Low frequency of extrapair paternity in the polygynous great reed warbler, Acrocephalus arundinaceus

We carried out DNA fingerprinting on 553 young (130 broods)great reed warblers (Acrocephalus arundnaceus) in 1987–1991.In the study population, where 40% of the males become polygynous,there was a low frequency of extrapair fertilizations (EPF).When data from all five years were pooled, 3.1% of the youngwere sired by extrapair males (EPF-males) and 5.4% of the broodscontained extrapair young. We found no cases of extrapair maternity;young with 6–17 mismatched DNA bands (n= 17) had highband sharing with their putative mothers (range = 0.52–0.72)but low band sharing with their putative fathers (range = 0.24–0.40).In broods exposed to EPF, on average 53% of the young were siredby EPF-males. We found the genetic father to each of the illegitimateyoung. In all cases the same EPF-male sired all extrapair youngin a brood. Broods containing EPF-young tended to be initiatedlate during the breeding season. Breeding attempts were ratherevenly distributed over two months, thus this breeding asynchronywould have facilitated EPFs. There was no difference in EPFfrequency between broods where the pair males had left theirfemales unguarded during parts of their fertile periods andbroods where males guarded throughout the fertile periods. Nestswith extrapair young had significantly shorter mean distanceto the closest male neighbor and more male neighbors within100 m than nests without extrapair young. We found no indicationthat females engaged in EPF to get parental care from the EPF-males,or because they were forced to copulate with extrapair males.The low frequency of EPF suggested that females did not seekgenetic diversity to their brood. We cannot rule out the possibilitythat females engaged in EPF to insure fertility. However, datasupporting this hypothesis were weak. Instead, our data supportthe conclusion that females engaged in EPF to increase the geneticquality of their offspring, and that females may have used malesong repertoire size as a cue when choosing EPF partners.     

Strategic paternity assurance in the sex-role reversed Eurasian dotterel (Charadrius morinellus): behavioral and genetic evidence

Sex role reversal in birds is usually associated with paternalcare of both eggs and chicks. This pattern of care typicallyleads to the potential rate of reproduction of males being lowerthan that of females. Hence, operational sex-ratio theory predictsthat each male should be under strong selection to avoid beingcuckolded. A male should, therefore, guard his female partner(s)from extrapair copulation attempts by other males. Furthermore,the sexual conflict theory of copulation behavior predicts thatin species with extensive paternal care the male should controlthe temporal pattern of copulations—copulations shouldoccur both frequently and throughout the prelaying period. Wetested these predictions in the Eurasian dotterel (Charadriusmorinsllus), in which the male usually provides all the parentalcare. In accordance with the first prediction, male dotterelsdid "guard" their pair-female prior to egg-laying. Contraryto the second prediction, however, copulations were not frequentand did not occur throughout the pre-laying phase-despite frequentsolicitation by the female, copulations only occurred immediatelyprior to egg-laying. Nevertheless, male-initiated courtshipwas both coincident with the pattern of copulations and morelikely than female-initiated courtship to result in copulation.Our results do, therefore, appear to agree with the centralprediction of the sexual conflict theory that males should controlthe pattern of copulations. We suggest that male dotterels willcopulate only after several days of being paired because theyface a duel risk of cuckoldry from both extrapair copulationand rapid mate switching. We tested the realized incidence ofcuckoldry using DNA fingerprinting. Only 4.6% (2/44) of chickswere not the genetic offspring of the caring male correspondingto 9.1% (2/22) broods affected. The rate of extrapair paternityin the dotterel is, therefore, relatively low compared to thatin many other avian species. We conclude that male dotterelssuccessfully protect their paternity of the brood for whichthey care through a combined strategy of mate guarding and strategictiming of copulations.     

Male shrikes punish unfaithful females

The costs to females of participating in extrapair copulationsis an interesting but hitherto neglected topic in behavioralecology. An obvious potential cost to females is male physicalsanctions. However, although retaliation and punishment by malepartners has been proposed as a basic cost for female extrapairbehavior in theory, it has not been experimentally demonstrated.We studied the breeding biology of the lesser gray shrike (Laniusminor) and combined field observations and a field experimentto show that (1) there is a high intrusion rate during the female'sfertile period, and extrapair copulations occur in this population;(2) by detaining females during the fertile phase, males wereinduced to retaliate physically against their partners, therebyincreasing costs related to female extrapair behavior; and (3)there were no obvious costs to males of punishing their mates.DNA fingerprinting reveals that extrapair paternity is rareor absent in this population. Although we cannot conclude thatmonogamy at the genetic level is the result of male retaliation,we do show that male physical sanction is a cost that deceptivefemales have to assume. Males' strategies based on coercionshould be considered when explaining variation in extrapairpaternity across species.     

