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Some aspects of the ecology of lesser snow geese wintering on the Fraser River estuary

University of British Columbia

A study of the winter feeding ecology of lesser snow geese was conducted on the Fraser River Estuary of southwestern British Columbia. Food habits analyses indicated a strong preference for the rhizomes of Scirpus americanus and S. paludosus. Based on frequency of occurrence at least 75 percent of the diet is made up of the below ground organs of these species. Measurements of the food supply indicated a total standing crop of 516 mt of rhizomes. Rhizome density varied significantly between each marsh unit from a high of 77.3 g.m⁻² on Lulu Island to a low of 30.2 g.m⁻² on Westham Island. Rhizome density decreased significantly in April from that observed in October and December. Standing crop was minimum in poorly drained depressions and along the outer reaches of the lower zone where soil aeration is limited. Crude protein content was similar from rhizomes within single marsh units regardless of variations in water cover, microrelief or season. However, significant differences were found to occur between locations. Mean crude protein and crude fiber levels were 11.8 and 22.4 percent, respectively. Patterns of spatial and temporal distribution were analyzed in an attempt to determine probable causation. Aerial photo counts and ground checks were conducted to quantify dispersal. Multivariate Nominal Scale Analysis was used to analyze the data. The complete model accounted for up to 48.4 percent of the observed variance. Seasonal changes in hunting pressure, social stimulation, food supply and passive predator avoidance strategy were the most important factors influencing distributional patterns. Diurnal activity profiles were recorded between the hours of 05.00 and 24.00 from late autumn to early spring to identify the areas of greatest importance for feeding and resting. Access to food supply, as regulated by changes in tide height and flock location, was the most influential determinant of feeding and sleeping periodicity. Feeding intensity was greatest at Brunswick point when water depth was between 20 cm above and 10 era below the stem base of Scirpus americanus and least at Reifel Refuge, when water depth was either deeper than 20 cm or shallower than -10 cm at the upper and lower edges of the community, respectively. Because of the tidal fluctuations, a polyphasic rhythmicity was displayed by snow geese rather than the more commonly reported illumination-controlled, diphasic routine. Nocturnal feeding occurred regularly whether hunting season was open or closed. The mean proportion of each day engaged in feeding was 29 percent. Feeding intensity by snow geese, even during periods of optimal food accessibility, amounted to a mean of only 52 percent of the total activity profile indicating a combination of relatively efficient feeding techniques and a highly nutritious diet. Analysis of the responses of snow geese to various forms of active disturbance clearly showed differences in strategy and gradients of intensity of hazard avoidance. Helicopters released escape at the greatest distances, presumably because of their slow, erratic flight patterns. Small, fixed-wing wheel-planes presented the least irritative image of all aircraft observed. Bald eagles were the most common avian predator to create disturbance. Hunters and other humans in the marsh produced extreme wariness and open approach was not possible closer than 100 m. Non-human mammals were treated with great tolerance. Both aircraft and eagles caused circular escape flights whereas humans and gunshots produced linear flights. The mean duration of normal flight was 33 seconds. Flights resulting from disturbance were significantly greater at 85 seconds. Metabolizable energy (ME) and passage rate were calculated from experimental trials on caged snow geese. ME for rhizomes of Scirpus americanus was 1.43 kcal/g. Mean digestibility was 28 percent. The rate of passage of the rhizome diet did not vary with a change in the amount of food contained in the G.I.tract. Mean retention time was relatively constant at 120 minutes. The internal organ dimensions of 90 hunter-killed geese were collected from October to April. Analysis showed no age or sex-related differences. Gizzard size, small intestine length and dry matter capacity were shown to increase significantly from first arrival. The mean full gut capacity was 28.8 g dry matter. Direct late season measurements indicated a range of from 25.4 g to a maximum of 37.7 g. A feeding strategy was proposed to account for the relatively small amount of time actually engaged in feeding. Total daily energy expenditure for winter was calculated from the activity budget, environmental temperatures and levels of disturbed and undisturbed flight to be 269 kcal. From mean passage rate and alimentary tract capacity an intake of 294 g per day was estimated. At 1.43 kcal/g this level of intake represents 420 kcal ME per day, well above the calculated energy expenditure. Length of effective feeding period is presented as a possible explanation for the traditional mid-winter migration which occurs during late January and early February each year when, for several days, the effective feeding period falls below 9 hours. Daily consumption per bird was 0.3 kg per day. The total number goose-days on the Fraser during winter of 1974-75 was 744,000, representing a removal of 167 mt, or 32 percent of the total standing crop. The population size for that year was the smallest since 1947. A relative scale of importance for feeding was developed by incorporating measurements of the size of each marsh plus the use-intensity and feeding intensity at each unit. In descending order of importance per unit area are Brunswick Point, Lulu Island, Westham Island and Sea Island.

Text

2010-02-16

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http://hdl.handle.net/2429/20262

Animal Science

University of British Columbia

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