The Invisible Birds
1979
Preston, Frank W.
In a search for birds in the bush or other objects of natural history, geology, and many other things, two observers in a unit of time, or one observer in two units of time, will in most cases see > one observer in one unit. This has been known for many decades, presumably for ages. Some recently available data from the late David Lack and from Milton B. Trautman make it possible to state the matter quantitatively, treating the observations as a matter of chance sighting. Both Lack and Trautman were dealing with birds, but the theory is applicable to many other objects of search. In this paper I shall use the term "birds" to cover asteroids, mineral deposits, caterpillars, or other objects of search. We are concerned to discover how many observers can usefully be employed in a search (this depends on the efficiency or skill of the searchers), or conversely, to estimate, given the number of observers, how many objects will be missed. Reasonably skilled observers will miss 50% of the objects when observing alone, but miss only 13% of the objects when observing in 3—person census parties. The fraction of species missed similarly drops from 15% to <5% as the party size increases from one to three.
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