Polygyny and Sexual Selection in Red-Winged Blackbirds:by William A. Searcy, Ken Yasukawa
The purpose of this book is to explain why red-winged blackbirds are polygynous and to describe the effects of this mating system on other aspects of the biology of the species. Polygyny is a mating system in which individual males form long-term mating relationships with more than one female at a time. The authors show that females choose to mate polygynously
The purpose of this book is to explain why red-winged blackbirds are polygynous and to describe the effects of this mating system on other aspects of the biology of the species. Polygyny is a mating system in which individual males form long-term mating relationships with more than one female at a time. The authors show that females choose to mate polygynously because there is little cost to sharing male parental care in this species, and because females gain protection against nest predation by nesting near other females. Polygyny has the effect of intensifying sexual selection on males by increasing the variance in mating success among males. For females, polygyny means that they will often share a male's territory with other females during the breeding season and will thus be forced to adapt to frequent female-female interactions.
This work reviews the results of many studies by other researchers, as well as presenting the authors' own results. Studies of red-winged blackbirds have ranged from long-term investigations of reproductive success and demography, to research on genetic parentage based on modern molecular methods, to a variety of experimental manipulations of ecological circumstances and behavior. Since the red-winged blackbird is one of the best studied species of any taxa in terms of its behavior and ecology, the authors have a particularly extensive body of results on which to base their conclusions.
Originally published in 1995.
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Polygyny and Sexual Selection in Red-winged Blackbirds
By William A. Searcy, Ken Yasukawa
PRINCETON UNIVERSITY PRESSCopyright © 1995 Princeton University Press
All rights reserved.
The behavior of red-winged blackbirds in the wild has been studied as extensively as that of any species of bird in the world. Given the traditional prominence of birds in ethology and behavioral ecology, this means that "redwing" behavior is as well known as that of any species of any taxa. That so much attention has been paid to red-winged blackbirds is due in part to certain practical advantages they offer for field study: they occupy open habitats, where they are easy to observe, and in most locales their nests are easy to locate, so that reproductive success can be followed at least over the short term. Additionally, the species is abundant over a large area where ornithologists are also thick on the ground. The popularity of redwings for study can also be attributed in part to their having been chosen for investigation by a few early workers in the 1950s, just as the current wave of interest in evolutionary studies of behavior and ecology was getting underway. It is generally true that the more that is found out about any species, the more new questions arise, so that interest in one species can snowball; this type of "runaway" (with apologies to Sir Ronald A. Fisher) is what has happened with red-winged blackbirds. The single most important reason for the interest in red-winged blackbirds, however, is that they are polygynous, and highly polygynous at that. Polygyny, in which one male has several mates, is a rare mating system in birds. The vast majority of bird species are socially monogamous, and very few indeed of the polygynous species show the degree of polygyny found in redwings. This puts red-winged blackbirds at the center of two of the hotter areas in behavioral ecology: mating systems and sexual selection.
Mating systems are categories of breeding-season social organization, and as such they sum up a great deal of the behavior and ecology of a species. The study of polygyny, especially polygyny in birds, has been central to the development of mating-systems theory (Wittenberger 1981), a body of theory that attempts to explain the evolution of mating systems within species, and their distribution across species. As the best-known of polygynous birds (the only possible rival being the pied flycatcher), it is natural to use red-winged blackbirds to test theories about the evolution of polygyny; one of our principal objectives in this book will be to use redwings for this purpose. We recognize that it is unlikely that a single hypothesis will explain the occurrence of polygyny in all species with this mating system (Searcy and Yasukawa 1989), so theories need not stand or fall solely on how well they fit red-winged blackbirds. Still, it is logical to test polygyny theories first on those species for which the most complete data are available.
A second objective will be to explore the effects of the mating system, acting through sexual selection, on other aspects of the species' ecology and behavior. Sexual selection exercises a peculiar fascination for contemporary biologists, perhaps because sexual selection can account for the occurrence of so many traits that seem in some ways nonadaptive, such as bizarre ornaments and display behaviors. As Darwin (1871) was the first to point out, sexual selection acts more strongly in polygynous than monogamous species, making any polygynous species a natural laboratory for the study of the effects of sexual selection. It will be our contention that in red-winged blackbirds, the polygynous mating system has a pervasive influence on the biology of the species, in large part because polygyny enhances sexual selection, but for other reasons as well.
In this introductory chapter we will first define the various categories of mating systems found in birds. Second, we will describe the natural history of red-winged blackbirds, in sufficient detail to justify their assignment to the category of polygyny. Third, we will give a historical development of some of the principal hypotheses on the evolution of polygyny, which will be tested against the redwing data later in the book. Finally, we will introduce some of the concepts from the theory of sexual selection that we will employ later.
