Disturbance does not have a significant impact on waders in an estuary close to conurbations: importance of overlap between birds and people in time and space.

Abstract

Disturbance of wildlife is a potential cause of conservation concern, not least to overwintering waders Charadrii inhabiting estuaries close to conurbations where human recreational and economic activities are often concentrated. Disturbance from people on and alongside intertidal foraging areas could make it more difficult for birds to survive until spring in good condition by reducing the time available for foraging, by increasing energy requirements and by displacing birds to poorer foraging areas. We adopted a two-part approach to testing whether such significant impacts occurred in a Special Protection Area where disturbance risk was high because of its small size and close proximity to conurbations. In part one, we recorded over the whole estuary during stages of the tidal cycle when part or all of the intertidal zone was exposed and so accessible to waders (i.e. on receding, low and advancing tides): (i) the numbers and activities of people on the intertidal flats and on the adjacent land in those places where people were visible to waders in the intertidal zone, and (ii) the numbers of waders present and disturbed into flight, the flight distance and flight duration in the ‘overlap’ areas where people did disturb waders. People occurred on < 25% of the 938 ha of intertidal flats but most waders foraged on mudflats whereas most people were on sandflats. People on land were visible to foraging waders along < 35% of the 16.5 km of shoreline. Waders and people were therefore substantially separated in space. Within overlap areas, people and waders were often frequently separated in time: for example, people on land mostly disturbed waders when only the upper shore levels were exposed. The average overwintering wader spent <0.1% of its foraging time during daylight flying away from people and the additional energy expenditure was equivalent to <0.02% of its daily requirements. The comparison made in part two between our study area and two comparable estuaries showed that the number of visits each day to the overlap areas would have to have been 29 or 43 times greater for disturbance to have lowered the birds’ body condition and winter survival. Both parts of the study therefore suggested strongly that the amount of disturbance was too trivial to have a significant impact on waders. It is concluded that: (i) to properly assess disturbance risk to waders, both extensive and intensive observations must be made on the behaviour of people and birds to quantify the extent to which they overlap in space and time, and (ii) it should not be assumed that an estuary’s close proximity to conurbations, and the presence of large numbers of people in the vicinity of the SPA, necessarily implies a significant disturbance risk to waders.