Polygyny Threshold Model Research Articles

Poor reproductive success of polygynously mated female birds with obligatory bi-parental care: a result of deceptive behaviour of males?

Social polygyny usually benefits males by increasing the number of offspring, whereas it is detrimental for females as they must share the resources provided by their mate. An intersexual conflict may exist in animals with obligatory bi-parental care, such as birds of prey, in which females incubate and brood, whereas males provision food for their families. Long-term ringing data from Eurasian kestrels (Falco tinnunculus) breeding in nest-boxes and data on density indices of main prey animals (voles) were collected during 1985–2013 in western Finland to study polygynous behaviour. Of 1294 males, 54 (4.2%) were encountered at two (53) or three (1) nests during the same breeding season. Polygyny occurred more frequently during years of high vole abundance. The distances between nests of corresponding primary and secondary females were greater (median 1010 m) than the distances from nests of primary females to the nearest vacant nest-box (median 455 m). Twenty-eight (53%) of 53 secondary females had nearest available monogamous male within 2 km from their nest-boxes, indicating that mating options were available. Secondary females produced 30% less fledged offspring than simultaneously laying monogamously paired females. The abundance of prey animals is apparently alleviating the effort of males mating with multiple females. Spacing out the nests of primary and secondary females implies deceptive behaviour in the nest-site selection of polygynous males. Contradicting the polygyny threshold model, reproductive success of secondary females was significantly reduced in comparison to monogamous females laying simultaneously. These results show that secondary kestrel females apparently made a maladaptive choice, likely because they were deceived to accept polygynous mating status during the courtship feeding period.Significance statementAs dedicated parental effort of both the male and the female is vital to ensure the offspring survival amongst animals with obligatory bi-parental care, polygyny should be inherently a maladaptive mating strategy for females. However, regular social polygyny has been documented in at least 10% of bird species from ten orders. Previous studies on breeding success of polygynous birds of prey indicate reduced offspring production of secondary female partners with no apparent cause for females to choose polygynous males over other mating options. We showed that polygyny in Eurasian kestrels is frequent when food is abundant facilitating males to provision their two or more females during courtship feeding. Polygynous males space out their two nests thus attempting to hide their mating status from their secondary partners which suffer from their mate choice in form of poor reproductive success. Therefore, amongst the “cost of polygyny to females” hypotheses, the deceptive behaviour of males during courtship feeding appeared to be an apparent explanation for maladaptive mate choice of secondary females.

Social polygyny in the Eastern Bluebird (Sialia sialis)

I recorded social polygyny in an Eastern Bluebird (Sialia sialis) population located in central Wisconsin. A male simultaneously tended nests with 2 different females at his second and third broods. The polygyny threshold model is not an adequate explanation of this observation. Polyterritoriality cannot be ruled out. Multiple brooding behavior likely created an opportunity for an early breeding male to exhibit social polygyny. Food supplements and breeding asynchrony likely mitigated the costs of sharing a male.

The neglected role of individual variation in the sexy son hypothesis

The polygyny threshold model predicts that a female chooses an already-mated male if the direct benefits of higher territorial quality compensate for the cost of mate sharing. The sexy son hypothesis (SSH) expands this framework from direct to indirect benefits assuming that polygynously-mated females give birth to sexy and subsequently polygynous sons. Although the SSH has generated much debate, empirical evidence is inconclusive. I argue that lack of support could be related to the SSH formulation of the polygyny threshold as a population-level mechanism making the female choice of a polygynous male either advantageous or disadvantageous regardless of any other difference between potential mates. Using a simulation approach, I show that individual variation in males’ quality should be considered because, otherwise, the empirical validation of the hypothesis is hampered, giving rise to patterns that are apparently irreconcilable with the SSH predictions.

Correction to: ‘Greater wealth inequality, less polygyny: rethinking the polygyny threshold model’

Author(s): Ross, Cody T; Mulder, Monique Borgerhoff; Oh, Seung-Yun; Bowles, Samuel; Beheim, Bret; Bunce, John; Caudell, Mark; Clark, Gregory; Colleran, Heidi; Cortez, Carmen; Draper, Patricia; Greaves, Russell D; Gurven, Michael; Headland, Thomas; Headland, Janet; Hill, Kim; Hewlett, Barry; Kaplan, Hillard S; Koster, Jeremy; Kramer, Karen; Marlowe, Frank; McElreath, Richard; Nolin, David; Quinlan, Marsha; Quinlan, Robert; Revilla-Minaya, Caissa; Scelza, Brooke; Schacht, Ryan; Shenk, Mary; Uehara, Ray; Voland, Eckart; Willfuhr, Kai; Winterhalder, Bruce; Ziker, John | Abstract: With apologies, an incorrect author list was given in the article. An additional author, Chris von Rueden, (Jepson School of Leadership Studies, University of Richmond) also contributed to this work. Author Ryan Schacht's affiliation was out-of-date. His correct affiliation is: Department of Anthropology, East Carolina University.

