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Table of Contents
Great Circle Hypotheis
Magnetoclinic Hypothesis
Magnetic-Latitude Hypothesis
Compass Bearings Hypothesis
Suns' Azimuth Hypothesis
Expansion-Contraction Hypothesis
Always Advance Hypothesis
Never Go Back Hypothesis
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Analysis of Field Data for the 1987 Monarch
Butterfly Migration in Calhoun, Georgia
 Table XIV
Mean Vanishing Bearing for Danaus plexippus in North Winds
Descriptive statistics for the September 28 to October 10, 1987,
observations in Calhoun, Georgia
|
Statistic |
Value |
Comments |
1)
Sample size |
121 |
Records with non-numercical records for
vanishing bearing or wind (e.g. N instead of 360°) were not included in
the analysis. |
2) Sum of
sines |
-16.25161 |
Divide by sample size to get mean sine (line 4). Also
needed when pooling data from other studies. |
3) Sum of
cosines |
-111.19490 |
Divide by sample size to get mean cosine (line 5).
Also needed when pooling data from other studies. |
4) Mean sine |
-0.13431 |
Values for lines 4 and 5 are used with
sample size (line 1) to calculate length of mean vector (line 6). |
5) Mean cosine |
-0.91897 |
6)
Length of mean vector |
0.92873 |
An index of dispersal of
bearings. Used to determine values for lines 9 and 13. |
7) Sine of
mean vector |
-0.14462 |
Values for lines 7 and 8 are obtained by
dividing the values from lines 4 and 5 by the value for line 6. The resulting
sine and cosine are used with a Trigonometry Table of sines and cosines to
extrapolate the angle of the mean vector, in this case the Magnetic mean bearing (line10
). |
8) Cosine of
mean vector |
-0.98949 |
9) Angular
deviation |
±21.4° |
Determined from Tables that convert mean vector length
(line 6) into angular deviation (or circular standard deviation). |
10)
Magnetic mean bearing |
188° ±21.4°
(West) |
Descriptive statistic for the True mean
bearing and the dispersion around the mean for the sample. |
11) Magnetic declination
|
-2ºW |
Subtract magnetic declination (variation) to obtain True
bearing |
12) True
mean bearing |
186° ±21.4°
(West) |
Descriptive statistic for the True mean
bearing and the dispersion around the mean for the sample. |
13) 95% confidence
intervals |
±4° |
95% Confidence Intervals (C.I.) are extrapolated from a
chart using values from lines 1 and 6. |
Comments:
Mean vanishing bearing was very similar to those recorded
for D. plexippus flying in N winds in southern Ontraio (i.e.
185°±26°, N = 53, CI ±8°).
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