Population Of Red-winged Blackbirds Research Articles

Experimental reduction of haemosporidian infection affects maternal reproductive investment, parental behaviour and offspring condition.

When hosts have a long coevolutionary history with their parasites, fitness costs of chronic infection have often been assumed to be negligible. Yet, experimental manipulation of infections sometimes reveals effects of parasites on their hosts, particularly during reproduction. Whether these effects translate into fitness costs remains unclear. Here, we present the results of an experimental study conducted in a free-ranging population of red-winged blackbirds (Agelaius phoeniceus) naturally experiencing a high prevalence of haemosporidian infections, with more than 95% of breeding adults infected with parasites from one or more haemosporidian genus. To assess effects of infection during reproduction, we manipulated adult red-winged blackbird females' parasite burden by administering an anti-haemosporidian medication before onset of egg-laying. Experimental reduction of infection resulted in significant benefits to mothers and their offspring. Medicated females laid heavier clutches, invested more in incubation and provisioning behaviour, and produced more fledglings than control females. Nestlings of medicated females had higher haematocrit, higher blood glucose, and lower reactive oxygen metabolites than nestlings of control females. Overall, our results provide evidence that, even in a species with high prevalence of infection, parasites can lead to decreased maternal investment and offspring quality, substantially reducing fitness.

Egg rejection behavior does not explain the lack of cowbird parasitism on an eastern North American population of Red-winged Blackbirds

BackgroundRed-winged Blackbirds (Agelaius phoeniceus), hereafter red-wings, are much less frequently parasitized by Brown-headed Cowbirds (Molothrus ater) in eastern North America than in central North America and had not been recorded as hosts in our study area in southeastern Pennsylvania. Although hosts of Old World cuckoos (Cuculidae) often show geographic variation in egg rejection behavior, cowbird hosts typically exhibit uniform responses of all acceptance or all rejection of cowbird eggs. Thus, geographic variation in cowbird parasitism frequencies might reflect a different behavioral response to parasitism by hosts where only some populations reject parasitism. In this study, we tested whether egg rejection behavior may explain the lack of parasitism observed in our eastern red-wing population, which may provide insight into low parasitism levels across eastern North America.MethodsWe parasitized red-wing nests with model cowbird eggs to determine their response to parasitism. Nests were tested across three nest stages and compared to control nests with no manipulations. Because rejection differed significantly by stage, we compared responses separately for each nest stage. We also monitored other songbird nests to identify parasitism frequencies on all potential hosts.ResultsRed-wings showed significantly more rejections during the building stage, but not for the laying and incubation stages. Rejections during nest building involved mostly egg burials, which likely represent a continuation of the nest building process rather than true rejection of the cowbird egg. Excluding these responses, red-wings rejected 15% of cowbird eggs, which is similar to rejection levels from other studies and populations. The overall parasitism frequency on 11 species surveyed in our study area was only 7.4%.ConclusionsEgg rejection behavior does not explain the lack of parasitism on red-wings in our eastern population. Alternatively, we suggest that cowbird preference for other hosts and the low abundance of cowbirds in the east might explain the lack of parasitism. Future research should also explore cowbird and host density and the makeup of the host community to explain the low levels of parasitism on red-wings across eastern North America because egg rejection alone is unlikely to explain this broad geographic trend.

2. Correlations between Urbanization and Reproductive Fitness in Agelauis phoeniceus (Red-Winged Blackbird)

Urbanization has complex and diverse effects on the environment. How do animals respond to such dramatic changes to their habitat? In this study, I investigated the relationship between the degree of habitat urbanization and reproductive success in Red-Winged Blackbirds. I located and sampled two populations of Red-Winged Blackbirds: one rural population located at the Queen’s University Biological Station in Elgin, Ontario, and one urban population located at Belle Park in Kingston, Ontario. I mapped nesting territories from both populations and evaluated them for parameters such as water, marshland, and vegetation cover, as well as distance to roads, trails, and buildings. I also took physical measurements from adult birds, eggs, and nestlings of each territory to evaluate reproductive fitness. With this data, I will compare the degree of habitat urbanization with reproductive success. I hope to see meaningful relationships that might reveal how breeding Red-Winged Blackbirds respond to urbanized landscapes, and what key components of nesting territory might influence their reproductive success. With our society currently experiencing the largest wave of urban growth in history, it is crucial to understand the effects of this growth on other species and how we can most effectively preserve ecological value in urban landscapes.

