COLOUR-RINGING OF GREAT CORMORANTS IN DENMARK, 1995-2000

Thomas Bregnballe

National Environmental Research Institute, Kalø, Grenåvej 12, DK-8410 Rønde, Denmark.
E-mail: tb@dmu.dk

Research on Great Cormorants Phalacrocorax carbo sinensis in Denmark has primarily been based on studies of individuals ringed with coloured darvic rings with engraved codes, readable from a long distance. Most chicks have been ringed in the breeding colony on the island of Vorsø, located 30 km SSE of Århus. Ringing in this colony was initiated in 1977 and has continued annually with ringing of an average of 592 chicks per year during 1979-1994. Ringing has become increasingly difficult in recent years in the Vorsø colony, but we continued to ring on average 349 chicks per year until 2000 to keep a ringed population in the colony.

Ringing with colour-rings has also been carried out in other breeding colonies in Denmark. In the first period, 1982-1991, an average of 753 chicks were ringed per year, and in the second period, 1996-2000 (except 1998) an average of 1132 chicks were ringed annually. In the first period, ringing took place in colonies all over the country, whereas in the second period, ringing has been concentrated to the colonies in SW Kattegat, where Vorsø is the old "mother colony". The recent ringing scheme in SW Kattegat is mainly aimed at studying the extent to which young Great Cormorants disperse to colonies other than their natal colony. It is the intention to compare the dispersal of young birds in the second half of the 1980s and early 1990s with the dispersal observed in 2000-2002. The development of the breeding population in Denmark and Northern Germany has undergone a transition from growth and colonisation of new sites to stabilisation. Since many colonies have stabilised despite high reproductive outputs, the stabilisation does not seem to have occurred because of a decline in the production of potential recruits. It will be studied whether or not the transition from growth to saturation within the region of SW Kattegat has affected the decision of young birds to remain in their natal region (where they may have to postpone age at first breeding) or to disperse to still unsaturated colonies elsewhere in and outside Denmark. I would like to use this opportunity to encourage observers to look for colour-ringed Great Cormorants in the colonies they visit. Thus searches for ringed Great Cormorants in Denmark in the spring of 2000 have documented that at least some Great Cormorants have emigrated from The Netherlands, Germany and Sweden and settled to breed in Denmark. It would also be interesting to know to what extent Great Cormorants of Danish origin disperse to and settle to breed in other European countries.

Another aim of ringing Great Cormorants in Denmark has been to obtain knowledge about when and to which areas individual birds migrate. Between 1977 and 1994, we received records of sightings from outside Denmark of 2,872 individuals, many of which had been resighted several times (Bregnballe & Gregersen 1995, Bregnballe et al. 1997). Although the proportion of individuals sighted of those estimated to be alive dropped after 1989/90, we still receive many valuable resightings. Between July 1994 and December 1999, 576 individuals of Danish origin were observed outside Denmark. Most resightings were made in Germany (227 individuals, 532 resightings), The Netherlands (111 individuals, 262 resightings) and France (104 individuals, 263 resightings). Thanks to a new group of observers in Belgium, who initiated searching for ringed birds in a standardised way and with a constant effort in the autumn of 1997, valuable data on duration of stay and site fidelity have been collected in Belgium (57 individuals and 533 resightings). Recoveries of ringed Great Cormorants found dead indicate that Danish Great Cormorants have shifted their wintering areas from the central Mediterranean towards the Atlantic coast during the 1990s. Continued ring reading at staging and wintering areas may add to the understanding of such shifts and improve our understanding of how increased population size affects migratory behaviour. For example, does an increased Cormorant density lead to less fidelity of individuals to areas used in autumn and winter, and to shorter stop-overs at staging sites? We thank all observers who searched for colour-ringed Great Cormorants and supplied the database with valuable information.

The ringing and subsequent resightings of colour-ringed Great Cormorants have in valuable ways added to our understanding of population dynamics and migration. Some of the more recent publications from the Danish ringing scheme deal with topics such as survival (Frederiksen & Bregnballe 2000a,b), prospecting of young birds in colonies (Schjørring et al. 1999), age at first breeding (submitted), post-breeding dispersal (Bregnballe & Rasmussen 2000), drowning in fishing gear (Bregnballe 1999), and site fidelity towards breeding sites (Schjørring et al. 2000) and wintering sites (to be submitted).

All resightings of colour-ringed Cormorants of Danish origin should be sent to the National Environmental Research Institute, Att. T. Bregnballe, Kalø, Grenåvej 12, DK-8410 Rønde, Denmark.

References

Bregnballe, T. & Gregersen, J. 1995. A decline in resightings of Danish Great Cormorants. Corm. Res. Group Bull. 1: 16-22.

Bregnballe, T., Frederiksen, M. & Gregersen, J. 1997. Seasonal distribution and timing of migration of Cormorants Phalacrocorax carbo sinensis breeding in Denmark. Bird Study 44: 257-276.

Bregnballe, T. 1999. Seasonal and geographical variation in net-entrapment of Danish Great Cormorants Phalacrocorax carbo sinensis. Dansk Ornitologisk Forenings Tidsskrift 93: 247-254.

Bregnballe, T. & Rasmussen, T. 2000. Post-breeding dispersal of Great Cormorants Phalacrocorax carbo sinensis from Danish breeding colonies. Dansk Ornitologisk Tidsskrift 94.

Frederiksen, M. & Bregnballe, T. 2000a. Factors affecting survival and colony fidelity of adult Cormorants: a combined analysis of recoveries and resightings. Journal of Animal Ecology 69.

Frederiksen, M. & Bregnballe, T. 2000b. Mortality, emigration or delayed return: Diagnosing a decline in return rate of one-year-old Cormorants. Journal of Animal Ecology 69.

Schjørring, S., Gregersen, J. & Bregnballe, T. 1999. Prospecting enhances breeding success of first-time breeders in the Great Cormorant, Phalacrocorax carbo sinensis. Animal Behaviour 57: 647-654.

Schjørring, S., Gregersen, J. & Bregnballe, T. 2000. Sex difference in criteria determining fidelity towards breeding sites in the Great Cormorant. Journal of Animal Ecology 69: 214-223.

Thomas Bregnballe, National Environmental Research Institute, Kalø, Grenåvej 12, DK-8410 Rønde, Denmark. E-mail: tb@dmu.dk