Individual male Yellow Warblers (Dendroica petechia) exhibit differences in their level of parental investment, which correlate with the amount of brown streaking on the breast. We tested two alternative hypotheses to explain these patterns. The age effect hypothesis proposes that the variation in plumage and parental behaviour is an effect of differences in male age. This was tested by analyzing the nest visit rates and plumage scores of banded males present in the study in more than 1 year. The age effect hypothesis is rejected because there was no evidence of significant within-male variation due to increasing age, nor of any consistent directional change in either parental effort or plumage pattern between years. The territory quality hypothesis proposes that the quality of territory occupied has an important proximate influence on male parental effort. By observing the same territories over different years we were able to compare nest visit rates of different males with different plumage scores while holding the effect of territory quality constant. Analysis of variance revealed that male plumage rank had a highly significant effect on nest visit rate, while the effect of territory occupied was not significant. Rejection of these two alternative hypotheses provides further support for the hypothesis that phenotypically different males are using alternative and irreversible lifetime reproductive strategies.
This study was done near the Queen's University Biological Station, 50 km north of Kingston, Ontario, during May and June from 1982 to 1986. Two study areas were used in each year. Area 1 (Camelot) was an extensive wetland consisting of large areas of lowlying shrubs (Spirea) and isolated stands of large willow bushes (Salix). Area 2 (sugarbush) was an upland area with scattered shrubs and trees and a marsh-like border along Lake Opinicon. These are the same areas used by Studd and Robertson (1985a, 1985b).