Acceptance of Simulated Cowbird Parasitism in a Northern Population of Red-Winged Blackbirds

Red-winged Blackbirds (Agelaius phoeniceus) may be more likely to desert nests parasitized by Brown-headed Cowbirds (Molothrus ater) when they interact with the parasite, because desertion frequencies are higher at naturally parasitized nests than at artificially parasitized nests in some brood-parasitic hosts. We experimentally parasitized blackbird nests in southern Manitoba with cowbird model eggs and presented taxidermic mounts of adult female cowbirds near their nests. We observed no desertion, which is possibly attributable to constraints of a shorter breeding season at this latitude. We also compared rejection frequencies for blackbirds when a mounted female cowbird was placed near the nest to rejection frequencies recorded when (1) no model egg was added, (2) a model egg was added but no mounted cowbird was presented, and (3) a model egg was added plus presentation of a mount of a non-parasitic and non-threatening species, the Fox Sparrow (Passerella iliaca). Blackbirds struck the cowbird mount more often than the sparrow mount, but responses to the cowbird mount did not influence acceptance or rejection of the model egg. Blackbirds apparently engage in the least costly form of anti-parasite defense (aggression) and accept the costs of acceptance instead of those of renesting during a short breeding season.

Modeling Wetland Blackbird Populations as a Function of Waterfowl Abundance in the Prairie Pothole Region of the United States and Canada

Blackbirds share wetland habitat with many waterfowl species in Bird Conservation Region 11 (BCR 11), the prairie potholes. Because of similar habitat preferences, there may be associations between blackbird populations and populations of one or more species of waterfowl in BCR11. This study models populations of red-winged blackbirds and yellow-headed blackbirds as a function of multiple waterfowl species using data from the North American Breeding Bird Survey within BCR11. For each blackbird species, we created a global model with blackbird abundance modeled as a function of 11 waterfowl species; nuisance effects (year, route, and observer) also were included in the model. Hierarchical Poisson regression models were fit using Markov chain Monte Carlo methods in WinBUGS 1.4.1. Waterfowl abundances were weakly associated with blackbird numbers, and no single waterfowl species showed a strong correlation with any blackbird species. These findings suggest waterfowl abundance from a single species is not lik...

Long-term decline in a red-winged blackbird population: ecological causes and sexual selection consequences

Habitat loss and large-scale climate phenomena are widely implicated as causing decline in animal populations. I examined how both factors contributed to a precipitous decline in an Ontario red-winged blackbird (Agelaius phoeniceus) population using 16 years of data collected between 1974 and 1995. The decline was manifested as an almost 50% reduction in mean harem size, which reduced the opportunity for sexual selection threefold. Regional hay production, which should affect recruitment into the study population, also declined substantially. Correlation between blackbirds and hay may be coincidental, however, because annual changes in harem size were not associated with annual changes in hay production. This study coincided with an unprecedented positive phase of the North Atlantic Oscillation (NAO). Changes in harem size were correlated with winter NAO index values, suggesting that winter mortality contributed to the population decline. Positive correlation between harem size change and male return rates also supported the winter mortality hypothesis. Continued declines will cause this blackbird population to change from socially polygynous to socially monogamous. Study of red-winged blackbird winter ecology is needed to identify the proximate causes of mortality, whereas breeding studies can explore the consequences of relaxed sexual selection.

Tests of spatial and temporal factors influencing extra‐pair paternity in red‐winged blackbirds

Extra-pair paternity (EPP) is a widespread and highly variable reproductive phenomenon in birds. We tested the effects of habitat, spatial factors, and timing of breeding on the occurrence of EPP in red-winged blackbirds (Agelaius phoeniceus). We used PCR-amplified microsatellites to assess the paternity of 1479 nestlings from 537 broods on 235 territories over four breeding seasons. Over 4 years, 40% of nestlings were extra-pair. At least 27% of actual sires were non-neighbours, suggesting that males or females interacted over longer distances than in other populations of red-winged blackbirds. The level of EPP was significantly clumped within broods and males but not within females across broods. EPP was negatively related to the area of a male's territory. The spatial proximity of a female's nest to the territory boundary had no effect on total EPP, but tended to increase the probability of an EPP by a nearby male. We found no influence on EPP of the type of habitat on the territory or the level of nesting activity nearby. The time in the season when a nest was started and the synchrony of breeding also had no significant effect on the level of EPP. The age of the male, the age of his neighbours, and the interaction between the two had no effect on total EPP. However, older males were less likely to have an offspring sired by a neighbour on their territory. Males with older neighbours were also less likely to have offspring sired by a neighbour, particularly if they were new territory owners. The high variability in who gained and lost paternity, and the limited impact of spatial and temporal factors influencing it, have some interesting implications for theories seeking to explain mating patterns.

