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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 1998 Green Darner Dragonfly Migration
at UTM, Mississauga, Ontario, Canada
Table III e
Descriptive statistics of the September
12, 1998 Observations of Anax juinius migration at the Mississauga
Campus of the University of Toronto |
Statistic |
Value |
Comments |
1)
Sample size |
13 |
Observations 4 and 5 were not
included in the analysis*. |
2) Sum of
sines |
-11.6925 |
Divide by sample size to get mean sine (line 4). Also
needed when pooling data from other studies. |
3) Sum of
cosines |
+1.75941 |
Divide by sample size to get mean cosine (line 5).
Also needed when pooling data from other studies. |
4) Mean sine |
-0.89942 |
Values for lines 4 and 5 are used with
sample size (line 1) to calculate length of the mean vector (line 6). |
5) Mean cosine |
+0.13534 |
6)
Length of mean vector |
0.90955 |
An index of dispersal of
bearings. Used to determine values for lines 9 and 13. |
7) Sine of
mean vector |
-0.98887 |
Values for lines 7 and 8 are used with a
Trigonometry Table of sines and cosines to extrapolate the mean angle, in this case
the Magnetic mean bearing (line10 ). |
8) Cosine of
mean vector |
+0.14880 |
9) Angular
deviation |
±25.6° |
This value is determined from Tables that convert mean
vector length (line 6) into angular deviation (or circular standard deviation). |
10)
Magnetic mean bearing |
279° ±25.6°
(West) |
Descriptive statistic of the True mean
bearing and the dispersion around the mean for the 13 dragonflies in the sample. |
11) Magnetic declination
|
-10º |
Subtract magnetic declination (variation) to obtain True
bearing |
12) True
mean bearing |
269° ±25.6°
(West) |
Descriptive statistic of the True mean
bearing and the dispersion around the mean for the 13 dragonflies in the
sample. |
13) 95% confidence
intervals |
±18° |
values from lines 1 and 6 are used to extrapolate 95% (or
99%) Confidence Intervals (C.I.) from appropriate Tables. |
* Observation 4 and 9 were of monarch butterflies (Danaus plexippus).
Comments
To the best of my knowledge, these are the first descriptive
statistics of the flight tactics of A. junius in southern
Ontario. The site is 5 km inland from Lake Ontario and the dragonflies were flying
on a track that was rotated about 50° to the right of the shoreline. The next
lake that the dragonflies would encounter, if they continued to fly W, would be the
southern section of lake Huron. There was abundant supply of prey
available as the dragonflies frequently broke off straight flight, appeared to chase and
capture insects (which were often visible when the dragonflies were overhead),
then immediately resumed straight flight. It is important to emphasise that
this mass movement was nearly invisible from the ground. If I had not been using
binoculars to scan for D. plexippus, I probably would not have noticed that there
were large numbers of dragonflies aloft. Question - why were they flying
west?
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