Settlement patterns, territory size, and breeding density in the song sparrow (Melospiza melodia)

Abstract

In this study we investigated one possible proximate factor underlying year-to-year changes in mean territory size in song sparrows. We tested the hypothesis that birds defend smaller territories if they settle more or less at the same time, than if they settle on territories asynchronously. This idea is supported by some anecdotal field evidence in birds. To carry out the test, we did two types of removal experiments: (a) removing all the territory holders in an area at the same time and (b) removing territory holders one at a time, with 3- to 4-day intervals between each removal. Both types of removal were done in October 1972, coinciding with the fall peak of territorial activity, and in March 1973, coinciding with the spring peak. All the empty territories were rapidly refilled by juveniles that had previously been non-territorial; thus the breeding density of song sparrows is limited by territorial behavior. The replacement males were bird's that had been dominant in winter flocks of juveniles. For the two simultaneous-removal areas, territory size decreased significantly after resettlement, while for the successive-removal and control areas there was no such change. This shows that settlement patterns can influence mean territory size, and may be a proximate mechanism underlying annual fluctuations in size of territories.