Greater wealth inequality, less polygyny: rethinking the polygyny threshold model.

Monogamy appears to have become the predominant human mating system with the emergence of highly unequal agricultural populations that replaced relatively egalitarian horticultural populations, challenging the conventional idea—based on the polygyny threshold model—that polygyny should be positively associated with wealth inequality. To address this polygyny paradox, we generalize the standard polygyny threshold model to a mutual mate choice model predicting the fraction of women married polygynously. We then demonstrate two conditions that are jointly sufficient to make monogamy the predominant marriage form, even in highly unequal societies. We assess if these conditions are satisfied using individual-level data from 29 human populations. Our analysis shows that with the shift to stratified agricultural economies: (i) the population frequency of relatively poor individuals increased, increasing wealth inequality, but decreasing the frequency of individuals with sufficient wealth to secure polygynous marriage, and (ii) diminishing marginal fitness returns to additional wives prevent extremely wealthy men from obtaining as many wives as their relative wealth would otherwise predict. These conditions jointly lead to a high population-level frequency of monogamy.

Chick growth and survival in northern lapwings Vanellus vanellus indicate that secondary females do the best of a bad job

The polygyny threshold model predicts that monogamous and secondary females on average settle at the same time and have similar reproductive success. This is not generally found. Incorporating varying female competitive strength into the model, changes the predictions to state that secondary females should breed later and show a reduced success compared to that of monogamous and primary females. We examined if this was the case by investigating growth and survival in chicks of northern lapwings Vanellus vanellus from mothers of monogamous, primary and secondary mating status. Chicks where monitored from hatching to the age of 15–18 d. Growth and survival in secondary chicks was lower than in monogamous and primary chicks. Primary chicks survived significantly better than secondary chicks. Survival of monogamous chicks was comparable to primary chicks and close to significantly higher than in secondary chicks (p = 0.086). Among surviving chicks, daily weight gain in monogamous chicks was significantly higher than in secondary chicks. Growth rates of primary chicks were comparable to monogamous chicks and tended to be higher than in secondary chicks (p = 0.11). Monogamous and primary females both bred significantly earlier than secondary females, and chick survival and body‐mass growth decreased significantly with hatching date. Given the premium on early breeding in lapwings, secondary females appeared to do the best of a bad job, and their later onset of breeding could have been caused by poorer condition and/or lower breeding experience. Additional costs might also have accrued from sharing breeding resources with primary females that presumably were stronger competitors.

Precopulatory Mate Guarding, Mating System and Pairing Parental Care in Hyale rubra (Peracarida; Amphipoda; Gammaridae)

Mating behaviour of the gammarid amphipod, Hyale rubra, was observed. H. rubra displayed precopulatory mate guarding: males clasped females with their gnathopods during copulation, forming a pair. Males also participated in embryo care during the incubation period. The population was small, and the sex ratio was almost equal. Energy allocation for mating effort and parental effort in the two sexes appear to be almost equal. The mating system was sequentially polygamous (or promiscuous) and there was conspicuous sexual dimorphism in the size of the gnathopod, which was used for mate guarding. However, there appears to be relatively weak sexual competition for mating opportunities despite conspicuous sexual dimorphism. H. rubra did not display territorial competition or external fertilization. Nevertheless, the male provided paternal care. Since H. rubra inhabit tide pools and live on algae, the polygamous mating system of the species can be explained by the polygyny threshold model. The evolution of mate guarding and parental care may have been favored by the species' low population density and harsh environments, an interpretation consistent with the optimality model.

