Emerging Themes and Questions in the Study of Avian Reproductive Tactics

Many researchers have explored the ramifications of the idea that extra-pair copulation (EPC) is a male reproductive tactic to obtain parentage while avoiding parental investment since this concept was advanced by Trivers in 1972. Consortship between males and their fertile mates has been interpreted almost exclusively in terms of mate guarding by males. Females have been thought to benefit little, if at all, from extra-pair activities. This mindset has persisted and influences our interpretation of patterns of reproductive success revealed by molecular markers. Here we briefly trace the historical development of this line of reasoning and the newer, contrasting view well represented in this volume that females as well as males have EPC tactics. We identify specific contributions made by authors in this volume, contrast their approaches, and discuss the implications of their results for the understanding of avian mating systems and the role of sexual selection in avian social evolution. Finally, we illustrate the richness of this collection of papers by expanding on key points. This volume had its origins in a symposium on "Avian Tactics for Extra-Pair Mating" organized by Patty Parker at the request of Thomas C. Grubb for the 1995 AOU meeting in Cincinnati, Ohio. Cognizant of the increasing number of substantial data sets showing that rates of extra-pair fertilization (EPF) are commonly much higher than was expected even a few years previously, Patty invited participants with such data sets, fully expecting to find that patterns of EPF would be interpreted in a variety of ways. What emerged in the symposium, however, was a clear and compelling empirical consensus: acquisition of multiple genetic mates is a female reproductive tactic in avian species having a diversity of social mating systems (monogamy, polygyny, promiscuity) and social organizations (cooperative breeders, territorial species, gregarious and colonial species). This consensus is reinforced by several recent papers (e.g., Gowaty and Bridges 1991; Kempenaers et al. 1992; Lifjeld and Robertson 1992; Wagner 1992; Burley et al. 1994, 1996; Lifjeld et al. 1994; Stutchbury et al. 1994). This idea provides a sharp contrast to the prevailing view, briefly discussed below as well as by several contributors to this volume (Johnson and Burley, Chapter 2; Ketterson et al., Chapter 4; Stutchbury and Neudorf, Chapter 5), that selection on males is the