3.1.3 Disturbance of birds during surveys
Colonial-nesting birds, especially American White Pelicans, Great Blue Herons, and Western Grebes are vulnerable to disturbance at colonies. Some colonies of Great Blue Herons are acclimatized to routine human activities (e.g., Stanley Park, Vancouver and Beacon Hill Park, Victoria), but others are not. For example, at Quamichan Lake (Duncan), in the most sensitive of nine heronries studied, adult herons flew from their nests, when a human observer approached within: a) 200 m before eggs had been laid; b) 100 m after eggs had been laid; and c) 10 m after chicks were present (Butler 1991). Butler (1991) recommended a 300 m buffer zone around heronries with no human activity during the breeding season.
American White Pelicans are extremely sensitive to overflights of aircraft at their nesting colonies (Bunnell et al. 1981). Direct counts of nest contents are also not recommended. Pelicans tend to flush excitedly into the water and then fly off. Such disturbance can cause delays in egg-laying or loss of eggs and/or young through chilling or overheating. This effect can also occur if the nesting islands are approached by humans in boats. Pelicans incubate eggs with the bottoms of their feet (Johnsgard 1993), so eggs maybe broken if incubating birds are flushed suddenly (Dunbar 1984). If people land on the nesting islands during the incubation period, or if incubating pelicans are put off their nests repeatedly by human visitors (e.g., fishermen) then abandonment of nests can occur (Campbell et al. 1990). For example, human disturbance caused abandonment of all 130 American White Pelican nests at Stum Lake during 1986 (Campbell et al. 1990). Visits to the colony should be restricted to times when young pelicans are to be banded. Banding programs should be carefully rationalized and planned to maximize data gathering potential and minimize negative impacts on survival of young. Pelicans are also sensitive to aircraft, especially helicopters, on foraging lakes and may take flight upon approach within 1 km (J. Cooper unpubl. data).
Grebes are less susceptible to casual disturbances, because they cover their eggs with vegetation before leaving the nest, and tend to return quickly after humans leave the area (J. Cooper pers. observation). However, continuous and increasing human activities (water-skiing, floatplanes, fishing) have been implicated in the permanent abandonment of Western Grebe colonies at Williams Lake, Swan Lake, and Vernon (Cannings et al. 1987; Campbell et al. 1990; Burger 1991). In addition, predation by Ring-billed Gulls on Western Grebe (and probably other grebes) is a problem at Salmon Arm and possibly at other locations (R.W. Campbell, pers. comm.). Putting adults off the nest may, therefore, contribute to loss of eggs by increasing exposure of nests to predators. Increasing the risk of lower reproductive success should be a serious concern to biologists working on species with small populations, like Western Grebe and American White Pelican.
Wildlife managers should consider if the benefits of the survey will offset potential damage to colony success.
Birds may spend varying amounts of time in wintering, staging, migration, and breeding areas. Surveys must cover the period (extreme early and late dates) when populations might peak. Surveys to determine population trends must be conducted over a number of years to account for short term fluctuations.
Factors such as weather, water level, tidal cycle, and disturbance can affect daily, seasonal, and annual use patterns. For example, Great Blue Heron colonies may move en masse to a new site (R.W. Butler pers. comm.), Eared Grebes may not breed in traditional marshes during years of low water level (unpubl. data in B.C. Nest Record Scheme), and storms may temporarily disperse local winter concentrations of Western Grebes (J.M. Cooper pers. obs.). Because of possible bird population shifts, surveys may need to be repeated to accurately assess normal bird use of the study areas.
Variability always exists among observers in experience, ability to identify and count birds at various distances, visual and auditory acuity, etc. (Verner 1985). When more than one person is involved in surveys, the members of the team should be of equal ability. If not, training is required. Data will not be comparable between observers of different quality. One method of minimizing bias is to rotate observers between or among surveys so that bias is distributed equally.