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	Western Sandpiper an indicator of wildlife sustainability in the Georgia Basin

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Map of Western Sandpiper Annual Count sites in the Georgia Basin of British Columbia

fraser_georgia_e.gif, 19KB

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Statistical Significance

Statistical significance is an essential tool to judge the claim that some trend is not just a random event. As an example, you would expect counts of sandpipers to differ each year and over time it is likely that the counts would go up or down. But how do scientists judge if such trends might be real or not? We employ statistical tests to the data. Some of these tests are quite involved but what is important here is how we decide that a trend is something of concern. An accepted rule is that an event such as a decline in sandpipers, is not considered to be random if the result occurs 95 out of 100 times. Statisticians refer to the probability described in various ways but most often as P<0.05. In other words, the probability (P) of this event occurring randomly is less than 5% (or 0.05). Similarly, P<0.01 refers to the probability that the event occurs at random less than 1% (or 0.01) of the time.

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Western Sandpiper Bird Count Methods

1.	Blocking Method - divides the entre flock into blocks of 100's or 500's along their length and summing the estimates.
2.	Density Method (used 1992-97) - estimated by counting the number of shorebirds in an estimated 10 m² patch of mud in a subjectively selected portion of a representative density of the flock and multiplying this by length and breadth of the flock.
3.	Strip Sample Method (used 1998-2000) - average number counted in a 1 m wide strip across the flock at about 100 m intervals and multiplied by the length of the flock.

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Graph Data - Spring Migrants

Graph data for the peak number of spring migrant Western Sandpipers counted on Roberts Bank, Fraser River delta. The regression analysis of the counts yielded the following linear relationship of y = 228899325 - 114401x, with a correlation of r²=0.73 and t-test of t=-3.7, at a probability of P=0.01 (from Butler and Lemon, 2001).

Year	Peak numbers of birds counted
1992	923,500
1994	1,125,000
1995	500,000
1997	177,200
1998	460,000
1999	223,500
2000	126,000

Graph Data - Juvenile Fall Migrants

The total number of juvenile Western Sandpipers per observer days on Sidney Island in August. The regression analysis of the counts yielded the following curvi-linear relationship of y = -8.11x²+ 32350.03x - 32255314.47 with a correlation of r²=0.95 (from Butler and Lemon, 2001).

Year	Number of birds per observer day
1992	302.7
1993	259.1
1994	277.3
1995	287.4
1996	316.6
1997	210
1998	227.4
1999	170.1
2000	59.9

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References cited in the Western Sandpiper indicator:

Butler, R. W. and M.J.F. Lemon. 2001. Trends in abundance of Western and Least Sandpipers migrating through southern British Columbia. Bird Trends 8:36-38, Canadian Wildlife Service, Ottawa.
Butler, R.W., T.D. Williams, N. Warnock, and M.A. Bishop. 1997. Wind assistance: A requirement for migration of shorebirds? Auk, 114:456-466.
Gratto-Trevor, C.L., V.H. Johnston and S.T. Pepper. 1998. Changes in shorebird and eider abundance in the Rasmussen Lowlands, NWT. Wilson Bulletin 110:316-325.
Liechti, F. 1995. Modelling optimal heading and airspeed of migrating birds in relation to energy expenditure and wind influence. J. Avian Biol., 26:330-336.
Morrison, R.I.G. 2001. Shorebird population trends and issues in Canada: an overview. Bird Trends 8:1-5, Canadian Wildlife Service, Ottawa.

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