Tactics and Vectors 98/99
                           

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Great Circle Hypotheis  

Magnetoclinic Hypothesis

Magnetic-Latitude Hypothesis

Compass Bearings Hypothesis

Suns' Azimuth Hypothesis

Expansion-Contraction Hypothesis

Always Advance Hypothesis

Never Go Back Hypothesis

 

 

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?