Cooperative nest defence in red–winged blackbirds: reciprocal altruism, kinship or by–product mutualism?

Male red-winged blackbirds (Agelaius phoeniceus) often cooperate with their neighbours in defending nests against predators. Some studies have suggested that this is an example of by-product mutualism, whereas others have suggested the possibility of reciprocal altruism. No study has addressed the possibility of kin-selected cooperation in nest defence in this species. Reciprocal altruism, kin selection and by-product mutualism are not mutually exclusive alternatives, but few studies of territorial neighbours have tested for multiple mechanisms simultaneously. We test for these three possibilities in a population of red-winged blackbirds. We used simulated defections to test for reciprocal altruism. We used analysis of microsatellite loci to test for kin selection between adult male neighbours. We also used microsatellite loci to test for by-product mutualism resulting from nest defence of offspring sired on neighbouring territories. We found that male red-winged blackbirds cooperate in nest defence primarily as a form of reciprocal altruism. Experimental males reduced their level of nest defence relative to controls following simulated defection by a neighbour. In contrast to some earlier studies, we found no evidence for by-product mutualism: males did not defend nests where they had sired extra-pair offspring. We also found no evidence for kin selection: males were no more cooperative with more closely related neighbours. Considered alongside the results from other studies, our study suggests that mechanisms stabilizing cooperation in red-winged blackbirds may vary among populations.

Lethal Control of Red-Winged Blackbirds to Manage Damage to Sunflower: An Economic Evaluation

In the northern Great Plains of the United States, conflicts between red-winged blackbirds (Agelaius phoeniceus) and sunflower (Helianthus annuus) growers have intensified since the late 1960s due to the expanded commercial production of sunflowers. We studied the potential population effects of the removal of up to 2 million red-winged blackbirds annually under a 5-year program of baiting during spring with DRC-1339 (3-chuloro-4-methalalanine)-treated rice. We also examined whether lethal control, in combination with current levels of breeding-habitat management, would be cost-effective in decreasing depredation of sunflower crops during late summer. We evaluated the cost-benefit ratio for 4 culling scenarios involving (1) variable annual culls, not exceeding 2 million birds, with and without density compensation (i.e., a positive density-dependent response) on adult survival; and (2) culls of 2 million birds annually with and without density compensation. We constructed a red-winged blackbird population model represented as an age-based matrix and calibrated to stable growth. We assumed a total population of 27 million birds on 1 April (week I ), representing the red-winged blackbird breeding population staging in eastern South Dakota and migrating into North Dakota. Under each culling scenario, we reduced the stable red-winged blackbird population (equally for females and males) and projected the population through week 23 of the annual cycle (2 Sep). We then evaluated the associated costs of the management relative to potential sunflower crop losses, assuming $0.07 in damage per bird and 4% loss to other factors. Variable annual culls, likely the more biologically realistic model scenarios, yielded mean annual removals of 1,240,560 (SE = 12,328) birds with density compensation and 1,231,620 (SE = 28,811) birds without density compensation, with cost-benefit ratios of 1:2.3 and 1:3.6, respectively. Annual intrinsic rates for the model population over the 5-year period ranged from -1.4 to -4.8%. Considering potential variability in the effectiveness of the cull and the combination of direct and indirect costs, we contend that the realized benefits to sunflower growers by lethal control of red-winged blackbirds via spring baiting, in combination with current nonlethal management efforts, would likely be negligible.