On the evolution of polygyny: a theoretical examination of the polygyny threshold model

The polygyny threshold model states that if costs incurred are less than the benefits gained from mating polygynously in terms of breeding-situation quality, then polygyny is favored and could evolve. We constructed mathematical models and computer simulations to evaluate this hypothesis. In the basic model, there is a single locus with two alleles, which regulates whether the female is receptive to polygyny. There are two breeding situations of differing quality on which males randomly assort. Females then select a mate based on the associated breeding situation and whether the male already has mates. This basic model is extended mathematically to include a cost for the initial female of a male with multiple mates and again to include gene expression in males. The computer simulations extend the basic model to multiple loci and alleles and to multiple breeding situations. The results presented here suggest that the polygyny threshold model is valid in a population genetic context: if the fitness of females that actually mate polygynously is greater than the fitness of monogamous females on poorer breeding situations, polygyny evolves. However, this approach reveals interesting dynamics not apparent from the verbal model. If the trait is expressed in males and females, then polygyny can evolve even if females mating polygynously have a lower fitness than females mating monogamously. In the multiple breeding-situations model, the polygyny allele increases to some equilibrium value above which it experiences no selection. Surprisingly, as the cost of polygyny increases, the equilibrium frequency of the polygyny allele also increases. The difference between this evolutionary model and the ideal free distribution is discussed. Copyright 2003.

Predicting polygynous settlement while incorporating varying female competitive strength

The rationale behind the polygyny threshold model is that the breeding situation quality (BSQ) males have to offer females varies, and that differences in BSQ offset females' costs of sharing with other females, thus favoring polygynous settling. It predicts that the first chosen territories become polygynous first, and that breeding success of secondary and contemporary monogamous females is similar. This is not generally found. Testing of the polygyny threshold model (PTM) assumes that females are competitors and distribute ideally free around available breeding resources, a condition probably not often met. If sharing a male is costly, and competitors differ in quality, weaker individuals should experience degrees of competitive exclusion. Setting competitive abilities proportional to arrival order, we use an individual-based interference-competition model to examine settlement patterns. Shifts in the ratio of variance in interfemale competitiveness to interterritorial differences in BSQ result in various settlement patterns, with different predictions concerning settlement order and fitness returns. We find support for the novel predictions from data on northern lapwing (Vanellus vanellus), starling (Sturnus vulgaris), pied flycatcher (Ficedula hypoleuca), and blue tit (Parus caeruleus). We suggest that before testing polygyny predictions, an evaluation of the settlement sequence should be made, which may help to generate more accurate predictions. We argue that violation of the equal female assumption may explain much of the discrepancy between predictions and empirical findings in previous tests of the PTM, and that secondary females in general have lower success than do monogamous breeders because they are of lower quality. Copyright 2003.

Polygyny in the dusky warbler, Phylloscopus fuscatus: the importance of female qualities

The polygyny threshold model states that secondary females gain benefits from high territory quality that outweigh the costs of sharing a male. We aimed to test this prediction using the dusky warbler as a model species. We first showed that neither the shifted sex ratio hypothesis nor the no-cost models were likely to apply to our study. Secondary females settled in territories of higher quality (high food abundance, low predator density) and had a nonsignificantly higher reproductive success than simultaneously settling monogamous females. However, there were strong indications that these two groups of females differed intrinsically. Secondary females were older than late monogamous females, and while they replaced lost clutches more often, they also suffered from a nonsignificantly higher winter mortality. Consequently, it was impossible to tell whether differences in reproductive success were caused by differences in territory quality or in female qualities. Our study suggests that female choice may also depend on characteristics that are specific to the individual, and may therefore be more sophisticated than allowed for in traditional polygyny models. In other words, the polygyny threshold may be not the same for all individuals. We suggest that prior breeding experience may help older females to profit more from the benefits and to suffer less from the costs of polygyny than young females.

Experimental confirmation of the polygyny threshold model for red-winged blackbirds.

The polygyny threshold model assumes that polygynous mating is costly to females and proposes that females pay the cost of polygyny only when compensated by obtaining a superior territory or male. We present, to the authors' knowledge, the first experimental field test to demonstrate that females trade mating status against territory quality as proposed by this hypothesis. Previous work has shown that female red-winged blackbirds (Agelaius phoeniceus) in Ontario prefer settling with unmated males and that this preference is adaptive because polygynous mating status lowers female reproductive success. Other evidence suggests that nesting over water increases the reproductive success of female red-winged blackbirds. Here we describe an experiment in which females were given choices between two adjacent territories, one owned by an unmated male without any over-water nesting sites and the other by an already-mated male with over-water sites. Females overwhelmingly preferred the already-mated males, demonstrating that superior territory quality can reverse preferences based on mating status and supporting the polygyny threshold model as the explanation for polygyny in this population.

