Measurement of Territory and Home Range Size in Birds

Abstract

At some time during the annual cycle, most vertebrates restrict their activities to a definite area which may be termed the home range. If all or part of the home range is defended against other individuals of the same species, the guarded area is called a territory, according to current usage. Territoriality is an important mechanism which reduces intraspecific competition; it is especially pronounced in nest-building animals (birds, certain fish, insects, etc.) which have complicated behavior patterns requiring highly co6rdinated actions during reproductive periods. Establishment of territories and home ranges produces characteristic intrapopulation distribution patterns which have important bearing on the choice of census methods (See Odum, 1953, Chap. 6). Examination of the voluminous literature on territorialism in birds reveals that many qualitative details have been worked out. The kinds of territory, such as are listed in the comprehensive review by Nice (1941), have been classified, and the means of establishment and defense of the territory area have been described for many species. On the other hand, many quantitative aspects of territorialism have been scarcely considered. For example, the effect of various populations and habitat factors on the size and configuration of the defended or occupied areas is virtually unknown. A relation between territory size and food supply is often postulated, but we know of no case where the available food supply and the size of territory have both been accurately measured. It is true that numerous estimates made of territory size have been published, but it is evident that many are but crude approximations, often based on less than a dozen spot observations. Very little confidence can be placed in the comparison of measurements made by different investigators because of the great variation in procedures used and in the intensiveness of the observation. It is becoming quite evident that quantitative comparisons of territoriality cannot proceed until sound, consistent methods of measurement are developed. In this paper the problem of measurements of territory and home range size is critically examined, and a method for standardizing measurements is proposed. The method is illustrated by data obtained in a study of territoriality of seven southeastern species. These data are also used to test certain concepts developed by mammalogists in their parallel studies of home range.