Seasonal variation in ejaculate traits of male red-winged blackbirds (Agelaius phoeniceus)

Stefan Lüpold, Tim R. Birkhead, David F. Westneat

Research output: Contribution to journalArticlepeer-review

28 Scopus citations


Many reproductive traits, including ejaculate characteristics, usually show remarkable seasonal variation, but the potential for such dynamics in sperm morphology has been overlooked. Several studies have revealed high within-male repeatability in sperm morphology, but samples have typically been collected within a few hours or days, and the consistency of sperm morphology over longer periods remains unexplored. Here, we tested whether ejaculate traits, including sperm morphology, sperm number, and sperm velocity, exhibit seasonal phenotypic plasticity in a long-lived seasonal breeder, the red-winged blackbird (Agelaius phoeniceus). We found absolute and/or relative flagellum length and sperm velocity to increase across the season, whereas sperm numbers within ejaculates declined. Sperm morphological traits were further positively associated with harem size or the total number of offspring that fledged in each male's territory, suggesting that sperm morphology is likely to be linked to male reproductive quality. The underlying mechanisms of these patterns of seasonal variation remain unresolved, but we discuss our results in the context of dynamics of reproductive hormones, testicular structures and function, and reproductive behavior.

Original languageEnglish
Pages (from-to)1607-1617
Number of pages11
JournalBehavioral Ecology and Sociobiology
Issue number12
StatePublished - Dec 2012

Bibliographical note

Funding Information:
Acknowledgments We thank J. M. Fraterrigo, J. Homan, G. M. Linz, L. Reinhardt, and A. Trutsch for their help in the field and R. Montgomerie and two anonymous referees for valuable comments. SL was supported by the Janggen-Poehn Foundation, the Swiss National Science Foundation, a Sheffield University Overseas Research Student Award, a Lauff Research Award, and an NSF LTER Graduate Research Award; TRB, by a grant from the Leverhulme Trust; and DFW, by the U.S. National Science Foundation and the University of Kentucky.


  • Phenotypic plasticity
  • Seasonal variation
  • Sperm competition
  • Sperm morphology
  • Sperm number
  • Sperm velocity

ASJC Scopus subject areas

  • Ecology, Evolution, Behavior and Systematics
  • Animal Science and Zoology


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