Aarhus University Seal / Aarhus Universitets segl

Morten Frederiksen

Variation in seasonal use of a Great Cormorant colony

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The amount of fish that a cormorant colony consumes during a breeding season is under debate. In relation to this, we studied factors affecting residence time in the Great Cormorant colony on the island of Vorsø.
We used date of first and last observation of colour-ringed individuals to study individual variation and long-term changes in timing of arrival and departure.
Daily observations of individuals were made from a tower inside the colony, located next to a freshwater pond used for roosting and bathing. In the present analyses we excluded individuals observed on only one day. We used a Generalized Linear Mixed Model and corrected for year effects when testing for relationships with age and sex.
Year, sex and age
The mean date of arrival differed between years (Fig 1), mainly because weather conditions in late winter affected how local wintering birds timed their arrival.
Males of all ages arrived earlier (Figs 1 and 2) and departed earlier than females resulting in equal residence times among sexes. On average adult males arrived 1 week before females.
1-year-olds never bred but appeared as prospectors in the colony. A variable proportion of 2-year-olds attempted breeding and they arrived on average 19 days earlier than 1-year-olds (Fig. 3). The 3-21-year-olds arrived on average 24 days before the 1-year-olds.
The mean residence time of adults varied slightly between years during 1985-1991 but declined markedly over the following five years and remained low thereafter (Fig. 4). These changes were partly due to an advancing date of departure from the colony (Fig. 5).
The breeding colony increased from 1500 pairs in 1982 to 5000 pairs in 1991. Breeding success subsequently decreased and so did breeding numbers. The observed changes in residence time and timing of departure were probably related to an increasing shortage of food during the early 1990s.
Original languageEnglish
Publication year2018
Publication statusPublished - 2018
Event14th International Seabird Group Conference - University of Liverpool, Liverpool, United Kingdom
Duration: 3 Sep 20186 Sep 2018
https://www.liverpool.ac.uk/international-seabird-group-conference/

Conference

Conference14th International Seabird Group Conference
LocationUniversity of Liverpool
CountryUnited Kingdom
CityLiverpool
Period03/09/201806/09/2018
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