Abstract
The Baltic/Wadden Sea flyway population of Common Eider Somateria mollissima has been in decline since the
early 1990s yet the numbers breeding in Denmark were stable during 1990-2010. A new breeding survey was conducted
during 2018-2022 and the results are reported here. A total of 226 sites were surveyed covering almost all known and
potential Danish breeding sites. For 55 of these sites, counts were made only of males staying near the breeding islets.
To estimate nests numbers for these sites we multiplied male numbers by 0.55 (based on 53 sites where both male and
nest counts had been conducted in the same year). A total of 13 848 nests was thus obtained of which 90.2% originated
from nest counts and 9.8% from male counts. Corrected for unrecorded nests, we estimated the breeding population
in the 2020 survey to be 17 000 pairs (range: 16 500-17 500 pairs) which is markedly lower than the 24 500-25 500 pairs
estimated to have been breeding in 2010. Our result corresponds to a decline of 31.9% (4.6% per year). There was a large
geographical variation in the development of nest numbers ranging from growth or stable/slow decline in the north
(Limfjorden), west (Vadehavet) and central parts (N Fyn, N Sjælland) of Denmark to the more severe declines that occurred
in the southern and eastern regions comprising the Baltic Sea and the belt-areas. Among the larger colonies, the most
marked decline was recorded at Saltholm in Øresund where numbers had declined from 4351 nests in 2008 to 1365 nests
in 2021, corresponding to 37.5% of the national decline of 8000 pairs over the last decade. There were indications that
non-breeding was more frequent in the years of the 2020 survey than in previous surveys. For a number of sites, high
numbers of females were recorded on the water near to islands where low numbers of nests were found compared to
earlier years. A likely stressor – besides foxes – that may have prevented Eiders from breeding is the White-tailed Eagle
Haliaeetus albicilla. White-tailed Eagles may stress Eiders during the pre-nesting foraging period, during egg-laying, and
during incubation. White-tailed Eagles were observed in or close to several of the surveyed Eider colonies (the maximum
recorded was 28 eagles at the largest Eider colony on Saltholm). Female Eider carcasses left by White-tailed Eagles were
recorded on nine breeding islets. Disturbance by eagles may also have led to higher predation of Eider eggs by gulls. This
recent decline recorded in the size of the Danish Eider breeding population aligns with the decline estimated earlier for
the Baltic segment of this flyway population. Emerging pressure from a predator such as the White-tailed Eagle, which
directly kills adult females and indirectly affects reproduction, appears to be one of the factors that already have and will
continue to affect the development of the Eider breeding population in Denmark.
early 1990s yet the numbers breeding in Denmark were stable during 1990-2010. A new breeding survey was conducted
during 2018-2022 and the results are reported here. A total of 226 sites were surveyed covering almost all known and
potential Danish breeding sites. For 55 of these sites, counts were made only of males staying near the breeding islets.
To estimate nests numbers for these sites we multiplied male numbers by 0.55 (based on 53 sites where both male and
nest counts had been conducted in the same year). A total of 13 848 nests was thus obtained of which 90.2% originated
from nest counts and 9.8% from male counts. Corrected for unrecorded nests, we estimated the breeding population
in the 2020 survey to be 17 000 pairs (range: 16 500-17 500 pairs) which is markedly lower than the 24 500-25 500 pairs
estimated to have been breeding in 2010. Our result corresponds to a decline of 31.9% (4.6% per year). There was a large
geographical variation in the development of nest numbers ranging from growth or stable/slow decline in the north
(Limfjorden), west (Vadehavet) and central parts (N Fyn, N Sjælland) of Denmark to the more severe declines that occurred
in the southern and eastern regions comprising the Baltic Sea and the belt-areas. Among the larger colonies, the most
marked decline was recorded at Saltholm in Øresund where numbers had declined from 4351 nests in 2008 to 1365 nests
in 2021, corresponding to 37.5% of the national decline of 8000 pairs over the last decade. There were indications that
non-breeding was more frequent in the years of the 2020 survey than in previous surveys. For a number of sites, high
numbers of females were recorded on the water near to islands where low numbers of nests were found compared to
earlier years. A likely stressor – besides foxes – that may have prevented Eiders from breeding is the White-tailed Eagle
Haliaeetus albicilla. White-tailed Eagles may stress Eiders during the pre-nesting foraging period, during egg-laying, and
during incubation. White-tailed Eagles were observed in or close to several of the surveyed Eider colonies (the maximum
recorded was 28 eagles at the largest Eider colony on Saltholm). Female Eider carcasses left by White-tailed Eagles were
recorded on nine breeding islets. Disturbance by eagles may also have led to higher predation of Eider eggs by gulls. This
recent decline recorded in the size of the Danish Eider breeding population aligns with the decline estimated earlier for
the Baltic segment of this flyway population. Emerging pressure from a predator such as the White-tailed Eagle, which
directly kills adult females and indirectly affects reproduction, appears to be one of the factors that already have and will
continue to affect the development of the Eider breeding population in Denmark.
| Translated title of the contribution | Status over den danske ynglebestand af Ederfugl 2020 |
|---|---|
| Original language | English |
| Journal | Dansk Ornitologisk Forenings Tidsskrift |
| Volume | 118 |
| Issue | 1 |
| Pages (from-to) | 18-31 |
| ISSN | 0011-6394 |
| Publication status | Published - Feb 2024 |