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Management

WSNL 1997-1

P. Becker et al.

COLONY SITES AND METHODS

The studies of these species were carried out at four important breeding sites in the Dutch (Griend, 2,500 - 3,300 pairs in 1993-1995) and German Wadden Sea (Baltrum 43 - 65 pairs, Minsener Oldeoog 1,800 - 2,100 pairs and Trischen 340 - 2,050 pairs). On Griend, Minsener Oldeoog and Trischen, the terns use dunes; on Griend and Baltrum the salt marshes are also used as breeding habitat. In representative parts of the colonies, groups of nests were fenced and the fate of the eggs or chicks was checked every two or three days if weather conditions were favorable (see Becker & Finck 1986, Exo et al. 1996 for methods).

RESULTS

On Trischen, in the three years no chick hatched owing to heavy egg predation by gulls (Tab. 1). The clutch size was low, as most clutches were already depredated before completion. Comparing the three other colonies, clutch size on Griend was lowest in each of the three years. Hatching success was high and showed no large differences between years or sites. Fledging success varied highly between years and sites and had values between 0 and 68%, corresponding 0 and 1.8 chicks per pair, resp. (Tab. 1). The average reproductive output during the three years was 0.7 chicks fledged per pair on Griend, 1.0 on Baltrum, 0.9 on Minsener Oldeoog, and 0.0 on Trischen.

The fate of the chicks (Tab. 2) may give information about the causes of this interyear and intersite variability. Main causes for the chick losses on Baltrum were predation and flooding. On Minsener Oldeoog, predation varied among years and was strong only in 1993, when Herring Gulls had taken all chicks alive. Similarly, as on Minsener Oldeoog, most chicks starved on Griend.

WSNL 1997-1

P. Becker et al.

DISCUSSION

Although during the 1980s and early 1990s the breeding population of the Common Tern in the German Wadden Sea was relatively stable, strong intersite differences in breeding success are obvious. This shows the different ecological value of several Wadden Sea islands and areas for fecundity and maintenance of the population, and the necessity to investigate several sites in a monitoring program.

The three-year average values of the reproductive output of Common Terns were higher on Minsener Oldeoog (0.9 chicks per pair) than on Griend (0.7) corresponding to long-term data at these sites (Minsener Oldeoog 1981-1990, 0.8, Becker 1992; Griend 1981-1991, 0.4, Stienen & Brenninkmeijer 1992; this figure is possibly underestimated as no enclosures were used). On the Jade, from 1980 to 1987, several colonies produced on average 0.8 chicks per pair per year (Becker 1991), a value assumed to be in the order of magnitude sufficient to maintain the population which was stable during the 1980s. On Griend, the more or less constant colony size despite the low breeding success over years, is astonishing as immigration from other colonies seems to be low.

During the three years of our study, only the Common Terns on Minsener Oldeoog (0.9) and Baltrum (1.0) reached the average success mentioned above. Our data elucidate, that the breeding outcome at Trischen was impaired mainly by predation. Consequently, the colony on Trischen decreased from 2,050 pairs in 1993 to 340 in 1995. Owing to the unfavorable conditions on Trischen, presumably many pairs resettled and joined a new growing colony at the Elbe estuary (Neufeld) increasing during the same period from 251 to 1,181 pairs (Landesamt für den Nationalpark Schleswig-Holsteinisches Wattenmeer).

But in some years, the more successful colonies on Minsener Oldeoog and Baltrum also suffer from predation (Tab. 2). As those years are singular events, this must be caused by special circumstances. On Minsener Oldeoog, 1993 was the only year from 1981 to 1996, when gulls had taken all chicks. The lower clutch size in 1993, but also other parameters not presented here, like the late onset of egg laying and the low growth of chicks, indicate an insufficient food availability. The main food of Common Terns in the Wadden Sea are clupeoids, Herring and Sprat, and the IBTS- Herring Index in 1993 was also low (Dornheim & Wegner 1996).

We assume that the benthic food sources of Herring Gulls were reduced in this year, too, so that this species switched diet towards young birds. In 1993, this happened not only on Minsener Oldeoog, but also on Scharhörn and Nigehörn (pers. communication), yet not on Griend, where Herring Gulls were culled, and not on Baltrum where Herring Gulls nested at greater distance from the terns which bred within a large Black-headed Gull colony (6,000 pairs). More research and the inclusion of the gulls into the studies is needed to understand the interrelations between factors like predation and food availability. It may be that on Trischen the food situation for both species groups, terns and gulls, generally was also poor. In this way, increasing interactions and predation may have led to low quality as a breeding site over the years and finally to the abandonment of Trischen by the terns.

Also on Griend, a poor food situation may have caused smaller clutches, the starvation of many chicks and low reproductive success over the years. As Stienen & van Tienen (1991) show, on Griend, Common Tern chicks were mainly fed on shrimps and crabs of low energetic value, in contrast to the clupeoid diet found on the Jade (Frank 1992).

