Compensating for the costs of polygyny in hen harriers Circus cyaneus

In polygynous species, the adults are faced with a dilemma during chick rearing. Males must decide how to distribute food between their females and food allocation patterns are often highly unequal. In turn, the females that receive less food from males have to decide how much time to invest in additional hunting. If they spend more time hunting, then they leave their young exposed to weather and predators. However, if they stay at the nest, they increase the risk of their chicks starving. One way that birds may compensate for reduced provisioning is by increasing the size of prey caught. We tested this hypothesis by comparing prey deliveries to nests of hen harriers, Circus cyaneus, with females of different breeding status. As expected, male harriers delivered less food items to the nests of polygynous females, and especially their secondary, or β females. However, both sexes were able to compensate by delivering larger items and there was no difference in the overall mass of food delivered to nests. Moreover, females spent a similar amount of time at the nest, irrespective of status, and there were no overall differences in breeding success. Our results show that polygynous female harriers can compensate for the costs of polygyny, but we suggest that their ability to do so will vary according to the abundance of both large prey and predators.     

Higher rate of nest loss among primary than secondary females: infanticide in the great reed warbler?

An individually marked population of great reed warblers Acrocephalus arundinaceus was studied between 1984 and 1991 in South Central Sweden. The fates of 279 nests were analysed for the 25 days following clutch initiation. The average frequency of nest loss (43%) did not differ significantly between years or between four periods of the breeding season. After a breeding failure, 70% of the females laid a replacement clutch, most of them together with the same male. On polygnous territories, males assisted the female who first hatched young regardless of her initial mating status (Fig. 2). According to the acutal status of the females attending the nests, for each day nests were classified as being of either monogamous (M), primary (P) or secondary (S) status. From egg-laying to fledging the rate of nest loss decreased among nests of primary status whereas it increased among nests of monogamous and secondary status. During the egg-laying period, the rate of nest loss was 3 times higher among nests of primary than among nests of monogamous and secondary status (Fig. 3). Thus, the high loss level among nests of primary status during the laying period was closely associated with the presence of a female with a less advanced nest on the territory. All nests were situated in reed beds above deep water and most of them at a height at which possible inter-specific nest predators would have caused disturbance to the nest itself. The suspicion that secondary females committed sexually selected infanticide was supported by an experiment with dummy eggs that revealed bill peck markings identical to those obtained from great reed warblers (Fig. 4).     

Extra-pair paternity in willow ptarmigan broods: measuring costs of polygyny to males

Willow ptarmigan are one of only three monogamous grouse species in North America. However, in some populations between 5 and 20% of individuals pair polygynously. It has been suggested that monogamy may be maintained by the high cost of polygyny to males. We have used DNA fingerprinting to assess the actual reproductive success of both monogamous and polygynous adults. We determined whether or not the putative parents were the biological parents of the chicks from 38 broods. Of these clutches 30 were from monogamous matings, and 8 were from bigamous matings. Of the 207 chicks from monogamous matings 96% were within-pair offspring, compared to 67% of the 49 chicks from bigamous matings. All extra-pair offspring chicks resulted from extra-pair fertilizations (EPFs), and there were no instances of intraspecific nest parasitism. Mate guarding by monogamous males seems to be a highly effective method for maintaining genetic monogamy, as the only cases in which EPFs occurred were when the resident female left the territory for a few days or when a second female visited the territory. Our results support the notion that certainty of parentage may be one factor constraining willow ptarmigan males to be monogamous.     