There are two types of definitions for mating systems: social and genetic. Social definitions stress the social relationships that form between males and females breeding together: how many associations each individual forms simultaneously or within a season, how long associations last, and what kinds of cooperative behavior each sex performs, especially in terms of parental care. Genetic definitions stress the transmission of genes to offspring: the relative numbers of males and females transmitting genes in one season, or the number of members of the opposite sex with which each individual shares parentage within a season. Previously it was thought that the two sets of definitions were congruent, so that (for example) a male and female that formed a monogamous social relationship would also share gametes only with each other. It is interesting to consider why we so readily assumed that sexual relations would be more straightforward in animals such as birds than they are in humans, but at any rate it has by now been shown that they are not. Instead, social relationships do not always reveal how gametes are shared, in humans and nonhumans alike, so social and genetic definitions of mating systems must be kept separate.
The social definitions we will use are modified from Wittenberger (1979, 1981). Polygyny is defined as the prolonged association and mating relationship between one male and two or more females at a time. The only modification from Wittenberger here is that we do not specify an "essentially exclusive" mating relationship, which recent genetic evidence indicates may be rather rare. The other major systems are monogamy, the prolonged association and mating relationship between one male and one female at a time; polyandry, the prolonged association and mating relationship between one female and two or more males at a time; and promiscuity, in which associations between males and females are always brief. Finally, in rare cases there can be a prolonged association and mating relationship between two or more males and two or more females; such a system is called polygynandry.
These definitions stress social relationships and therefore have implications for social behavior such as intrasexual aggression, pair bonding, and parental care. They are stated so as to apply to individual mating associations rather than to populations or species. Thus one might find an occasional polygynous association (say one male and two females) in a population in which most associations are monogamous; this of course is actually the typical pattern in most "monogamous" species. By a convention proposed by Verner and Willson (1966), a population is termed polygynous if 5% or more of its males form polygynous associations. This rather lax criterion would seem to skew the classification toward polygyny, but even so over 90% of bird species still qualify as monogamous (Verner and Willson 1966, Lack 1968).
Wiley (1974) proposed genetic definitions of mating systems that hinge on the relative numbers of males and females contributing gametes to offspring within a breeding system. Polygyny, for example, is defined as a system in which more females than males contribute gametes. These definitions apply only to populations and not to individual associations. An individual-level, genetic definition for polygyny is a system in which one male contributes gametes to zygotes of more than one female within a season, whereas each female shares gametes with only one male, at least within a single breeding attempt. In this classification, monogamy means that one male and one female share gametes only with each other; and polyandry means that one female uses gametes from more than one male, whereas each male shares gametes with only one female. Promiscuity has no place in this classification. According to genetic definitions, polygynandry occurs when both males and females share gametes with multiple members of the opposite sex.
The Mating System of Red-winged Blackbirds
Red-winged blackbirds are members of the subfamily Icterinae, the New World blackbirds, in the family Emberizidae. The species' common name describes the appearance of adult males, which are a glossy black except for a red "epaulet," bordered in yellow, at the bend (wrist) of the wing. Males attain this "adult" plumage in the prebasic molt in August of their second year. Prior to this molt, one-year-old males are much browner overall, with smaller epaulets that are orange or red-orange rather than red. One-year-old males are also slightly smaller than "adult" (i.e., older) males; for example, wing lengths of one-year-olds and adults averaged 124 and 131 mm, respectively, in a Washington State population, and 121 and 128 mm in an Indiana population. We will refer to one-year-old males as "subadults," though this term is somewhat misleading in that such males actually seem to be sexually competent (Wright and Wright 1944, Payne 1969). Adult females are even smaller than subadult males, averaging 107 mm in wing length in Washington and 102 mm in Indiana. Females attain adult plumage, more or less, in their first prebasic molt in August of their natal year. Females in the adult plumage are dark brown above and brown streaked with white below. Females lack the conspicuous epaulets of adult males, but many do have a splotch of orange or red, of variable size and intensity, in the same position.
The preferred breeding habitat of red-winged blackbirds is marsh; this is where breeding densities and reproductive success tend to be highest. In arid regions, especially in western North America, redwings nest almost exclusively in marshes, but in wetter areas in the Midwest and East they breed in grassland and old fields as well. In northern populations, both sexes migrate south for the winter, and males return north to the breeding areas in advance of the females. Immediately upon return, males establish individual territories if they can, but there is always a surplus of males that are apparently unable to find positions in appropriate habitat. Subadult males are greatly over-represented in this surplus population, but there are many adult male "floaters" as well. In some cases, adult males remain as nonterritorial floaters throughout their lives.
Adult females begin arriving in northern breeding habitats some weeks after the first males. Early-arriving females usually delay a few days or weeks before nesting, whereas later arrivals may begin nesting almost immediately. Once a female settles on a territory, she is committed to a residence of about six to seven weeks if her nesting attempt is successful: approximately three or four days to build the nest, three to five days to lay eggs, 11 or 12 days to incubate, 11 days with young in the nest, and perhaps another two weeks feeding fledglings on the territory. Second-nesting ("double clutching") in the same season is rare among females who succeed in raising young on their first attempt, at least in northern marshes. Unsuccessful females usually renest at least once, and most renesting occurs on the original territory, but some females do switch territories between attempts. Females give parental care by building nests, incubating eggs, brooding young, feeding nestlings and fledglings, and defending eggs and young against predators. Males do not build, incubate, or brood, they may or may not feed nestlings and fledglings, and they perform varying amounts of defense against predators. We know of no cases in which two or more males shared a territory or in which nestlings or fledglings were fed by more than one male, although it is common for several males from neighboring territories to "mob" a potential predator together.