Decline of the Red-Winged Blackbird Population in Ohio Correlated to Changes in Agriculture (1965-1996)

Based on North American Breeding Bird Survey (BBS) data since 1966, Ohio has traditionally hosted 1 of the highest breeding season densities of red-winged blackbirds (Agelaius phoeniceus) of any U.S. state or Canadian province. However, from 1966 through 1996, breeding populations of red-winged blackbirds in Ohio showed a marked decline (x% change/yr in birds per route = -3.9), with breeding population indices decreasing by over 53%. Because the red-winged blackbird successfully adapted to habitats created by agricultural expansion over the last century and became a recognized pest of crops such as corn (Zea mays), understanding the decline of this species in Ohio is important from both ecological and damage control perspectives. We examined 35 crop and climatic factors relative to their relationship with the observed breeding population trend for the red-winged blackbird in Ohio 1966 to 1996. Each year, we found that the area of non-alfalfa (Medicago sativa) hay harvested, the combined area of corn and soybeans (Glycine max) harvested, the area of non-alfalfa hay cut by 30 May of the index year (1966-1996), and the area of hay (all types) cut by 30 May of the year prior to the index best explained the variance in the breeding population trend of the red-winged blackbird in Ohio. Given our findings, we suggest that a long-term population trend for this abundant bird in Ohio is negatively associated with the efficiency and expansion of modern agriculture.

BREEDING SYNCHRONY AND NEST PREDATION IN RED-WINGED BLACKBIRDS

Three previous studies have disagreed on whether breeding synchrony increases nest success through reduced nest predation in Red-winged Blackbirds (Agelaius phoeniceus) in eastern North America. We used data from an 11-yr study of redwings in eastern Ontario, Canada, to duplicate the analyses of the one study (by D. F. Westneat) that did show that nesting synchrony reduced predation. We found no evidence that nest predation and nest success were affected by nesting synchrony. We did find significant effects of female age, clutch size, and clutch initiation date on nest success, but overall, our independent variables explained very little variation in nest success. We hypothesize that the difference between our results and Westneat's may be attributable to differences in the principal nest predators. Mammals appeared to be important nest predators in our study, as is typical for eastern populations of Red-winged Blackbirds. Mammals appeared to be less important nest predators in Westneat's study, probably because that study area was fenced and the grass between ponds was mowed. Red-winged Blackbirds mob avian predators, but have limited ability to defend nests from mammals. Group mobbing of avian predators could result in a positive relationship between nesting synchrony and nest success when birds are the most important nest predators. The greater importance of avian nest predators in western than in eastern populations of Red-winged Blackbirds could account for some differences in social polygyny between those populations. However, our results indicate that nesting decisions by female redwings in eastern populations should not be affected by the nesting activity of other females. Thus, social polygyny in this population does not arise through an advantage of group living.

Does Red-Cockaded Woodpecker Excavation of Resin Wells Increase Risk of Bark Beetle Infestation of Cavity Trees?

WEATHERHEAD, P. J., AND S. B. McRAE. 1990. Brood care in American Robins: Implications for mixed reproductive strategies by females. Animal Behaviour 39:1179-1188. WESTNEAT, D. F. 1987. Extrapair fertilizations in a predominantly monogamous bird: Genetic evidence. Animal Behaviour 35:877-886. WESTNEAT, D. F. 1990. Genetic parentage in the Indigo Bunting: A study using DNA fingerprinting. Behavioral Ecology and Sociobiology 27:6776. WESTNEAT, D. F., AND E. M. GRAY. 1998. Breeding synchrony and extrapair fertilizations in two populations of Red-winged Blackbirds. Behavioral Ecology 9:456-464. WESTNEAT, D. F., AND P. W. SHERMAN. 1997. Density and extra-pair fertilizations in birds: A comparative analysis. Behavioral Ecology and Sociobiology 41:205-215. WESTNEAT, D. F., P. W. SHERMAN, AND M. L. MORTON. 1990. The ecology and evolution of extra-pair copulations in birds. Current Ornithology 7: 331-369.

A test of the good-genes-as-heterozygosity hypothesis using red-winged blackbirds

Brown recently proposed that the ‘‘good genes’’ that females pursue when choosing mates may be individual heterozygosity because more heterozygous mates sire offspring with higher fitness. Further, because heterozygosity might enhance developmental stability, males with more heterozygosity are recognized by the reduced fluctuating asymmetry (FA) of their bilaterally paired traits. We used a point sample of 67 male red-winged blackbirds (Agelaius phoeniceus) to test two predictions of this hypothesis: (1) males with more heterozygosity have higher fitness, and (2) males with more heterozygosity have lower FA. We identified 7 polymorphic loci from an initial screening of 16 enzymes; 32 individuals were completely homozygous, and 35 individuals were heterozygous at at least 1 locus. Larger and older males realized higher mating success in this population, but neither size nor age was related to heterozygosity. Heterozygous males were not in better condition than homozygous males, nor were they less infected by hematozoa, lice, or mites. Among 1-year old males, epaulet length did not differ between homozygotes and heterozygotes, but among older males, heterozygotes did have longer epaulets. Homozygotes and heterozygotes did not differ in their mean FA scores for nine individual characters. Although the two groups of males did differ in composite FA, heterozygous males were less symmetrical. Interestingly, this difference was attributable to a single allele at the PGM-3 locus. Combined with previous results showing that FA was generally unrelated to male health, viability, parental care, social dominance, or mating success, the present results indicate that Brown’s hypothesis does not explain mate choice or male quality in this population of red-winged blackbirds.