Bill morphology reflects female independence from male parental help.

The study of territorial polygyny in birds has been influential in the development of the theory of social mating systems. Alternative female mating options have been studied within the framework of the polygyny-threshold model and later as the outcome of conflicts of interest between individuals. However, little attention has been given to variations between individual females, and how this affects their mating behaviour. Here, we test the hypothesis that some females are better adapted to raise nestlings without male assistance, and thus to mate polygynously. Specifically, we investigate whether intraspecific variation in female bill morphology is related to mating behaviour. This hypothesis is derived from earlier studies showing that, in both intra- and interspecific comparisons, uniparental care by females is correlated with the catching of larger prey items than when both parents provision the young. Using the polygynous dusky warbler (Phylloscopus fuscatus) as a model species, we found that, in accordance with our prediction, females with deep bills were more likely to mate as a secondary female. Moreover, regardless of mating status, females with deep bills settled in territories with more food and they received less male assistance in feeding their offspring. We argue that females with stronger bills are better adapted to exploit the abundance of large food items in rich territories and thus to raise young on their own. Our results demonstrate the importance of studying variations between individual females, and provide evidence for an extended version of the 'constrained-female hypothesis'. As bill depth is a highly heritable trait, our study strongly suggests that variation in female mating behaviour is not only related to ecological factors and female condition (as shown elsewhere) but also to heritable morphological traits.

Paternal investment and the human mating system

Paternal investment has long been considered responsible for the evolution of predominantly monogamous marriage in humans. However, male–male competition resulting in mate-guarding and male coercion could be equally important. In this review, I use a comparative approach to examine the effect of variation in human paternal investment on our mating system. I conclude paternal investment is important but so too is mate-guarding. I propose a model of our mating system incorporating both factors. Variation in the mating system is explained by variation in male resource control and contribution, resulting in ecologically imposed monogamy or polygyny, as predicted by the polygyny threshold model, as well as variation in male–male competition for status, resulting in socially imposed monogamy or polygyny.

Nest and mate choice in the red bishop (Euplectes orix) : female settlement rules

We investigated female settlement in a colony of red bishops (Euplectes orix), a territorial and highly polygynous weaverbird widely distributed over sub-Saharan Africa. An earlier study showed that male reproductive success is mainly determined by the number of nests a male builds in his territory, which appeared to be a good indicator of male quality. Because males provide no parental care or food resources within the territory, females sharing a territory do not compete for material resources and might therefore be expected to settle preferentially in territories of males that build many nests to gain the possible genetic benefit of high-quality offspring. An analysis of female settlement, however, revealed that females did not show a preference for territories of males with many nests and that the distribution of female breeding attempts with regard to the number of vacant nests within a territory could be explained best by random female settlement in 3 out of 4 years. Females settled more often than expected by chance (in 3 out of 4 years) in territories already containing occupied nests, indicating that resident females did not discourage settlement of additional females. However, sharing a territory with other females might impose costs in terms of an increased predation risk because nests in territories that contained other nests with young suffered from higher predation than nests in territories that did not contain other nests with young. Females therefore might trade the possible benefits of settling in territories of males with many nests against the costs of sharing a territory with other females. This might result in the mating pattern found with random female settlement and male reproductive success being directly proportional to the number of nests built. We discuss possible implications of this mating pattern for sexual selection on males. Key words: Euplectes orix, female choice, male quality, predation, random female settlement, sexual selection, weaverbirds. [Behav Ecol 11:378–386 (2000)] I n polygynous species with resource defense polygyny, territorial males provide resources that are important for reproduction such as food or nest sites within the territory to attract more than one female. Females often have to make a choice between a male without mate and an already mated male. If there is variation in the amount or quality of the resources provided by the males, it might pay for the female to choose an already mated male with, for example, a better territory, instead of an unmated male, because the advantage of higher territory quality outweighs the disadvantage of sharing the territory and the available food within the territory with another female. If the quality differences among territories are large, this will lead to high variation in male mating success (polygyny threshold model; Verner and Willson, 1966; Orians, 1969), and female mating preferences can be explained by direct benefits (e.g., better offspring survival resulting from higher food resources within the territory). In contrast, in polygynous species, where males provide females with little material resources, such direct benefits of female mate choice are less likely to fully explain female mating preferences. Instead, indirect mechanisms as proposed by ‘‘good genes’’ models of sexual selection may play a more important role. We studied sexual selection in the red bishop (Euplectes orix), a sexually dimorphic and polygynous weaverbird from Southern Africa. Males in breeding plumage establish and defend territories in reedbeds or bullrush stands around water, where they construct several nests during a breeding season to which they try to attract females. Most territories contain one or more empty nests at any time during the breeding