The average female red-winged blackbird, then, remains on a territory, associating with the male owner, for several weeks. This period qualifies as a prolonged association in avian terms, thus satisfying part of our social definition of polygyny. Furthermore, the relationship does involve mating; observations show that the majority of females copulate most often with the owners of their nesting territories (see below). The final criterion in our social definition of polygyny is that males form multiple associations simultaneously, and of course this is the key feature of the redwing mating system.
The females nesting on the territory of an individual male are referred to as his "harem," a term with unnecessary and unintended connotations but still a convenient one. Harem sizes are calculated in various ways, none of which are ideal. In Box 1.1 we demonstrate the four most commonly used estimates of harem size and discuss their relative merits. Table 1.1 shows mean and maximum harem sizes for various populations of red-winged blackbirds, along with the method of estimation used. Mean harem sizes range from 1.7 up to 6.2, and the overall maximum recorded harem size is 15. In all these populations, the great majority of male-female associations are polygynous. For example, the population in Table 1.1 with the lowest mean harem size (1.7) is a mixed marsh-upland population in New York studied by Westneat (1993a). In that population, 50 of 72 nesting females (69%) were involved in polygynous associations, as were 21 of 43 mated males (49%). In the Searcy and Yasukawa (1983) study of a marsh population in eastern Washington, 489 of 492 females (99%) bred in polygynous associations, as did 96 of 107 territorial males (90%). Other populations mostly fall within this range. Clearly, social polygyny is the norm in this species.
We can compare the degree of polygyny found in red-winged blackbirds to that found in other socially polygynous species. In many such species, maximum harem size is only two, and mean harem sizes for territorial males are approximately one (see Table 1.2). In these species, most male-female associations are not polygynous. For example, in indigo buntings only 21% of the females and 10% of the males mated in polygynous associations (Carey and Nolan 1979). Figures for marsh wrens are 46% of females and 25% of males mating polygynously (Verner and Engelsen 1970), and for great reed warblers 50% of females and 27% of males (Catchpole 1986). Thus, in these species the average adult does not mate polygynously. Of course, there do exist other species, besides redwings, for which polygyny is the norm rather than the exception; species with comparable (but not greater) levels of polygyny include fan-tailed warblers, yellow-headed blackbirds, and corn buntings (Table 1.2). Clearly, if we can speak in terms of varying degrees of polygyny, red-winged blackbirds are among the most polygynous of bird species.
So far we have been applying a social definition of polygyny to red-winged blackbirds; what about a genetic definition? Social and genetic criteria for mating systems would be completely congruent for redwings if all the females nesting on any territory copulated exclusively with the owner of that territory. To the extent that females perform copulations with males other than the territory owner ("extrapair copulations," or "EPCs"), and these copulations are successful in siring offspring, the social and genetic definitions are not congruent. Occasional EPCs were noted by early workers (Allen 1914, Beer and Tibbits 1950), but it was not until 1975 that hard data began to appear on the magnitude of the phenomenon. These first data came from an ingenious study in which Bray et al. (1975) sterilized territorial male redwings using vasectomy. The intent of the study was to test the effectiveness of male sterilization in population control. To the surprise of all, nearly half of the clutches on territories of sterilized males had at least one fertile egg. Sperm storage by females might provide a partial explanation for the fertility of these clutches, but Bray et al. minimized the importance of sperm storage by including only clutches started at least five days after vasectomy.
The vasectomy data made it clear that substantial numbers of EPCs must occur but gave little indication of the relative frequency of extrapair versus intrapair copulation. This problem was addressed by two separate studies that directly observed copulations by marked individuals in the same population in central Washington (Monnett et al. 1984). Both studies found extrapair copulations to be less frequent than intrapair ones, amounting to 12% of the 58 copulations in one study (Monnett et al. 1984) and 16% of 56 copulations in the other (Emily Davies pers. comm.). A more recent study on a New York State population found an even lower incidence of EPCs, which constituted 6% of 71 total copulations (Westneat 1992a). The authors of all these studies made careful attempts to obtain an unbiased sample of copulations, but there is still a real possibility of bias in that extrapair copulations may be more rapid or more cryptic than intrapair ones. Also, it may be that the timing of EPCs is biased either toward or away from each female's period of maximum fertility. Thus data are also needed on which males are actually fertilizing eggs.
Excerpted from Polygyny and Sexual Selection in Red-winged Blackbirds by William A. Searcy, Ken Yasukawa. Copyright © 1995 Princeton University Press. Excerpted by permission of PRINCETON UNIVERSITY PRESS.
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