Morphological Adaptation and Digestion in Relation to Raptor Feeding Ecology

• Main Author: Barton, Nigel William Hugh
• Format: Thesis
• Language: English
• Published: ProQuest Dissertations & Theses 1992
• Subjects: Accipiter gentilis; Falco peregrinus; peregrine falcon
• Online Access: http://theses.gla.ac.uk/75321/; http://theses.gla.ac.uk/75321/1/13815503.pdf

1. An integral part of the study was to relate intestinal morphology to digestive efficiency in raptors. The source of morphology data was carcasses handed in by the public. The environmental conditions and time for which carcasses were exposed were unknown, as were the storage procedures following collection. The validity of using gut morphology data from carcass analysis was tested by assessing the extent to which small intestine length and weight in two-week old cockerels (Gallus gallus) changed under different experimental conditions of time and temperature post-mortem. Intestine weight decreased significantly with increases in time and temperature. Intestine length changed to a lesser extent and was chosen as the preferred measure when restricted to using carcass data. 2. Having determined which measure of gross gut morphology to use, data from Falconiformes and Strigiformes were used to quantify interspecific differences in small intestine length, the region of the gut responsible for food absorption. The study assessed the influence of predatory behaviour and prey type on morphological adaptations of the flight musculature and gut. Falconiform species were categorised as either 'attackers' or 'searchers' depending on the degree to which active, powered pursuit is required for prey capture. Attacking species feed predominantly on avian prey, requiring extreme agility, speed and acceleration for prey capture. Searchers feed largely on relatively slow-moving mammals and carrion. Weight minimisation is very important in terms of flight energetics and it was hypothesised that attackers would minimise the weight of internal organs which are not important for flight, such as intestinal mass. Searchers which do not require such agility and acceleration for prey capture would be expected to have longer, heavier intestines. It is further considered whether the absolute length or weight of the gut is important or whether it is the weight associated with gut contents that influences the size of the digestive tract. A skeletal body-size measure was determined to enable calculation of intestine length independent of body-size and shape differences. Attacking species were found to have a snail intestine which was up to 50% shorter than found in searchers of equivalent body- size. Strigiformes which locate prey by active flight also had intestinal tracts shorter than expected. It is hypothesised that these interspecific differences in gross gut morphology result in corresponding differences in digestive efficiency. 3. The size of the small intestine, stomach, liver, kidney and heart were compared between species and considered in relation to hunting strategy and body size for several raptor species. The extent to which these organs are affected by differences in body condition and parasite burden was examined. No relationship was found between parasite burden and intestine length. There was a strong correlation between body condition and organ size. Condition, fat content and parasite burden were shown to be related. Attacking species were found to have a small stomach and intestine for their size; searchers had large, heavy digestive organs. The more active owl species also had a lighter digestive tract. The scaling of intestine length, area and volume with body-mass was discussed. 4. It was hypothesised that the relatively long small intestine found in searchers such as the Red Kite (Milvus milvus) and Common Buzzard (Biiteo buteo) is adaptive and results in increased digestive efficiency, whereas a short digestive tract as found in the Sparrowhawk (Accipiter nisus), Peregrine Falcon (Falco peregrinus) and Goshawk (Accipiter gentilis) is selected for higher flight performance but results in reduced digestive efficiency. In order to test this hypothesis, it was necessary to find an appropriate method to measure digestion. The study aimed to test the suitability of titanium dioxide as a nutritional marker for measuring digestive efficiency in raptors. Such a method would enable a larger sample of birds to be used since it would allow the use of birds which could not be tethered under experimental conditions and those which are permanently kept in large aviaries. Birds which had been trained by falconry techniques were used to compare the use of a marker with results based on total faecal collection. Titanium dioxide is supposedly inert. However, complete recovery of the marker was not achieved and titanium dioxide was determined not to be a suitable marker for digestion studies in raptors. Total faecal collection was therefore used throughout the remainder of the study for measuring food passage and digestive efficiency. (Abstract shortened by ProQuest.).