• Weatherhead, Patrick J.
  • Robertson, Raleigh J.


The relationships between harem size, territory size, nest site suitability, and fledging success were studied for a breeding population of redwinged blackbirds (Agelaius phoeniceus). Reproductive success was correlated negatively with female breeding density within territories and positively with territory quality as determined from nest site parameters. Harem size was not correlated with differences in either territory area or quality. From these results we suggest that some females were attracted to nest in territories in which their chances of successful reproduction were lower than if they had chosen other available territories. To explain the selective advantage of such choices, we postulate that females were choosing mates which would enhance their ultimate fitness rather than their immediate reproductive success.


Study Areas

The study areas were located within a lo km radius of the Queen'sUniversity station, located On Lake Opinicon, about km north Of Kingston, Ontario. five large marshes from 6000 to l6 000 m2 in area) and five small marshes (from 600 to mZ in area). The predominant vegetation Of most areas was cattail (Typha latifolia) with some areas of sedge, sweet gale (Myrica gale), alder (Alnus rugosa), loosestrife (Decadon verticillatus), grasses, and willows (Salix spp), The habitat surrounding the marshes ranged from open hayfields to dense stands of mixed deciduous and coniferous woods.


The study period in both 1974 and 1975 began at the onset of the breeding season in the 1st week of May and ended at the completion of the final active nest in July. Nesting information was collected using the method described by Robertson (1972). In addition, male territories were mapped based on regular observation of areas frequented and defended by particular males. Harem size was taken as the maximum number of females nesting within a territory at any given time as determined by the number of active nests. Successful nests were defined as those which fledged at least one young. After the completion of breeding all marshes were surveyed and mapped and territory areas computed using a polar planimeter. Since female redwings forage largely off the territory Robertson 1972; Holm 1973) the quality of nest sites is likely to be the most important component of the quality of a territory. In addition, it was also assumed that females should choose the best available nest sites in a territory and thus an assessment of relative territory qualities could be made based only on nest sites actually used. The relation of nest success within each of the four measured parameters of nest sites (water depth, nest height, support vegetation, and nest cover density) was computed independently. Based on these relations each nest was assigned a quality score with the mean of all nest scores in a territory determining the territory quality score. All four parameters were measured for each nest when the nest was first discovered. Nest cover density was assigned to one of three categories: dense if at several metres dis-only partially concealed, and open if the nest had very little concealment.