The Effect of Aggregated Nesting on Red-Winged Blackbird Nest Success and Brood Parasitism by Brown-Headed Cowbirds

We examined temporal and spatial nesting aggregations in a prairie-nesting population of Red-winged Blackbirds (Agelaius phoeniceus). In particular, we were interested in the effects of aggregated nesting on blackbird nest success and brood parasitism by Brown-headed Cowbirds (Molothrus ater). Most cowbird parasitism occurred early in the breeding season during the peak in Red-winged Blackbird nesting. As a result, more parasitized nests were active simultaneously with other nests than were unparasitized nests, even when we controlled statistically for the effects of nest initiation date. The probability of parasitism across the entire study site did not decrease on days when many nests were initiated, suggesting no "swamping" effect of aggregated nesting on cowbirds. However, individual parasitized nests were less synchronous with their nearest neighbors and farther from the nearest simultaneously active nest than were unparasitized nests. These data are consistent with the hypothesis that brood parasitism by Brown-headed Cowbirds may select for aggregated nesting in Red-winged Blackbirds as a result of greater parasitism pressure on isolated nests. Nests that succeeded in producing at least one fledgling were initiated significantly earlier in the breeding season than were nests that failed. Nest aggregation had no effect on nest success; successful nests were no different from failed nests in the number of simultaneously active nests, their distance to nearest simultaneously active nests, the laying interval with their nearest neighbors, and the distance to their nearest neighbors. These data suggest that nest predation (the primary cause of nest failure), in contrast to nest parasitism, does not necessarily select for aggregated nesting, or that predation and nest aggregation are related only at high nest-densities.

Impact of Brood Parasitism by Brown-Headed Cowbirds on Red-Winged Blackbird Reproductive Success

We examined brood parasitism by Brown-headed Cowbirds (Molothrus ater) on a prairie-nesting population of Red-winged Blackbirds (Agelaius phoeniceus) from 1984 to 1997 and the extent to which parasitism affected host reproductive success. During this 14-year period, 2-32% of redwing nests were parasitized per year. Two measures of cowbird parasitism, the proportion of nests parasitized and the number of cowbird eggs laid per year, significantly increased over time. Increases in these estimates of the intensity of parasitism were independent of annual changes in nesting density of Red-winged Blackbirds. Clutch size was reduced in parasitized nests, most likely as a result of host egg removal by cowbirds, but hatching success was similar between parasitized and unparasitized nests. Red-winged Blackbirds abandoned more than one-third of all parasitized nests. We estimated the cost of abandonment in terms of increased failure rate and decreased fledgling production of re-nesting attempts. When we excluded abandoned nests from the analysis, there was no difference in the probability of success between parasitized and unparasitized nests. Nestlings from parasitized nests were similar in mass and tarsus length to nestlings from unparasitized nests, but parasitized nests produced fewer redwing fledglings per successful nest. The success of unparasitized nests was negatively related to the intensity of cowbird parasitism, suggesting that cowbirds are predators as well as brood parasites of redwing clutches.

Breeding synchrony and extrapair fertilizations in two populations of red-winged blackbirds

We tested the relationship between synchrony of breeding and the frequency of extrapair fertilizations (EPFs) in two populations of red-winged blackbirds known to differ in female extrapair behavior. We found no association between the number of simultaneously fertilizable females (temporal neighbors) and EPF rate in either population, although a significant difference between populations in the direction of this relationship (positive where females initiated extrapair copulations and negative where males initiated them) suggested a modest difference in the influence of synchrony. Males losing offspring to EPFs tended to have more fertilizable females at that time than the actual sires in some analyses but not in others. We also tested several assumptions underlying two competing hypotheses for the effects of synchrony. We found no evidence that females pursued extrapair copulations more often when other females were synchronous. Rather, females were more likely to gain EPFs with extrapair males whose social mates were not yet building their nests. Synchrony also did not consistently affect male pursuit of extrapair copulations or achievement of EPFs. These results suggest that timing of breeding has some effects on extrapair activity, but that those effects are both relatively weak and influenced by other factors that vary between years or populations.