Patterns of Nest Predation Contribute to Polygyny in the Great Reed Warbler

According to the polygyny threshold model, females are compensated for the cost of sharing a territory with other females by breeding in territories of higher quality than those of monogamously mated females. In the polygynous Great Reed Warbler (Acrocephalus arundinaceus), the variation in territory quality may be associated with nest site characteristics or food supply. In this study, we examined the probability of nest predation in Great Reed Warblers in relation to an indirect measure of territory quality (territory attractiveness rank as indexed by settlement order) and to several variables of nest site structure. Artificial nests with attached soft plasticine model eggs were placed in Great Reed Warbler territories of different attractiveness. Marks in the soft model eggs were compared to marks from possible predator species. Both inter- and intraspecific predators were found to prey upon eggs in artificial nests. Marsh Harriers (Circus aeruginosus) predominantly preyed upon artificial nests located in territories of low attractiveness, in low-density reed beds, and early in the season. Bitterns (Botaurus stellaris) and Water Rails (Rallus aquaticus) showed a preference for artificial nests located inside the reeds. When all types of predators of artificial nests were pooled, the probability of nest predation was lower in more attractive territories. This pattern was corroborated in an analysis of predation on real nests. In general, polygyny occurred in territories with a low rate of nest predation, whereas unmated males held territories where the risk of predation was high. The low rate of nest predation in attractive territories on average compensated secondary females for the cost associated with sharing a male (i.e., reduced male assistance when feeding nestlings). The polygyny threshold model has been around for several decades, however, only few studies have identified the factors that may compensate secondary females for the cost of sharing a mate. In the present study we were able to show that the patterns of nest predation can be an important territory quality parameter that compensate secondary females, and, hence, contribute to the occurrence of avian polygyny.

PATTERNS OF NEST PREDATION CONTRIBUTE TO POLYGYNY IN THE GREAT REED WARBLER

According to the polygyny threshold model, females are compensated for the cost of sharing a territory with other females by breeding in territories of higher quality than those of monogamously mated females. In the polygynous Great Reed Warbler (Acrocephalus arundinaceus), the variation in territory quality may be associated with nest site characteristics or food supply. In this study, we examined the probability of nest predation in Great Reed Warblers in relation to an indirect measure of territory quality (territory attractiveness rank as indexed by settlement order) and to several variables of nest site structure. Artificial nests with attached soft plasticine model eggs were placed in Great Reed Warbler territories of different attractiveness. Marks in the soft model eggs were compared to marks from possible predator species. Both inter- and intraspecific predators were found to prey upon eggs in artificial nests. Marsh Harriers (Circus aeruginosus) predominantly preyed upon artificial nests located in territories of low attractiveness, in low-density reed beds, and early in the season. Bitterns (Botaurus stellaris) and Water Rails (Rallus aquaticus) showed a preference for artificial nests located inside the reeds. When all types of predators of artificial nests were pooled, the probability of nest predation was lower in more attractive territories. This pattern was corroborated in an analysis of predation on real nests. In general, polygyny occurred in territories with a low rate of nest predation, whereas unmated males held territories where the risk of predation was high. The low rate of nest predation in attractive territories on average compensated secondary females for the cost associated with sharing a male (i.e., reduced male assistance when feeding nestlings). The polygyny threshold model has been around for several decades, however, only few studies have identified the factors that may compensate secondary females for the cost of sharing a mate. In the present study we were able to show that the patterns of nest predation can be an important territory quality parameter that compensate secondary females, and, hence, contribute to the occurrence of avian polygyny.