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

We tested the relationship between synchrony of breeding andthe frequency of extrapair fertilizations (EPFs) in two populationsof red-winged blackbirds known to differ in female extrapairbehavior. We found no association between the number of simultaneouslyfertilizable females (temporal neighbors) and EPF rate in eitherpopulation, although a significant difference between populationsin the direction of this relationship (positive where femalesinitiated extrapair copulations and negative where males initiatedthem) suggested a modest difference in the influence of synchrony.Males losing offspring to EPFs tended to have more fertilizablefemales at that time than the actual sires in some analysesbut not in others. We also tested several assumptions underlyingtwo competing hypotheses for the effects of synchrony. We foundno evidence that females pursued extrapair copulations moreoften when other females were synchronous. Rather, females weremore likely to gain EFFs with exirapatr males whose social mateswere not yet building their nests. Synchrony also did not consistentlyaffect male pursuit of exirapair copulations or achievementof EPFs. These results suggest that timing of breeding has someeffects on extrapair activity, but that those effects are bothrelatively weak and influenced by other factors that vary betweenyears or populations.     

Extrapair paternity in hooded warblers

We examined the role of extrapair fertilizations (EPFs) in themating system of the hooded warbler (Wilsonia citrina), a monogamoussongbird. DNA fingerprinting revealed that 8 of 17 (47%) femaleshad extrapair young in their first or second brood, and 23 of78 (29%) nestlings were the result of EPFs. Extrapair youngwere signifkandy more likely to occur in first broods than insecond broods. The proportion of EPFs within a brood was stronglybirnodal among broods: nests had 50% or more extrapair youngor none. In seven of eight broods where EPFs occurred, an adjacentmale neighbor was identified as the actual father. Male-likecoloration in females did not reduce the likelihood of havingextrapair young. Females with extrapair young did not receiveless parental care from their mates. All males who obtainedEPFs were mated to fertile females or were feeding offspringat the time they most likely mated with the extrapair female.Our results are consistent with the female control hypothesis,which predicts that females benefit from extrapair copulations(EPCs) and have some control over which males, if any, obtainEPCs. However, we could not reject the alternative hypothesisthat some male neighbors are particularly dominant and aggressiveduring EPC attempts, so females accept these EPCs to minimizecosts.     

Fertility assurance through extrapair fertilization,and male parental effort

Extrapair paternity (EPP) has been observed in many formally monogamous species. Male pursuit of extrapair fertilizations (EPF) is explained by the advantages of having offspring that receive essential paternal care from other males. Because females are capable of exercising a degree of control over the post-copulatory sperm competition, EPP’s persistence indicates that females benefit from EPF. Thus, EPP involves cooperation between mated females and extrapair males. On the other hand, mated males exhibit a spectrum of anti-cuckolding strategies. Hence, extrapair attributes of diverse species and populations reported in the literature are particular solutions of evolutionary games involving gender-specific cuckolding/anti-cuckolding strategies. Here we use game theoretical methods to study the effect of male paternal effort conserving strategies in situations where females seek EPF for reasons of genetic compatibility and/or in pursuit of genetic diversity for their offspring. Our results indicate that in these circumstances pursuit of EPF is the only evolutionary stable female strategy. Males, on the other hand, have two, mutually exclusive, evolutionary stable strategies: males that restrict parental care regardless of their mate’s fidelity, and males that never restrict parental care. That is, when females seek EPF for reasons of fertility assurance and/or genetic diversity, the conditional male strategy—therein the male’s parental efforts are based on his certainty of paternity—loses in competition with the unconditional strategies.     