The year with the highest breeding success at the three sites was 1994, when food supply of clupeoids seemed to be good, which was corroborated by high growth rates of chicks, by observations of chick feedings on Minsener Oldeoog (unpubl.) and by the high Sprat index in this year (Dornheim & Wegner 1996).

Tasker & Becker (1992) showed that on Trischen and Griend in 1988, Common Terns were relatively highly polluted with environmental chemicals, compared to other colonies. PCB- and mercury levels found were capable of impairing reproduction (Becker et al. 1993). The high values of hatching success at three of four colonies under investigation, however, give no indication of an impairment of hatching by toxic chemicals. Monitoring the contamination of seabirds is ongoing, and next year we will be able to provide actual information on the chemical burden of tern eggs during the mid 1990s which can be related to the data on reproduction.

WSNL 1997-1

P. Becker et al.

CONCLUSIONS

The results show that monitoring the reproductive success of terns is a powerful tool to provide useful information on changing environmental conditions and quality of breeding sites in the Wadden Sea - information which can be used as an early warning in management. If reproductive output remains low over the years as on Trischen, more detailed research into the causes should be initiated in time. The findings will indicate starting points of measures taken to improve the ecological situation in the respective Wadden Sea area. Therefore, we strongly recommend the governments to support the monitoring of seabirds' breeding success.

This monitoring is well evaluated, and should be carried out on reference sites which are distributed on the total area of the Wadden Sea and also by the other bird species recommended taking benthic food (Exo et al. 1996). Sites under investigation should include not only the Elbe estuary and the inner German Bight as a hot spot of pollution also in the future, but furthermore the Delta area adjacent to the Wadden Sea. Hence, the river Rhine is the other important pollution source for the Wadden Sea. It will be very instructive to relate the data of birds' reproduction to their chemical contamination, especially at these sites.

Acknowledgements:

The field work of the study 1993 - 95 was supported by A. Behnke, L. Krzywanski and S. Hotes. The Deutsche Forschungsgemeinschaft, Mellumrat and Statoil presented financial support, and the Vereniging Natuurmonumenten, the Landesamt für den Nationalpark Schleswig- Holsteinisches Wattenmeer and the Bezirksregierung Weser Ems, Nationalparkverwaltung Niedersächsisches Wattenmeer, issuing the licenses.

WSNL 1997-1

P. Becker et al.

REFERENCES

BECKER, P. H. (1991): Population and contamination studies in coastal birds: The Common Tern Sterna hirundo. In: Perrins, C. M., J. D. Lebreton & G. J. M. Hirons (eds.): Bird population studies: relevance to conservation and management: 433-460, Oxford University Press, Oxford.

BECKER, P. H. (1992): Seevogelmonitoring: Brutbestände, Reproduktion, Schadstoffe. Vogelwelt 113: 262-272.

BECKER, P. H., S. SCHUHMANN & C. KOEPFF (1993): Hatching failure in Common Terns(Sterna hirundo) in relation to environmental chemicals. Environ. Pollut. 79: 207-213.

DORNHEIM, H. & G. WEGNER (1996): Jungfischsituation in der Nordsee wenig ermutigend. Inf. Fischwirtsch. 43: 107-113.

EXO, K. M., P. H. BECKER, B. HÄLTERLEIN, H. SCHEUFLER, H. HÖTKER, A. STIEFEL, M. STOCK, P. SÜDBECK & O. THORUP (1996): Bruterfolgsmonitoring bei Küstenvögeln. Vogelwelt 117: 287-293.

FRANK, D. (1992): The influence of feeding conditions on food provisioning of chicks in Common Terns Sterna hirundo nesting in the German Wadden Sea. Ardea 80: 45-55.

FURNESS, R. W. & J. J. D. GREENWOOD (1993): Birds as monitors of environmental change. Chapman & Hall, London.

STIENEN, E. W. M. & A. BRENNINKMEIJER (1992): Ecologisch profiel van de visdief (Sterna hirundo). RIN-rapport 92/18: 1-128.

STIENEN, E. W. M. & P. G. M. VAN TIENEN (1991): Prooi- en energieconsumptie door kuikens van noordse stern (Sterna paradisaea) en visdief (S. hirundo) in relatie tot enkele abiotische factoren. Rijkinstituut voor Natuurbeheer Arnhem, Intern rapport 91/32: 1-37.

TASKER, M. L. & P. H. BECKER (1992): Influences of human activities on seabird populations in the North Sea. Neth. J. Aquatic Ecol. 26: 59-73.

TMEG (1993): Integrated monitoring program of the Wadden Sea Ecosystem. Report of the Trilateral Monitoring Export Group, May 1993 BMU 760/93, Bonn.

Authors address:

P.H.B.
Institut für Vogelforschung
An der Vogelwarte 21
D - 26386 Wilhelmshaven

A.B. & E.W.M.S.
IBN-DLO
Postbus 23
NL - 6700 AA Wageningen

D.F.
Nationalparkverwaltung
Niedersächsisches Wattenmeer
Virchowstr. 1
D - 26386 Wilhelmshaven

P.T.
Buchholzer Moor 8
D - 254712 Buchholz