ROSKILDE FJORD

Birds

The large bird population on Roskilde Fjord is the main reason that the fjord is now a high priority natural area. It was also the motivation for the studies that form the basis of the Roskilde Fjord Portal. The study of the fjord’s birds has gone through a number of phases, following on from one another. We have attempted to answer the following questions:-

  • Which species are found in the fjord?
  • How are they distributed?
  • How many birds are there?
  • How are the populations changing?
  • Why do populations change?
  • What can we do to influence trends in a positive (from our point of view) direction?

We have more or less answered the first four questions. As far as population change is concerned, we have been able to point to a number of causes:-

  • A change in diet (Black-headed Gull)
  • Predators – Rats (eat gulls, terns etc), Herring Gull (eat eggs and young of tern and other birds), and foxes (eat waders and young of other birds)
  • Disease (infection in dense colonies, bird cholera and so on)
  • Weather and climate (flooding of terns’ nests, and hard winters)
  • Pollution (cleaner water)
  • Disturbances (hunting and tourism)
  • Cattle trampling nests (tern etc)
  • Competition for breeding/roosting sites (tern/cormorant)
  • Conservation rules (no access in breeding season, and hunting laws)
  • Rehabilitation of natural environment (restoration of Rørmosen on Eskilsø)
  • Bio manipulation (rat control, and feeding)


The list is not exhaustive, but in order to increase our understanding of these conditions we need to carry out long-term monitoring. The question of what we can do, or wish to do, is to a certain extent of a political nature, and involves conflict areas such as hunting versus bird protection, recreational use versus conservation, the question of how far we should control developments in the natural world, and the question of what constitutes a positive trend.

The most important data used in the bird descriptions are the counts of breeding birds and of birds roosting in the middle of October and January.

It has been possible to measure the breeding success of swans. From 1977-2014 there has been weekly monitoring, during the breeding season, of Mute Swans – their egg-laying, hatching, and the survival rate of their young. Whooper Swans breed far away to the north east and in their case the percentage of young in January is a good gauge. For other species an estimate has been made, often associated with ringing.

Together with the species descriptions there are population development graphs for breeding birds, October birds and January birds, according to the presence of the various species at those times. There are no population development graphs for non-breeding birds with very small populations, or for breeding birds which it is impracticable to count (e.g. Red-breasted Merganser). The graphs for breeding birds show the numbers of pairs, and the graphs for October and January show the number of individuals. The breeding birds graphs include data from 1978-2002 and 2007-2014. The January graphs cover the period 1984-2004, and the October graphs cover 1984-2002. In the population graphs, where it is statistically reasonable (r2>0.3), there is a regression line which shows the trend. The X axis on all the graphs shows time from 1978-2014, which makes it easier to compare the different graphs.

There is also a map of the fjord showing where birds have bred. This is based on ‘Google Maps’, which enables you to zoom in and out, and shift between map and satellite photo. The breeding sites are marked with symbols showing the size of the colony and indicating whether the species has increased or declined there. The size of the symbol shows the size of the colony in relation to the total population on the fjord. The ‘expression’ on the symbol (a smiley) shows whether the population at that site is increasing, declining or neither. In the case of species with such a small population that it is not possible to comment on population development, their presence is indicated with a little dot. The following symbols are used:-

A happy smiley: a growing colony – i.e. the difference between the median size of the colony in the period 2000-2014 and that in the period 1978-99, is bigger than +2% of the median size of the colony in 1978-2014.

A neutral smiley: a stable colony – i.e. the difference between the median size of the colony in the period 2000-2014 and that in the period 1978-1999 is between -2% and +2% of the median size of the colony in 1978-2014.

A sad smiley: a colony in decline – i.e. the difference between the median size of the colony in the period 2000-2014 and that in the period 1978-1999 is less than -2% of the median size of the colony in 1978-2014.

A little dot: such a small occurrence that it is impossible to comment on population development – i.e. the median size of the population at this site is less than 2% of the median size of the total fjord population of the species in question in the period 1978-2014.

The size of the symbols indicates:-

A large smiley: the median size of the population at this site is greater than 10% of the median size of the total fjord population of this species in the period 1978-2014.

A medium smiley: the median size of the population at this site is between 5%-10% of the median size of the total fjord population of this species in the period 1978-2014.

A little smiley: the median size of the population at this site is between 2%-5% of the median size of the total fjord population of this species in the period 1978-2014.

A little dot: the median size of the population at this site is less than 2% of the median size of the total fjord population of this species in the period 1978-2014.

Roskilde Fjord Portal covers all the species that have bred in the fjord, and the species of which more than 250 specimens have been counted in the autumn and winter counts.

For breeding-birds charts (islands) click here (XLS file)

For breeding-bird charts (whole fjord) click here (XLS file) or here (PDF file)

For January counts click here (XLS file) or here (PDF file)

For October counts click here (XLS file) or here (PDF file)

Population trends


Changes in numbers of birds breeding on the fjord, or present in autumn or winter.



Breeding pairs - all species
October - all species
January - all species