Digestion Strategies of North Atlantic Seabirds

• Main Author: Hilton, Geoff M
• Format: Thesis
• Language: English
• Published: ProQuest Dissertations & Theses 1998
• Subjects: common guillemot; Lesser black-backed gull; North Atlantic
• Online Access: http://theses.gla.ac.uk/75406/; http://theses.gla.ac.uk/75406/1/13815589.pdf

Studies on digestion in North Atlantic seabirds are presented, with particular emphasis on the relationships between digestion and ecology. A negative relationship between the rate of digestion and digestive efficiency is shown to occur in an inter-specific comparison of eight North Atlantic seabird species. This relationship is interpreted as representing a trade-off between benefits of rapid digestion and benefits of high digestive efficiency. Digestion rate is related to gut morphology: species with small guts tend to have rapid digestion. The selection pressures which result in species adopting a given digestion strategy are considered. Species with opportunistic feeding habits, and which include low quality food in their diets, tend to have slow but efficient digestion, whereas species which specialise on highly digestible and energy dense fish prey tend to adopt a strategy of rapid but inefficient digestion. It is suggested that slow digestion and a large gut is a requirement for species consuming low quality prey. A modelling approach indicates that digestion strategy can also have a profound effect on time and energy budgets of seabirds. In terms of time and energy minimisation, rapid digestion is likely to be favoured when costs of flight to the foraging site are high (in energy or time). An ingestion bottleneck is identified, which limits feeding rates when the gut is full, and thus applies strong selection pressure on optimal feeding trip length. The responses to digestive challenges of a specialist piscivore (Common Guillemot) and an opportunistically feeding seabird (Lesser Black-backed Gull) are compared. Birds were acclimated to one fish diet, and then abruptly switched to a novel diet. There is evidence that switched birds have non-optimal digestion of the novel diet, when compared with birds which are acclimated to that diet. The costs of diet switching are greater for Common Guillemots. The digestive cost of eating a mixed diet of two different fish types, when compared to eating the same diets separately, is also examined. For Common Guillemots digestive efficiency is significantly lower on the mixed diet, but no such cost is apparent for Lesser Black-backed Gulls. Thus the decision to change between diets should be affected by digestive considerations, even when the difference between diets is slight. It appears that species which commonly eat a varied diet are less affected by such digestive challenges. The relationship between diet characteristics and retention time are examined in a range of seabird species. Different fish species are digested at different rates, and these differences tend to be consistent across seabird species. Ease of digestion, energy density and nutrient composition should be considered as separate attributes of a diet, all of which may affect optimal retention time. The criterion by which optimal retention time is set in seabirds is unclear: they may be net rate maximisers, or efficiency maximisers. Geographic variation in the gut morphology and other major body organs is demonstrated within six Icelandic seabird species. This variation is consistent among species, and is related to geographic variation in ecological conditions, namely diet, foraging range and climate. Such variation in body composition between areas has not previously been shown, and may be an important component of adaptation to local habitat.