Effect of human disturbance on the foraging behaviour of the oystercatcher Haematopus ostralegus on the rocky shore

Abstract

The aim of this thesis was to investigate the effects of human disturbance on animal foraging behaviour using oystercatchers foraging on the rocky shore as a model system. The primary focus of this thesis was the balance between vigilance and foraging, and the variation in this balance with changes in environmental factors and the application of experimentally controlled human disturbance. On the structurally complex rocky shore foraging and vigilance are generally mutually exclusive behaviours and so an individual must trade-off energy acquisition with predator avoidance. The extent of the trade-off was expected to vary spatially and temporally dependant upon an individual's needs and perceived predation risk. The foraging behaviour and prey selection of individual oystercatchers on the rocky shore was observed from September to March when the birds were most vulnerable to starvation. A preliminary experiment conducted in the winter of 2001-2002 used experimental and observational methods in an attempt to identify which types of human recreational activities had the greatest effect upon oystercatcher behaviour. In the winter of 2002-2003 changes in oystercatcher behaviour and prey selection with environmental factors such as the weather, temperature, wind speed, season and tidal state; and additional factors such as individual age, and the distance to and species of the focal oystercatcher's nearest neighbour were investigated. Oystercatcher foraging behaviour and prey selection before, during and after human disturbance was also observed in order to examine whether any losses to energy intake as a result of human disturbance could be compensated for by feeding more intensively, changing prey selection or lowering their baseline level of vigilance so that foraging time increased. Oystercatchers did not vary in their response to disturbance dependant upon the type of activity, but did vary spatially which could be a factor of the structural complexity of the shore. Human disturbance significantly reduced oystercatcher foraging as their vigilance increased, but oystercatchers returned to feeding at pre-disturbance levels almost immediately after the disturbance had ceased. Oystercatcher success rate on the rocky shore varied significantly with temperature and season, which may reflect an increase in feeding effort in response to the increased energetic costs of thermoregulation when colder temperatures ensue. Having another oystercatcher as a nearest neighbour significantly decreased oystercatcher success rate, although the distance separating an oystercatcher and it's nearest neighbour had no significant effect. Wind speed did not affect oystercatcher success rate but did significantly reduce peck rate, whilst an oystercatcher's age and the state of the tide (the amount of the shore that was uncovered) had no significant effect on oystercatcher behaviour. Prey selection varied with the state of the tide which could reflect prey availability. Oystercatcher energy intake over the time for which their rocky shore prey items were uncovered by the tide was just over half their estimated daily requirement, suggesting that feeding in supplementary feeding areas at high tide or at night may be an important part of the oystercatchers' foraging regime. Prey selection did not vary with disturbance, and no compensatory mechanisms were observed. It is possible that short-term disruptions to feeding double as digestive pauses or that there are potential constraints to energy intake rates such as the risk of bill damage, inexperience of foraging, interference, and prey availability