Ofiera, Nadine (2017) Spatial patterns of reproductive success in a tree-based colony of Great Cormorants (Phalacrocorax carbo). (Bachelor thesis), Christian-Albrechts-Universität Kiel, Kiel, Germany, 33 pp.

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Abstract

Foraging and especially breeding ecology in birds has been studied extensively in the past, highlighting various social and environmental factors. Great Cormorants (Phalacrocorax carbo) are a species of today’s interest as they are the subject of an ongoing dispute between fishermen and conservationists. These waterbirds, common all over Europe, are accused of severely depleting fish stocks and causing economic harm. However, a cormorant’s diet can change rapidly due to prey availability, fluctuating environmental factors and intercolonial competition. They have also been proven to show social behaviour, for example practicing group hunting and forming compass rafts to signal food patches. In this study, I examined breeding success in regard to nest height and within-colony location and also warm take-off to foraging flights. My study site is a pond close to the city of Plön in Schleswig-Holstein. The colony consists of about 500 breeding pairs. I chose three different locations within the colony and there, picked twenty nests each for observation. Overall breeding success, measured with number of chicks, is highest in the centre. While overall nest height correlates with higher chick numbers, an independent examination only showed a relevant difference at the South Location. Location within the colony is significantly correlated with higher reproductive success. During swarm take-off, the central breeders always started first. In summary, my findings suggest that within-colony location is more important for breeding success than nest height. My data also supports a central-periphery pattern of distribution (high quality birds nest high and central, whereas low-quality birds are on the lower edge spots) and suggests that the central breeders are the decision-makers for take-off. This study reveals that breeding pairs of Great Cormorants are influenced by various different social and environmental factors. It poses a novelty in the field with the finding that within-colony location, which had hitherto not been examined by other authors, is a highly important factor for reproductive success. Moreover, nest height is not always directly beneficial for chick survival. Additionally, central breeders seem to be the most experienced birds, leading the swarm take-off, which indicates group intelligence. I suggest further research on within-colony location differences regarding breeding success and would like to emphasize the importance of future studies examining social intelligence and behavioural ecology in waterbirds.

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