg to 1.0 kg which is hardly possible because the birds reach an under weight on the basis of which one has to conclude that the bird has to be dead.

 
Figure 2. Regression of average body weight of the adult birds of three nests in the period of 3-5.5 weeks old chicks

Interestingly, weight of birds in the second nest remains unchanged at a level of about 2.2 kg. By contrast, nests with the highest mortality (Table 1) of chicks showed a mayor decrease of the weight of adult birds during the measured period (Figure 2).

A strong decrease in the number of visits to nests by adult birds and a sharp decrease of their weights were associated with a high mortality rate of nestlings. Apparently the adults lost energy reserves during the raising of their chicks. An important question is why. We have to quantify the attainable fish stocks for Cormorants and to develop the knowledge about the allocation of the birds’ fishing grounds on the lakes. In 1998 a start was made with sampling fish stocks. Every 3 weeks fish was caught at 21 different sites evenly and strategically distributed over the study area. Every site is sampled by a 10 meter wide, 5 meter high drag net between two fishing vessels which is towed for half an hour over a length of about 2 km. In 1999 a second season of assessing the attainable fish stock is planned. The next step is to use transmitters in the Cormorants to find out where the exact fishing territories of the birds are situated.

References

Bregnballe, T., 1996. Udviklingen i bestanden af Mellemskarv i Nord- og Mellemeuropa 1960-1995. Dansk Or. Foren. Tidsskr. 90: 15-20.

Van Eerden, M.R. & M. Zijlstra, 1988. Aalscholvers Phalacrocorax carbo met kleurringen uit de Oostvaardersplassen. Limosa 61: 57-60.

Van Eerden, M.R., K. Koffijberg & M. Platteeuw, 1995. Riding on the crest of the wave: possibilities and limitations for a thriving population of migratory Cormorants Phalacrocorax carbo in man-dominated wetlands. Ardea 83(1) 1995 (p. 1-9).

Van Eerden, M. R. & B. Voslamber 1995. Mass fishing by Cormorants Phalacrocorax carbo sinensis at lake IJsselmeer, The Netherlands: a recent and successful adaptation to a turbid environment. Ardea 83: 199-212.

Van Rijn, S. & M. Platteeuw 1996. Remarkable fledgling mortality at the largest Great Cormorant Phalacrocorax carbo sinensis colony in the Netherlands. Cormorant Research Group Bulletin, No. 2, September 1996.

Zijlstra, M. & M.R. Van Eerden, 1991. Development of Cormorants Phalacrocorax carbo in the Netherlands till 1989. In: M.R. van Eerden & M. Zijlstra (eds.): proceedings workshop 1989 on Cormorants Phalacrocorax carbo. Rijkswaterstaat, Directorate Flevoland, Lelystad, pp 53-60.

Stef van Rijn & Mennobart van Eerden, Rijkswaterstaat Riza, P.O. Box 17, NL-8200 AA Lelystad, the Netherlands.