Patterns of extrapair mating in relation to male dominance status and female nest placement in black-capped chickadees

In sexually promiscuous animals, females may benefit by nestingclose to the edge of their partner's territory to facilitateextrapair copulations. In the present study, we describe theextrapair mating system of black-capped chickadees, Poecileatricapillus, and test whether nest locations are influencedby conspecific attraction to extrapair partners. We conducteda spatial analysis of female mating strategies by using microsatellitepaternity analysis in conjunction with geographic informationsystem (GIS) analysis of nest and territory locations. Extrapairoffspring comprised 52 of 351 offspring (14.8%) and were presentin 19 of 57 broods (33.3%). Females paired to males with lowdominance status in the previous winter's flock hierarchy weremore likely to engage in a mixed reproductive strategy thanwere females paired to males with high dominance status. Femaleshad extrapair copulations and extrapair fertilizations withhigh-ranking males more often than with low-ranking males. Notall extrapair copulations resulted in extrapair fertilizations.Females constructed their nests within 16.8 ± 1.0 m ofthe edge of their partner's territory, significantly closerto the edge of their nearest neighbor's territory than to thecenter of their own partner's territory. Extrapair males usuallyshared territory boundaries with cuckolded males. Females pairedto low-ranking males constructed nests near the territory edgesof neighboring high-ranking males. However, females did nothave extrapair copulations with the neighbor nearest to theirnest or even with the high-ranking neighbor nearest to theirnest. We conclude that conspecific attraction to neighbors mayinfluence nesting location in black-capped chickadees; however,it does not operate by facilitating extrapair copulations.     

Female mate preferences and subsequent resistance to copulation in the mallard

In the majority of socially monogamous bird species, femalessolicit or accept copulations from males other than their partner.Females may gain direct benefits from extrapair males, suchas greater access to resources, or indirect genetic benefitsthat will influence the future success of their offspring. However,one group of birds appears to be the exception to this generalrule; in the wildfowl (Anseriformes), all extrapair copulationsappear to be resisted by females. It has been suggested thatresistance behavior may be a strategy to allow females a greaterchoice of mates, either at the precopulatory level (to promotechoice of copulation partner) and/or the postcopulatory level(to promote multiple mating to increase their choice of sperm).This paper examines the function of female resistance behaviorin one of the dabbling ducks, the mallard (Anas platyrhynchos).Observations on a marked population of wild mallard and experimentswith captive birds found that although females showed a strongpreference for particular males that are the first to molt intotheir breeding plumage, male attractiveness did not influencefemale responses to pair or extrapair copulation attempts. Femaleresistance decreased the likelihood that copulation attemptswould end in successful insemination. The findings did not supportthe hypothesis that females resist copulations to promote femalechoice and the reasons why waterfowl may benefit from avoidingall extrapair copulations are discussed.     

The use of extrapair copulations for mate appraisal by razorbills, Alca torda

Razorbills (Alca Undo) engaged in extrapair copulations (EPCs)during two phases of their breeding cycle when fertilizationof eggs was not possible, suggesting that EPCs provide nongeneticbenefits. Females actively pursued extrapair mountings afterthey completed egg laying, the first monogamous species reportedto do so. Mountings were performed in mating arenas outsideof the breeding colony, where attendance by postlaying femalesindicated that they sought encounters with extrapair males whiletheir mates were incubating. Postlaying females always successfullyresisted insemination yet positioned themselves to receive mountings.These findings support the hypothesis that resistance to inseminationis a ploy used by females to appraise males. At the end of thebreeding cycle, when males escorted the fledgling to sea, femalesremained at the colony where they consorted and sometimes copulatedwith other males. Nonfertilizable extrapair copulations mayserve two social functions for razorbills: female appraisalof males for future fertilizable EPCs and the appraisal andacquisition of new mates by both sexes.     

Indiscriminate polyandry and male parental effort

Extrapair paternity involves cooperation between mated females and extrapair males. On the other hand, mated males exhibit a spectrum of anti-cuckolding strategies. Hence, extrapair attributes of diverse species and populations reported in the literature are particular solutions of evolutionary games involving gender-specific cuckolding/anti-cuckolding strategies. Here we use game theoretical methods to study the effect of male paternal effort conserving strategies in situations where females seek extrapair fertilizations (EPF) for reasons of genetic compatibility and/or in pursuit of genetic diversity for their offspring. In such cases, females cannot make a pre-copulatory selection of the optimal genetic partners, and therefore combine promiscuous copulation with the use of in copula and/or post-copulatory selection mechanisms to optimize the genetic endowment of their offspring—indiscriminate polyandry. Our results indicate that, when indiscriminate polygamy is constrained by the availability of extrapair male partners, there are three possible (parameter regime wise) evolutionary stable strategy solutions. (1) All females seek EPF, while all males restrict parental care. (2) All females seek EPF, while all males are unconditionally parental. (3) Females use a combination strategy where pursuit of EPF is mixed—on either a population, or an individual level—with genetic monogamy, while all males use a conditional paternal care strategy, which involves adjusting their parental efforts according to their certainty of paternity.     

