Territorial Behavior: The Main Controlling Factor of a Local Song Sparrow Population

Abstract

IT has been shown frequently that territories are compressible. It is also known that the average territory size for a species in a given area may be significantly influenced by local conditions. However, it has been doubted whether territories can ever shrink to a point beyond which they can become no smaller and thus set an upper limit to the numbers of that local population. This paper describes a situation in which a local passerine population, affected by favorable environmental conditions, could reach high densities for several consecutive years, and in which the upper limit was set by territorial behavior. It throws some light on how territorial behavior, combined with other factors, may regulate a given population. Conclusions are based in part on data already available; nevertheless, references are made to factors-food and nestling mortality-that have not yet been fully investigated. Song Sparrows (Melospiza melodia), the subjects of this study, defend a territory that normally includes mating, nesting, and feeding grounds, and thus falls into the territory category A of Nice (1941). This study, undertaken in order to find out the regulatory mechanism in a local Song Sparrow population, was started in the spring of 1960 and is still in progress. Some data concerning this population were also available for the period between 1957 and 1960 (R. Drent and G. van Tets, pers. comm.).