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The use of resources by otters (<I>Lutra lutra</I> L.) living along coasts and streams in Southwest Portugal was described, and related to their spatial and temporal patterns of availability. The use of freshwater along coasts was inferred from the spatial distribution of spraints, and from radio-tracking. Spraints were mostly concentrated at the mouth of streams that provided freshwater throughout the dry season, and where rest-sites were found within dense thickets of brambles. Rest-sites were considered a scarce resource, because they seemed to be associated with the few permanent sources of freshwater. Along the coast prey availability was highest in winter and lowest in later summer and autumn. The corkwing wrasse was the dominant prey, and the seasonal variation in diet was primarily related to fluctuations in its abundance: wrasse were consumed when they were most abundant, other prey being taken to compensate for lows in their availability. In streams there were large year-to-year fluctuations in fish abundance, and these were related to the severity of summer droughts. Spring was probably the period of maximum prey abundance. Eels and crayfish were the dominant prey, but cyprinids, toads and frogs were also important seasonally. Prey were consumed relative to availability, except eels, which were consumed less than expected during the peak abundance of crayfish. Crayfish and eels were more associated in spraints that expected by chance. The introduction of the American crayfish to Iberia contributed to increase food abundance from spring to early autumn. Breeding of otters occurred seasonally, but births were earlier on the coast (October-December) than inland (January-March), most probably as a consequence of the seasonal patterns of food availability. Litter sizes inland were larger than on the coast. Differences between otter habitats in Southwest Portugal and elsewhere were discussed. A conservation strategy for otters in this region was developed.

Turbidity has generally been viewed as having detrimental effects on fish; yet, many turbid habitats in the world are also abundant with fish. This phenomenon is often explained as fish enjoying reduced predation pressure in turbid habitats. This represents a trade-off situation where fish should select clear or turbid habitats that provide maximum net benefits. Because turbidity reduces light penetration, both predator and prey visual ranges are reduced, rendering both less efficient foragers. For this reason, I suspected that the benefits of a turbid environment would be greatest in the presence of predators and hypothesized that when predation risk is high, prey should prefer turbid water. Laboratory experiments showed that regardless of predation risk, fathead minnows (Pimephales promelas) preferred feeding in a turbid habitat. The presence of a predator, yellow perch (Perca flavescens) or black bullhead (Ameiurus melas), caused minnows to reduce feeding. There was on interaction between water clarity and predation risk, water clarity and predation risk, thus, appeared to affect the minnows’ habitat selection independently. The predator’s effect on the prey was the same whether in turbid or clear water. Using the prey distributions established in the lab experiment, key parameters and assumptions were identified for a computer model which simulated both prey and predator responses to turbid water and their interactions. The model predicted that prey would always prefer the turbid habitat when one was available. Predators generally used both clear and turbid habitats. Only when its foraging efficiency was reduced significantly did the predator show strong avoidance of turbid water. As the number of predators increased, predators used both clear and turbid habitat more evenly. Turbid environments seem to provide important habitats for small and juvenile fish. It may benefit small fish by reducing predator efficiency or reduce prey energy expenditure.

Turbidity has generally been viewed as having detrimental effects on fish; yet, many turbid habitats in the world are also abundant with fish. This phenomenon is often explained as fish enjoying reduced predation pressure in turbid habitats. This represents a trade-off situation where fish should select clear or turbid habitats that provide maximum net benefits. Because turbidity reduces light penetration, both predator and prey visual ranges are reduced, rendering both less efficient foragers. For this reason, I suspected that the benefits of a turbid environment would be greatest in the presence of predators and hypothesized that when predation risk is high, prey should prefer turbid water. Laboratory experiments showed that regardless of predation risk, fathead minnows (Pimephales promelas) preferred feeding in a turbid habitat. The presence of a predator, yellow perch (Perca flavescens) or black bullhead (Ameiurus melas), caused minnows to reduce feeding. There was on interaction between water clarity and predation risk, water clarity and predation risk, thus, appeared to affect the minnows’ habitat selection independently. The predator’s effect on the prey was the same whether in turbid or clear water. Using the prey distributions established in the lab experiment, key parameters and assumptions were identified for a computer model which simulated both prey and predator responses to turbid water and their interactions. The model predicted that prey would always prefer the turbid habitat when one was available. Predators generally used both clear and turbid habitats. Only when its foraging efficiency was reduced significantly did the predator show strong avoidance of turbid water. As the number of predators increased, predators used both clear and turbid habitat more evenly. Turbid environments seem to provide important habitats for small and juvenile fish. It may benefit small fish by reducing predator efficiency or reduce prey energy expenditure.

Pursuit behaviours are vital in predator-prey interactions and in courtship for many flying animals. Existing research on target-directed flight behaviours in insects, birds and bats has aimed at identifying simple geometric rules describing the pursuit-flight trajectories. However, these geometric rules are only part of the picture as they only consider the outcome of the commanded changes in flight kinematics, and not the underlying guidance laws (dynamics) which generate these commands. To intercept a target, a pursuer implements a guidance law using sensory feedback to determine the required change in flight velocity, and the resulting kinematics determines the flight geometry. Most of the research until recently has examined insect flight systems, as the ethics of working with birds of prey are more complex and measuring their wide-ranging flight trajectories is difficult. Studies of predator-prey pursuit in birds have only described the geometrical rules for target interception, therefore overlooking the guidance laws which implement them. Therefore the aim of this thesis is to complete the picture by identifying the guidance laws used by birds of prey as they pursue and intercept targets both in the air and on the ground. I used onboard cameras and GPS to study attack flights in peregrine falcons (Falco peregrinus), and high-speed ground photogrammetry for attacks in Harris' hawks (Parabuteo unicinctus), to show that two different raptor species effectively implement the same guidance law of pure proportional navigation for intercepting manouevring and non-manouevring prey-targets. Proportional navigation is a feedback law whereby the bird's line-of-sight rate is fed back, in order to command a turn-rate in proportion to the change in line-of-sight rate, with a constant of proportionality N. Harris' hawks were found to use this guidance law in its simplest case with an N of approximately 1. This amounts to a pure pursuit course, meaning the bird maintains a heading angle of zero at all times (its velocity vector points at the target). Peregrine falcons were found to use a variety of values of N resulting in a quicker path to interception. A remarkable feature of most bird of prey eyes is that they possess two regions of high visual acuity - the shallow and deep foveae. The deep fovea is optimised for long-range vision, and is directed at approximately 45° to the side of the head. It has been proposed that the head is held in line with the body for streamlining, while the body is turned in flight to fixate the image of the prey on the deep fovea, resulting in a curved trajectory. My results contradict this theory, as falcons were seen to use saccadic head movements to maintain the image of the prey in their field of view whilst flying along curved trajectories - suggesting a different visual strategy. I provide the first quantitative analysis of how birds are able to guide their flight for successful prey capture. Not only does this provide new insights into animal behaviour and evolution, but this research has many applications in engineering, where there is a wide and growing interest in vision-based approaches to guidance and control in both civil and military spheres.

Thesis (Ph. D.)--College of William and Mary. / Vita. Includes bibliographical references. Also available via the World Wide Web.

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