Reproductive promiscuity in the splendid fairy-wren: effects of group size and auxiliary reproduction

Extrapair fertilizations complicate our understanding of cooperativebreeding in a number of ways. For example, auxiliaries may reducethe costs of seeking extrapair fertilizations for breeding malesor females, and auxiliary males may themselves seek copulationswith the breeding female in their own group. We employed microsatellitemarkers to examine patterns of parentage in the cooperativelybreeding splendid fairy-wren (Malurus splendens melanotus).Our study population exhibited a relatively high level of extrapairpaternity (42% of 386 offspring) with considerable annual variation(range = 24–52%). Across years the proportion of offspringsired by extrapair males was significantly correlated with theaverage number of auxiliaries per group. Furthermore, the proportionof extrapair young within a brood was related to group composition;groups with multiple auxiliaries were twice as likely as groupswith zero or one auxiliary to contain extrapair young. Mostoffspring were sired by dominant breeding males, but auxiliarymales sired approximately 25% of all extrapair young (10% ofall offspring), and about half of these were cases in whichthe auxiliary male sired offspring in his own group. Within-groupsirings by auxiliary males were most common after replacementof the breeding female, and they also appeared to be more likelywhen the auxiliary was not related to the breeding male. Thus,the presence of auxiliary males increased the likelihood thatfemales would produce extrapair young, and although incest avoidancemechanisms usually prevent within-group copulations by auxiliarymales, a conflict of interest among group males arises whena new female joins the group.     

Covert extraterritorial behavior of female hooded warblers

Extraterritorial forays are one specialized tactic that femalesuse to obtain extrapair copulations (EPCs). By radio-trackingfemale hooded warblers (Wilionia citrina) during the breedingseason, we found that females made covert forays onto neighboringterritories. Almost 80% of females made extraterritorial foraysduring their fertile stages, but none visited territories duringthe nonfertile (incubation) stage, which is consistent withthe extraterritorial foray hypothesis. Females that left theirterritories visited from one to four neighboring territories,and they intruded at a rate of 0.6 territories/h. The propensityof females to foray from their territory did not correlate withbody size or age of the social mate. One female that left herterritory obtained an extrapair fertilization. Females may useforays to assess male quality as well as participate in extrapaircopulations. An alternative hypothesis for extra-territorialforays is that females may foray onto neighboring territoriesto obtain food during the energetically stressful period ofegg production. The extraterritorial foray hypothesis and foraginghypothesis may not be mutually exclusive. Females may obtainbenefits of foraging while at the same time assessing malesfor EPCs. Extraterritorial behavior of female birds may be morecommon than previously believed and in many cases may only beruled out through radio-tracking     

The effectiveness of mate guarding by male black-throated blue warblers

In many socially monogamous birds, males maintain close proximityto their mates during the fertile period. This is often consideredan effort on the male's part to prevent other males from copulatingwith his mate, but other functions have been suggested andthe effectiveness of males in preventing extrapair fertilizationshas come into question. Moreover, it is unclear whether mateguarding conflicts with other male activities, particularlythe pursuit of extrapair fertilizations. We examined mate guardingby male black-throated blue warblers (Dendroica caerulescens).Behavioral observations showed that males that guarded theirmates more closely were less likely to have extrapair youngin their nests. Moreover, the experimental detention of a malefor 1 h during the fertility risk period increased the probabilitythat a brood would contain extrapair young. Thus, male mate guarding was effective in reducing the risk of extrapair fertilization.Males with many opportunities for extrapair copulations appearedto guard their mates less and consequently had more extrapairyoung in their broods than males with few such opportunities.This suggests that mate guarding may conflict with the pursuitof extrapair fertilizations.     

Male share of provisioning is not influenced by actual or apparent loss of paternity in western bluebirds

