Studies on the Morphology, Feeding Behaviour and Breeding Biology of Skuas (Family Stercorariidae) With Reference to Kleptoparasitism

• Main Author: Caldow, Richard William Grier
• Format: Thesis
• Language: English
• Published: ProQuest Dissertations & Theses 1988
• Subjects: Arctic; Arctic skua; Great skua
• Online Access: http://theses.gla.ac.uk/77830/; http://theses.gla.ac.uk/77830/1/10999363.pdf

1) The kleptoparasitic success rates of light and melanic Arctic skuas did not differ significantly. Great skuas were significantly more successful than Arctic skuas in inducing their victims to release their fish, and in securing these dropped fish. 2) Both species of skua had significantly higher kleptoparasitic success rates than "gulls" chasing the same range of species at the same location. 3) Groups of Arctic skuas were significantly more successful in inducing their victims to release their fish than were solitary Arctic skuas. This was not the case for great skuas. Kleptoparasitic association between skuas seems to be an example of a selfish group. 4) Chases by great skuas were more likely to be successful if the victim was high above the water when the chase began and was slow to react to the attacking skua. Chases by Arctic skuas were more likely to be successful if the skua approached to within 1 metre of the victim and if the chase was towards the cliff. 5) Arctic skuas and great skuas differed in the locations at which they concentrated their kleptoparasitic efforts and in the methods of attack which they employed. Generally, Arctic skuas relied less on rapid descending stoops from above their victims than did great skuas and used flapping flight in a significantly higher proportion of chases than great skuas. 6) Arctic skuas rearing chicks had rates of energy expenditure equivalent to 3.3 - 3.9 times BMR. The rate of energy expenditure by breeding great skuas which were rearing chicks was equivalent to 5.0 - 5.8 times BMR. Energy expenditure was greater than in the mid-1970s when host availability was higher. 7) The daily energetic reward from kleptoparasitism was sufficient to meet the daily energy requirements of three pairs of great skuas and two pairs of Arctic skuas with normal brood sizes. A pair of Arctic skuas with three chicks had a negative energy balance. 8) Mean clutch size, egg volume and hatching success of Arctic skuas in 1987 were as high as in 1979. Mean clutch size and hatching success of great skuas were as high in 1987 as in the mid-1970s. 9) The growth rates of skua chicks in 1987 were not as high as in the mid-1970s. The fledging success of great skuas in 1987 was lower than in the mid-1970s. The breeding success of Arctic skuas was significantly lower in 1987 than in 1979 due to extremely heavy predation of Arctic skua fledglings by great skuas in 1987. 10) Breeding skuas spent considerably more time foraging during the latter part of the pre-fledging period in 1986 and 1987 than in the mid-to-late 1970s. In 1986 and 1987, great skuas frequently left their chicks unguarded and hence open to predation by neighbouring skuas. 11) Arctic skuas and great skuas were found to differ significantly in many aspects of their morphology from equivalently-sized gulls. On theoretical grounds these differences appear to enhance the kleptoparasitic flight performance of skuas relative to that of gulls. 12) It is suggested that these morphological differences between gulls and skuas, although beneficial to skuas' kleptoparasitic flight performance, are not the result of specialisations which evolved specifically for this purpose. It is proposed that skuas are morphologically pre-adapted to behaving kleptoparasitically as a consequence of their evolution to a more predatory way of life than gulls. 13) It is suggested that the highly kleptoparasitic nature of skuas is the result of morphologically and behaviourally pre-adapted species utilising this feeding technique when possible and profitable to do so. 14) The flight muscles of a great skua and a herring gull were not found to contain any "white" fibres. The pectoralis major and pectoralis minor muscles of the great skua had higher levels of oxidative and glycolytic enzyme activity than the equivalent muscles of the herring gull. 15) The possession of morphological adaptations which enhance the kleptoparasitic success rate of skuas could be of benefit in three ways. Firstly, with a higher success rate this feeding technique may be profitable enough to allow skuas to switch from other feeding techniques, in the face of a food shortage, and to continue to breed successfully when a lower success rate would preclude this possibility. Secondly, a higher success rate reduces the time and energy expenditure required to collect a given amount of food. Under conditions of food shortage, this could be of great importance in maintaining adult energy expenditure below some critical level above which parental survival may be jeopardised, and in minimising the likelihood that parents will have to leave their chicks unattended. Thirdly, the ability to feed successfully by kleptoparasitism may allow skuas to breed in areas of the world where, with a lower success rate, they could not.