Polygyny and male parental care in Savannah sparrows: effects on female fitness

Summary To determine the effects of male mating status on female fitness, we compared the reproductive success, survival, and future fecundity of female Savannah sparrows (Passerculus sandwichensis) mated to monogamous vs. polygynous males in a 5-year study on Kent Island, New Brunswick, Canada. The proportion of males with more than one mate varied from 15 to 43% between years and sites. Polygynous and monogamous males fledged young of equal size in every year of the study. Females who shared paternal care with other females laid as many eggs per clutch and clutches per season as monogamously mated females. In most years polygynously mated females showed no delay in laying a second clutch, and they suffered no reduction in fecundity the following year. Recruitment of a female's offspring into the breeding population was generally independent of her mating status. Fitness costs of being mated to a polygynous male were only apparent in one year of the study, during which females mated to polygynous males had higher over-winter mortality than those mated to monogamous males. That same year, young raised by polygynous males were only one-third as likely to survive to reproductive maturity (as inferred by returns) as those raised by monogamous males. A male's mating status had no effect on his own survivorship. A male's mating status did not necessarily reflect his contributions to raising nestlings, which may partially explain why monogamously and polygynously mated females had equal fitness. At 35 nests the proportion of food deliveries brought by individual males varied from 0 to 75%; on average, males brought fewer than 30% of all food deliveries. Yet parental care by polygynous males was no less than that of monogamous males, at least at the nests of their primary females. Secondary females tended to receive less male assistance during the nestling stage, but their reproductive success was indistinguishable from that of primary females. Females feeding young without male assistance made as many food deliveries/h as did pairs in which males brought at least 30% of all food deliveries. Unassisted females did not suffer diminished fledging success or produce smaller fledglings. The benefits of polygyny for male Savannah sparrows are clear: polygynous males recruit more surviving offspring into the breeding population than monogamous males. The fitness of females, on the other hand, appears to be unaffected by whether their mate was monogamous or polygynous except in occasional years. Polygyny may be maintained in this population by the constraints of a female-biased sex ratio, the inability of females to predict a male's paternal care based on his morphology or behavior, the poor correlation between a male's mating status and his assistance at the nest, and inconsistent natural selection against mating with a polygynous male. Correspondence to: N.T. Wheelwright     

Parental investment, adult sex ratios, and sexual selection in a socially monogamous seabird

Although most birds are monogamous, theory predicts that greater female parental investment and female-biased adult sex ratios will lower the polygyny threshold. This should result in polygynous mating, unless obligate biparental care or the spatial and temporal distribution of fertilizable females constrains a male’s ability to take advantage of a lowered polygyny threshold. Here we present data on the extent of male sexually dimorphic plumage, adult sex ratios and breeding season synchrony in three populations of a socially monogamous seabird, the brown booby Sula leucogaster. For one of these populations, San Pedro Mártir Island, we also present data on differences in male and female parental investment, mortality and probability of pairing. The extent of plumage dimorphism varied among populations. Sex ratios were female biased in all populations. On San Pedro Mártir Island, parental investment was female biased, females failed more often than males to find a mate, but there was no polygyny. We suggest that on San Pedro Mártir: (1) a period of obligate biparental care coupled with a relatively synchronous breeding season constrained the ability of males to take advantage of a high environmental polygamy potential and (2) the resulting socially monogamous mating system, in combination with the female-biased adult sex ratio, caused females to be limited by the availability of males despite their greater parental investment. Received: 18 November 1999 / Accepted: 24 January 2000     

Reproductive success of monogamous and polygynous pied flycatchers (Ficedula hypoleuca)

One of the most interesting aspects of the mating system of pied flycatchers is the regular occurrence of polygyny. Here we present data on the reproductive success of polygynously paired pied flycatcher males compared to monogamous males based on paternity analyses through DNA fingerprinting. Males paired with two females suffered a higher loss in reproductive output per female compared to monogamous males due to (1) a greater proportion of unhatched eggs in their broods, (2) greater nestling mortality and (3) a greater probability of being cuckolded. Nevertheless, the number of fledglings was significantly greater for polygynous males. Based on the number of nestlings that returned for subsequent breeding seasons, however, the reproductive success of monogamous and polygynous males did not differ significantly. These data raise the question as to why males attempt polygyny. Received: 16 August 1999 / Received in revised form: 15 March 2000 / Accepted: 18 March 2000     

Food and the deceptive acquisition of mates by polygynous male harriers

Summary Female northern harriers Circus cyaneus are polygynous, marsh-nesting raptors, whose mate choices are enigmatic. I determined the mate choice cues employed by females by correlating the order in which males were chosen with characters that 1) significantly influenced reproductive success; 2) were assessable prior to settlement; and 3) varied between breeding situations. Only nest sites and male provisioning performance met all these conditions: wet nest sites significantly (P<0.05) increased nest survival and high provisioning rates significantly (P<0.01) enhanced brood survival. The order in which females settled was strongly correlated with provisioning performance in both years (r _s -0.65, and r _s -0.84), but not with nest site quality. Females thus apperared to choose males principally on provisioning performance. Despite using the same cue, however, females choosing mated males reared only 0.28 young for every nestling raised by concurrently settling monogamous females (the lowest ratio recorded for any avian species). Extrapolation of male courtship provisioning patterns from clutch sizes and laying date indicated that females received similar proportions prior to and during egg-laying, but that males later preferentially fed females. Secondary females therefore chose mated males on the basis of a temporally changing and unreliable cue. The polyterritoriality, cooperative harems, skewed sex ratio, and sexy-son hypotheses were all inadequate in explaining polygyny in harriers. Female choice of mated males among harriers is best explained by the deceitful provisioning of food by males.     

