Compass Bearings Hypothesis
Suns' Azimuth Hypothesis
Expansion-Contraction Hypothesis
Never Go Back Hypothesis
Analyses of Pooled Field Data: Hypothesis Testing |
Hypothesis testing: Comparsions of the Mean Bearings
to theoretical bearings, or theoretical directions, for the pooled data for the 1978,
1979, and 1981 Monarch butterfly (Danaus plexippus) migrations in southern
Ontario |
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Mean Bearing ± 95% Confidence Intervals overlaps with the Upwind: |
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Wind |
Number |
Mean Bearing |
95% C.I. |
Bearing |
Sector |
North |
53 |
185° (S) |
±8° |
NO |
NO |
Northeast |
64 |
229° (SW) |
±7° |
NO |
NO |
East |
150 |
247° (WSW) |
±5° |
NO |
NO |
Southeast |
131 |
257° (WSW) |
±5° |
NO |
NO |
South |
15 |
237° (WSW) |
±39° |
NO |
YES |
Southwest |
35 |
143° (SE) |
±52° |
NO |
NO |
West |
20 |
128° (SE) |
±29° |
NO |
NO |
Northwest |
107 |
153° (SSE) |
±5° |
NO |
NO |
Population |
575 |
222° (SW) |
±5° |
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* Adapted from Gibo, D. L., 1990
Definitions of abbreviations and symbols: N = North, NE = Northeast, E = East, S = South, SW = Southwest, WSW = West-Southwest, etc., C.I. = Confidence Intervals.
Conclusions
- The null hypothesis is rejected for upwind bearings for all wind
conditions. In each case, the mean direction of displacement of the butterflies was
not aligning with the upwind bearing.
- The null hypothesis is rejected for upwind sectors (compass arcs) for all wind conditions except South winds. The butterflies have a significant tendency to fly upwind in South winds.
Comments
- The results were fairly clear. Migrating D. plexippus tend not to
fly directly upwind in southern Ontario. Even when the upwind direction is defined
in terms of wind sector (compass arcs), the butterflies tend to fly upwind only in South
winds.
- The length of a compass arc defining a wind sector was 45°. The downwind
bearing was the bearing at the midpoint of the downwind sector. For example, the
Bearing of a North wind was 360° and the downwind sector extends 22.5° on
either size, or from 337.5° to 022.5°.
- Defining D. plexippus that landed, usually on vegetation, as
non-migrants excluded most observations of individuals that were flying upwind.
- The results for the South wind group should be considered tentative because of
its small sample size, large angular variance and wide Confidence Intervals.
- Testing the Hypothesis, either by using the midpoint bearing for the the wind, or by using wind sectors, results in a reduction in precision. Ideally, the data should be grouped according to wind sector for each observation, upwind direction should be determined from the measurement of wind bearings made at the time of the observation.