The Cost of Polygyny: Definitions and Applications

The polygyny threshold model was introduced in the 1960s in order to explain why females in some species choose to mate vith already mated males. Since then, a number of complementary or alternative hypotheses have been suggested. By using a hierarchical testing approach Searcy and Yasukawa (1989) organised the models that proposed to explain the maintenance of territorial polygyny. Here, I point out that there is no true way to organise the models, and that the organisation itself may influence which explanation the observations mill support. In particular, I discuss Searcy and Yasukawa's distinction between cost and no-cost models. As an alternative to the hierarchical approach I suggest careful comparisons of those variables that inflict costs of sharing with those that provide compensation. While the hierarchical approach may put very similar systems in different model families, a cost and benefit evaluation might instead show that the difference is quantitative rather than qualitative. Also, an identical cost of polygyny can arise from several different combinations of costs of sharing and benefits of sharing. Hence, the hierarchical approach may put systems with different mechanisms behind polygyny into the same model family. (Less)

Mating systems and territory in Lapwings Vanellus vanellus

In a population of Lapwings Vanellus vanellus studied for 5 years near Bergen, southwester Norway, 23–41% of the males were polygynous, having two and sometimes three mates, whereas 50–77% were monogamous and 0–21% remained unmated. Bachelors held territories in or immediately adjacent to the study area each of the years. As predicted from the Polygyny Threshold model, primary females generally laid eggs earlier than monogamous females, but the overlap in laying dates was substantial between these two classes. The number of females and start of breeding on the territories were analysed in relation to nine variables pertaining to safety from predation and to food. Of these, territory size correlated most consistently with number of females (positively) and the laying of first egg (negatively). Numbers of females observed were not significantly different from numbers expected from territory size in 4 out of 5 years; however, earlier start of breeding on large territories indicated that females did not settle according to the Neutral Mate Choice model. The Female‐biased Sex Ratio hypothesis was refuted, unmated territorial males being available throughout the breeding seasons.

Female Mating Status and Reproductive Success in the Great Reed Warbler: Is there a Potential Cost of Polygyny that Requires Compensation

1. Using data from a 9-year study of the great reed warbler Acrocephalus arundinaceus L. carried out in Sweden, I tested the main assumption of the polygyny threshold model (that there is a cost of polygyny) and the main prediction (that this cost is compensated for). The main question was whether a female that chose a mated male (secondary females) experienced the same fitness as a female that at the same time chose an unmated male (primary females). The study includes a total of 192 breeding seasons of 104 individual females of which 40% settled with already mated males. 2. Eight potential correlates of fitness were examined; namely, clutch size, hatching success, hedging success, survival until breeding age, nest survival, mass of fledglings, mass of feeding females and annual survival rate of breeding females. I started each analysis by controlling for four potential confounding variables (year, study site, female age and breeding date). Then I compared the average fitness measure of primary and secondary females. 3. The only detected potential component cost of polygyny was a higher mortality among nestlings if a non-primary position within the harem was retained until hatching. This was supported by a natural experiment. Females that initially chose mated males but which achieved a primary position after failure of the originally primary female's nest enjoyed an increased fledging success, probably because of more male assistance. 4. The number of recruits, controlling for settling dates, for the two categories of females were almost identical (primary females 0 . 53 recruits per year, secondary females 0 . 51 recruits per year). Since secondary and primary females showed similar rates of survival until next breeding season the fitness of the two strategies appears to be equal. These findings strongly suggest that polygyny in the great reed warbler is best explained by the compensation models such as the polygyny threshold model. (Less)

Polygyny in the red-winged blackbird: do females prefer monogamy or polygamy?

Five hypotheses have been proposed to explain polygyny in the red-winged blackbird (Agelaius phoeniceus). We categorized the hypotheses into three groups based on female preference for unmated versus monogamously mated males: (1) the “polygyny threshold” model, “sexy son” hypothesis and the “asynchronous settlement” model, which assume that females prefer unmated males to mated males on breeding situations of homogeneous quality; (2) the “neutral mate choice” hypothesis, which assumes that females have no preference; and (3) the “cooperative female choice” model, which assumes that females prefer monogamously mated males to unmated males. We tested the direction of female preference in two field experiments. In both experiments, newly settling females were given a choice of two adjacent territories, one defended by an unmated male and the other by a monogamously mated male. Male mating status was randomized with respect to the variation in territory quality and male quality. Early in the breeding season, significantly more females settled with the unmated males than with the mated males. Although more females settled with the unmated males than with the mated males late in the breeding season, the difference was no longer significant. Female settlement late in the season appeared to be related to the tenure of the resident females: the new females avoided territories where the resident females were in early stages of their nesting, but settled on territories where the resident females were in late stages. The pattern of female settlement shows that females prefer unmated males to mated males. The preference is consistent with the polygyny threshold model, sexy son hypothesis and the asynchronous settlement model, and inconsistent with the neutral mate choice hypothesis and the cooperative female choice model. For this reason, the latter two hypotheses are unlikely to explain the occurrence of polygyny in our population of red-winged blackbirds.