Beyond Birding - Field Projects for Inquisitive Birders

By Thomas C. Grubb, Jr. , Department of Zoology, Ohio State University

The Boxwood Press; Pacific Grove; 1986

(Used with Permission)

Birds in the Classroom
The Bird Groups
Back to Home
Glossary
Themes
Bibliography


Preface

Chapter One - Ornithology as a Science

Chapter Two - Analytical Ornithology

Chapter Seven - When Do Great Blue Herons Give Up?

Chapter Fifteen - What Determines Individual Distance?

Chapter Twenty Three - What To do When You Know this Book

Appendix 2 - The Spearman Rank Correlation Test

Chapter 15 - What Determines Individual Distance?

Have you been struck by the uniform spacing of swallows resting on a telephone wire? It almost seems as if the birds measure the interval between where they alight and where their nearest neighbors are perching. Quite a few species show this spacing characteristic which is called individual distance, a rather precisely defined zone of intolerance to approach by conspecifics. Here we will evaluate the hypothesis that individual distance is determined by the reach of a perched bird, that conspecifics are not permitted inside the distance a resting bird can reach with its bill.

Numerous species congregate along linear perch sites. They range from the large gulls and cormorants loafing on a seawall or boathouse ridgeline to species as small as Bank Swallows resting on utility lines near their nesting colonies. This large variation in body size gives us material for a prediction. Since large birds can reach out farther with their bills, if our hypothesis is correct, there should be a positive correlation between body size and individual distance. The bigger the bird, the farther from its neighbors it perches or stands.

Begin this project by unrolling a spool of adhesive tape and sticking it to your kitchen floor or some other smooth clean surface. Using a ruler as a guide, blacken alternating 1-cm wide bands with a waterproof-ink marking pen. after the ink has dried, reroll the tape onto the spool.

Locate sites where birds of several different body sizes congregate in linear formations. Beside the possibilities mentioned above, Starlings sitting on fences around feedlots, doves perched on the sides of stock tanks, and House Sparrows resting on your clothesline come to mind. You may have other examples in your area. Unroll your tape and apply it on or just below and parallel to the surface the birds are using. If your birds appear unwilling to land on or just above the white-and-black tape, you can color the white bands a light brown to make them less conspicuous. Even then, it may be necessary to leave the tape in place for several days before your birds become accustomed to its presence.

After the birds are again perching nicely in a line on or just above the tape, you are ready to begin taking notes. Retreat some distance from the tape so you can view the perching line at an approximately perpendicular angle through binoculars or telescope. For each species, take 10 records of body width and 10 records of what appear to be minimum individual distance for each species, and add these to the table.

Now evaluate the prediction that individual distance is a positive function of body size. For each species, you have a pair of numbers, average body width and average minimum distance between neighbors, both taken to the nearest centimeter. Mark in Figure 15.1 the point corresponding to each pair of numbers. Are all the points arranged in a narrow band from lower left to upper right in the figure? If so, you have proven the prediction and failed to reject the alternative hypothesis. It is quite possible that your points are not so neatly arranged. Maybe they are in a broad band rather than a narrow one. Here, we are in doubt whether there really is a relation between body size and individual distance, or whether the weak correlation we see is just a product of random chance. To help you make a firm pronouncement on the hypothesis, you might with to analyze your results statistically in the fashion detailed at the close of this chapter.

 

 

 

 

Individual distance has been studied only slightly and there are opportunities for investigating a number of questions. For example, a few species are known to dispense with individual distance in very cold weather, with birds then huddling in contact with each other, presumably to keep warmer. What is the threshold temperature at which individual distance breaks down? Just after youngsters have fledged, they often sit next to their parents at loafing sites. Is the individual distance between parent and offspring smaller than the species average? Sometimes larger and smaller species intermingle. When neighbors differ in size, is the distance between them set by the larger bird, the smaller one, or is it some compromise distance? When individuals of the same species perch next to one another, is the distance between them larger when they face the same way than when they face opposite directions? If it is, can you formulate an explanatory hypothesis? We need a general hypothesis to explain why some species have individual distances while others do not and routinely perch in bodily contact. Deserts have very substantial fluctuations between day and night temperatures. Could it be that desert birds have thin plumage to keep cool during the day, and perch together in contact to stay warm at night?

Statistical analysis

The null hypothesis states that body size is not related to individual distance. The alternative hypothesis predicts a positive correlation between bird width and minimum individual distance. You can decide whether to reject the null hypothesis or the alternative hypothesis at the 5% level of confidence by testing your findings with the Spearman Rank Correlation Test explained in Appendix 2. Arrange your records in Table 15.2, then determine Spearman's r.

Table 15.1

Record Number

Body Width

Individual Distance

Record Number

Body Width

Individual Distance

Species:

Species:

1

2

3

4

5

6

7

8

9

10

Averages

Record Number

Body Width

Individual Distance

Record Number

Body Width

Individual Distance

Species:

Species:

1

1

2

2

3

3

4

4

5

5

6

6

7

7

8

8

9

9

10

10

Averages

Averages

Table 15.2

Species
Average Body Width
Average Minimum Individual Distance
Rank of Average Body Width
Rank of Average Minimum Individual Distance
Difference Between Ranks
Differences Between Ranks Squared

1

2

3

4

5

6

7

8