Approximately 45% of western bluebird (Sialia mexicana) femaleshave some chicks in the nest that are not sired by their socialmates. Extrapair fertilizations account for 42% of offspringin these nests and 19% of nestlings overall. I tested the hypothesisthat males reduce nestling provisioning when their certaintyof paternity or share of paternity is reduced. Capture and detentionof socially monogamous males for 1 h or 24 h during the layingperiod reduced males' copulatory access and their ability tomate guard, increasing the frequency with which extrapair malesintruded and attempted to copulate with resident females. Malesdetained during laying did not reduce their share of feedingtrips compared to control males detained during incubation,compared to unmanipulated males, or compared to males that werecaptured but not detained. Males detained on territory for 1h during the laying period did not reduce their share of feedingtrips when they observed male intrusion, nor when they observedtheir mates accepting extrapair copulations. Males that witnessedtheir mates accepting extrapair copulations did not reduce theirshare of risk in provisioning. Genetic fingerprinting at nonexperimentalnests indicated that males also failed to reduce their feedingcontributions when their estimated share of paternity was reduced,even when a helper male was present to reduce the impact onnestlings. These results suggest that male western bluebirdsdo not make significant adjustments in their share of provisioningwhen they have evidence of partial paternity loss. Togetherwith prior results, this study suggests that western bluebirdmales use an all-or-none rule, contributing approximately halfof the parental provisioning at nests, as long they have somecopulatory access to the female during egg laying.     

Do female pied flycatchers seek extrapair copulations with familiar males? A test of the incomplete knowledge hypothesis

In birds with biparental care, great variation exists in thefrequency of extrapair paternity. Several hypotheses have beenproposed to account for this variation. We tested the incompleteknowledge hypothesis, which states that females are constrainedin their knowledge of male quality and that this influencestheir willingness to engage in extrapair copulations (EPC).By selective removal and release of female pied flycatchersFicedula hypoleuca, we created a situation where females finallysettled with a social mate close to the site where a formersocial mate was breeding. According to the incomplete knowledgehypothesis, this would lower the threshold for females to seekextrapair copulations in cases where their former social matewas of higher quality than the one finally chosen. The hypothesiswas not supported because manipulation of female settlementdid not increase frequency of extrapair paternity, not evenin cases where the female nested close to the previous mateand the current mate apparently was of lower quality becausehe was younger and more dull colored. However, we found that when extrapair paternity did occur, the cuckolder tended tobe a familiar male (i.e., the female's initial social mate).     

Breeding density, cuckoldry risk and copulation behaviour during the fertile period in raptors: a comparative analysis

Extrapair copulations (EPCs) and fertilizations (EPFs) occur in many socially monogamous bird species, including raptors. In this group of species, males invest heavily in reproduction through the feeding of female and young, so the cost of cuckoldry is particularly high. The feeding of females by males, characteristic of most species, conflicts with mate guarding, so raptors are expected to use frequent within-pair copulations (WPCs) for paternity assurance. In this study, I reviewed information on copulation behaviour of diurnal raptors, and used regression analyses and phylogenetic comparative analyses to investigate relations between density, EPC frequency and WPC behaviour. EPCs occurred in most raptor species studied (68%, N=19), mostly during the presumed fertile period of females. EPC frequency, measured as the percentage of females engaging in EPC but not the percentage of extrapair copulations, was positively related to breeding density. The rate and relative duration of WPCs were also positively related to breeding density, but only WPC rate was positively related to EPC frequency (percentage of females). Frequently copulating species had relatively larger testes, underlying higher sperm production capacities. The results support the hypotheses that sperm competition intensity increases with breeding density, and that male raptors rely on frequent copulations to ensure paternity. Despite the risk, EPFs rarely occurred in raptors, suggesting efficient paternity guards.     

Genetic monogamy in blue-headed vireos and a comparison with a sympatric vireo with extrapair paternity

Based on the breeding synchrony hypothesis, we predicted, intwo congeners that nest in simiilar habitat but differ in nestingsynchrony, that blue-headed vireos (Vireo solitarius) wouldhave fewer extrapair fertilizations (EPFs) thaii red-eyed vireos(V. olivaceus EPFs were rare in blue-headed vireos (1/37 nestlings),but common in red-eyed vireos (11/19 nestlings). We studiedthe behavior of blue-headed vireos to determine what factorscould promote genetic monogamy. We found no evidence that malesmate guarded to prevent extrapair copulations from occurring.Males did not follow fertile mates closely when mates left thenest (14–25% of female departures) and, during the egg-layingperiod, males were often alone on the nest (22.3 mm/h). Femaleblue-headed vireos, but not red-eyed vireos, obtain direct benefitsfrom social mates such as nest building and incubation (49.1%of the total), and they assess male quality long before becomingfertile. Female blue-headed vireos spent more time incubatingwhen their mates had low incubation effort. Furthermore, maleincubation effort was positively correlated with nest survivalduring incubation. We discuss the evolution of genetic monogamyand sex role convergence in blue-headed vireos in relation toasynchronous breeding.     

