An examination of the ecological correlates and evolution of polygyny in marsh wrens in Delta, British Columbia.

Abstract

In this study, I examined the occurrence of polygyny in marsh wrens (Cistothorus palustris) in Delta, B.C. Using four field seasons of data, I found in this population that males were moderately (19%) to highly (66%) polygynous, that in three years of the study (1979, 1992, 1993) polygynous males fledged more young than did monogamous males, and that the breeding sex ratio was female biased in three out of the four years of the study (1979, 1982, 1993). In order to examine if the observed male reproductive success was an accurate measure of male fecundity, I used DNA fingerprinting to assess paternity and therefore male realized reproductive success for the 1992 and 1993 breeding seasons. In order to ascertain the importance of predation of breeding nests on female choice of a breeding situation and male and female reproductive success, I compared predation rates on polygynous and monogamous nests. In order to test all of the possible reasons for polygyny occurring simultaneously I used a multi-modeled approach. I found that the skewed sex ratio model explained the occurrence of polygyny in 1979, 1982, and 1993. Additionally in 1979, 1982, and 1992, the polygyny threshold model also explained polygyny in the Delta population. In 1993, I found that the random settlement model was also supported by the results of this study. Overall in this population of marsh wrens the largest factors affecting the occurrence of polygyny was the skewed sex ratio of the breeding population in favor of females. Secondly, determination of which males became polygynous was most likely affected by female choice of a breeding situation, with females apparently preferring males whose territories were further from the upland edge of the marsh.