Do female red-winged blackbirds benefit genetically from seeking extra-pair copulations?

Female pursuit of extra-pair copulations was investigated in a western population of red-winged blackbirds,Agelaius phoeniceusPotential genetic benefits to females were considered in this study; potential material gains were examined in Gray (1997Anim. Behav.53, 625–639). In this population, females initiate courtship and copulation outside their social bond, and between 18 and 43% of all breeding females engaged in extra-pair copulations each breeding season. The peak of extra-pair copulation occurred 2 days before the first egg was laid and 1 day closer to egg laying compared to the peak of within-pair copulation, suggesting that extra-pair copulations conferred a genetic advantage to females. Females that copulated with extra-pair males improved their reproductive success by hatching a significantly greater proportion of eggs and fledging a significantly greater proportion of young than females that did not copulate with extra-pair males. Improved hatching success was probably due to female avoidance of temporary male sperm depletion. Improved fledging success resulted from significantly lower depredation rates of clutches laid by females that copulated with multiple males. In addition, a lower proportion of nestlings tended to starve in broods containing extra-pair fertilized offspring. I found no evidence that participation in extra-pair copulations was costly to females. Male aggression towards females was rare, and males were never observed chasing or force-copulating with a mate immediately following an extra-pair copulation. These results show that the selection pressure on females to engage in extra-pair copulations is strong, and they support new theories maintaining that extra-pair copulations are an important aspect of female reproductive success.

Female control of offspring paternity in a western population of red-winged blackbirds ( Agelaius phoeniceus )

Extra-pair copulations, which occur when individuals that have formed social relationships to breed copulate outside their pairbond, now are recognized as an important component of reproductive success in many species. In situations where both males and females benefit from extra-pair copulations without incurring much risk, an inevitable conflict arises between pairbonded mates. In this study I investigated the conflict of interest between male and female reproductive strategies in a western population of red-winged blackbirds (Agelaius phoeniceus). Female red wings in this population initiate extra-pair copulations, which resulted in a 35% rate of extra-pair fertilization. Females initiated the majority (78%) of extra-pair copulations away from their nesting territory where pairbonded individuals typically copulate, and females that engaged in extra-pair copulations spent a significantly greater amount of time off the marsh during peak fertilization compared to females that did not. In addition, females that nested in areas with a large number of potential extra-pair partners produced significantly more extra-pair fertilized young compared to females that nested on marshes with few male neighbors. Males’ strategies to protect paternity were limited primarily to patrolling territory boundaries and to opportunistically preventing extra-pair copulations off the marsh when they were visible. In this population females appear to use behavioral means to control nestling paternity, which in turn directly affected their mate’s reproductive success, and males were restricted to using strategies that were largely ineffective at preventing the threat of extra-pair paternity.

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.

MEASURING THE EFFECTS OF PAIRING SUCCESS, EXTRA-PAIR COPULATIONS AND MATE QUALITY ON THE OPPORTUNITY FOR SEXUAL SELECTION.

Sexual selection can act through variation in the number of social mates obtained, variation in mate quality, or variation in success at obtaining extra-pair fertilizations. Because within-pair fertilizations (WPF) and extra-pair fertilizations (EPF) are alternate routes of reproduction, they are additive, rather than multiplicative, components of fitness. We present a method for partitioning total variance in reproductive success (a measure of the opportunity for selection) when fitness components are both additive and multiplicative and use it to partition the variance into components that correspond to each mechanism of sexual selection. Computer simulations show that extra-pair fertilizations can either increase or decrease total variance, depending on the covariance between within-pair and extra-pair success. Simulations also suggest that for socially monogamous species, extra-pair fertilizations have a greater effect than variation in mate quality or pairing status on the opportunity for selection. Application of our model to data gathered for a population of red-winged blackbirds (Agelaius phoeniceus) indicates that most of the variance in male reproductive success was attributable to within-pair sources of variance. Nevertheless, extra-pair copulations increased the opportunity for selection because males varied both in the proportion of their social young that they sired and in the number of extra-pair mates that they obtained. Furthermore, large and positive covariances existed between the number of extra-pair mates a male obtained and both social pairing success and within-pair paternity, indicating that, in this population, males preferred as social mates also were preferred as extra-pair mates.