Decoy presentations as a means to manipulate the risk of extrapair copulation: an experimental study in a semicolonial raptor, the Montagu's harrier (Circus pygargus)

Mate guarding and frequent copulations are two alternative paternity assurance strategies found in birds. In species with intensecourtship feeding, like raptors, the "frequent copulation"strategy is expected because male food provisioning conflictswith mate guarding. We evaluated experimentally the paternityassurance behavior of a semicolonial raptor, the Montagu'sharrier Circus pygargus, using decoy presentations to simulateterritorial intrusions. Breeding pairs were exposed to maleand female decoys at different periods during the female's reproductive cycle. Agonistic responses to decoys were intra-sexual,and the timing and intensity of male attacks toward male decoyssupported responses related to the risk of extrapair copulation(EPC): Male aggression peaked during the presumed fertile periodand almost disappeared after clutch completion. During thefertile period, copulation rate was significantly higher, andcopulations lasted longer, during male decoy presentations than during controls. Males also spent more time close to the femaleduring male decoy presentations compared to controls, bothduring the early prelaying and fertile periods, but not duringincubation. In the fertile period, males also increased presencetime close to the female in the hour following the removal of the male decoy. Conversely, female decoy presentations hadno significant effect on copulatory behavior or male presencetime. These results showed that the risk of EPC can be experimentallymanipulated by the means of decoy presentations, simulatingmale territorial intrusions, and that male Montagu's harriersincrease their short-term copulation frequency and female surveillancewhen they perceive themselves at an increased EPC risk.     

Fertility assurance through extrapair fertilizations and male paternity defense

Extrapair paternity has been observed in many formally monogamous species. Male pursuit of extrapair fertilizations is explained by the advantages of having offspring that receive essential paternal care from other males. Since females are capable of exercising a degree of control over the post-copulatory sperm competition, extrapair paternity cannot persist unless it confers fitness benefits on cuckolding females. Thus, extrapair paternity involves cooperation between mated females and extrapair males. On the other hand, paired males frequently exhibit strategies that minimize their loss of paternity and/or conserve paternal investment if paternity is lost. Hence, extrapair attributes of diverse species and populations reported in the literature are particular solutions of evolutionary games involving gender-specific cuckolding/anti-cuckolding strategies. Here we use methods of evolutionary game theory to study the role of male paternity guarding strategies in situations where females seek extrapair fertilizations for reasons of genetic compatibility and/or in pursuit of genetic diversity for their offspring. Our results indicate that in these circumstances pursuit of extrapair fertilizations is the only evolutionary stable female strategy. Males, on the other hand, have two, mutually exclusive, evolutionary stable strategies: full time pursuit of extrapair fertilizations and a compromise strategy wherein they protect in-pair paternity during their mate's fertile periods and pursue extrapair paternity the rest of the time. The relative merits of these two strategies are determined by the efficiency of male in-pair paternity defense, breeding synchrony, fitness advantages of extrapair over in-pair offspring, and the intensity of competition for extrapair fertilizations from floater males.     

Polygyny and extrapair fertilizations in eastern red-winged blackbirds (Agelaius phoeniceus)

Parentage of nestling red-winged blackbirds (Agelaius phoeniceus)from an eastern population was determinedusing DNA fingerprintingtechniques. Of 235 nestlings surveyed, 58 had fingerprints excludingthemale, but none excluded the female tending the nest. Data onpairing status during the female's fertilizable period was availablefor 232 offspring; 55 (25%of 1988 nestlings, 23% of 1989 nestlings)of those were sired through extrapair copulations. Of these55 offspring, 33 could be assigned to nearby territory holders;16 of the remaining nestlings may have been sired by nearbymales that were not captured. During both years, 44% of territorialmales had more than one female nesting simultaneously on theirterritory. The number of extrapair fertilizations gained bymales increased significandy with harem size in 1 year. Paternity(die proportion of nesdings on the territory sired by die territoryholder) showed a positive but nonsignificant increase widi haremsize in bodi years. There was no apparent cost in paternityfor males guarding two or more fertilizable females at the sametime. The broods of females that were fertilizable at die sametime anodier female was setding on die same territory tendedto have a greater proportion of extrapair fertilizations (0.37)than did die broods of odier females within harems (0.15). Establishedfertilizable females were chased significantly more by die territoryowner and by extrapair males when a new female was setding.There were no associations between a male's paternity or successat gaining extrapair fertilizations and his age or color-bandcombination. Overall, extrapair fertilizations had litde effecton die relationship between fledgling success and harem sizeand appeared to have a minimal impact on die overall intensityof sexual selection on males.