Change of topic from foraging strategies of albatrosses to foraging in cormorants

It has been generally accepted that cormorants find their prey when under water by way of their very keen vision, much in the way that aerial predatory birds detect and pursue prey from a distance. However our recent study has shown that the ability of cormorants to see when underwater is unexpectedly poor, with acuity about 60 times lower than that of an eagle. We have suggested, therefore, that when foraging, cormorants use behaviours that minimise their dependence on visual cues. We suggest that an important part of their repertoire involves probing with their beak at the substratum or into hollows among rocks and tree roots to disturb and flush out bottom-dwelling or hiding fish, and then rapidly capturing the fleeing prey, much as has been described recently for some insectivorous birds and some herons. We have called this strategy 'flush-foraging'.We propose to investigate this possibility by recording both the detailed movements and the energetic costs of the feeding behaviour of captive cormorants. We will be using birds from our existing colony, which we will supplement with new birds next year, and the purpose built cormorant facility here at the University of Birmingham. The existing group of birds will be trained to forage in a controlled but naturalistic situation around an artificial reef set up in a large swim tank. This tank is ideal as the birds are already familiar with it. Prey items will be presented in a random fashion to the birds as they forage around the reef. This will be achieved by firing dead fish from a series of water canons placed within the reef. We will place movement detectors on the back and neck of the birds and video them as they forage in order to determine whether or not the birds rapidly extend their neck to catch fast moving prey items at short range, rather than propelling the whole body towards the prey. We will vary the light level and turbidity of the water which will enable us to determine to what extent senses other than vision, such as detection of pressure waves, are used by the birds. Cormorants are unusual among diving birds in that they do not have water-proof feathers. Hence they become waterlogged within a relatively short period of time in the water and this could have a substantial effect on their buoyancy and, therefore, on the energy cost of diving . The new birds will be trained to dive from and back to a box positioned on top of the water in a tank. This will be used to determine the changes in buoyancy and rate of oxygen consumption during a bout of foraging. We will also determine the effect of diving in water and feeding on dead fish at different temperature on the ability of the birds to maintain their body temperature. Wing-spreading is a characteristic and enigmatic aspect of cormorant behaviour which often, but not always, follows periods of foraging. It has been suggested to have a range of functions including wing drying and food warming. We have already investigated these suggested roles behaviourally by manipulating food intake of diving captive birds and comparing the wing-spreading behaviour of fed and unfed animals. We found no evidence that feeding affects wing-spreading behaviour. When they are not on the diving tank in the buoyancy study, we propose to use the new birds and follow the protocol of our previous study. However, we are now able to monitor ambient weather conditions, heart rate and body temperature of the birds. From heart rate, we can estimate the rate of oxygen consumption, so that we will be able to relate the occurrence, physiology and energetics of wing-spreading to the prevailing weather conditions as well as to investigate the influence of the birds having been in water, with or without feeding.