Female reproductive success and nest predation in polyterritorial wood warblers (Phylloscopus sibilatrix)

Summary Polyterritorial polygyny in passerine birds, i.e., when already-paired males attract females in secondary territories, has been explained by male deception. Secondary females are assumed to make a poor choice, because they lack information about males' pairing status. To date, the deception hypothesis has focused mainly on the hole-nesting pied flycatcher. In this study of polyterritorial wood warblers, an open-nesting species, secondary females did not differ in number of fledged young compared with females of monogamous males, and they fledged even more young than primary females. Nest predation influenced the allocation of paternal care. Almost half of the secondary females had exclusive male assistance because nests of primary females were preyed upon.     

Nest site limitation and facultative polygyny in the ant Leptothorax longispinosus

Summary The ant L. longispinosus displays geographic variation in its pattern of facultative polygyny (Fig. 2). In nature, nest density and frequency of multiple queening are positively associated over three sites. A putative causal relation between availability of vacant nest sites and polygyny was examined in New York, where a plot was seeded with additional nest sites and monitored for 24 months. Both queen number and worker number per nest on the experimental plot were reduced relative to controls (Fig. 4, Fig. 5), indicating that scarcity of available nest sites influences the pattern of polygyny in this species. The observed demographic changes resulted from fractionation of existing colonies; adding nest sites induced polydomy. Although numbers of adult ants changed with addition of nest sites, the numbers of immatures were no different after 2 years (Table 1), suggesting that the population was undergoing growth to expand into the additional sites. These results are the first direct experimental evidence linking polygyny to an ecological parameter for any ant species.     

Female mate choice in the Pied Flycatcher Ficedula hypoleuca

Summary We have examined male and territorial factors which might influence female mate choice in the Pied Flycatcher (Ficedula hypoleuca). Mating success of males was largely determined by the order of arrival on the breeding grounds. Females actively selected those males that had been longest in the area. This is likely to be due to territorial cues, early males having the best territories. About 15% of the males become polygynous, and these are the males that arrived earliest.Male age was relatively unimportant for breeding success, but because old males tend to arrive earlier than yearlings, most polygynous males were old. On average, old males are somewhat darker than young males, but male colour also varies within males of the same age. Polygynous and monogamous males did not differ significantly in colour, and female breeding success was not correlated with the colour of their mates. We show theoretically that it does not pay for females to select young males to avoid polygyny unless polygyny frequency or the number of reliable monogamous males almost double. Hence our hypothesis of polygyny-by-deceit in the Pied Flycatcher remains realistic. Because of male polyterritoriality, females are unaware of the males' status when mating.     

Nestling provisioning in polygynous great reed warblers (Acrocephalus arundinaceus): do males bring larger prey to compensate for fewer nest visits?

Most studies of social polygyny in birds have examined male provisioning on the basis of the number of feeding visits. This may be misleading if males compensate for infrequent visits by bringing larger prey at each visit. We investigated nestling provisioning in the socially polygynous great reed warbler, Acrocephalus arundinaceus, in south Central Sweden in 1996–1997. We collected data on rate of feeding visits, prey size and the amount of biomass delivered by males and females. Males had lower rates of feeding visits and provided smaller prey to nestlings in secondary than in monogamous and primary nests. Secondary females had higher rates of feeding visits and brought larger prey than monogamous and primary females. These results confirm that secondary females face a potential cost of polygyny through a lower rate of male feeding, and that this cost was reinforced by the significantly lower male provisioning rate (biomass h^–1) at secondary nests. Secondary females compensated for the lack of male assistance by increasing their rate of feeding and bringing larger prey. As a result, offspring in nests of secondary females received as much food as did those in nests of primary females. Prey load size increased with the parent’s proportion of feeding visits, suggesting that parents use different feeding strategies depending on their amount of responsibility for nestling provisioning. We suggest that parents which take the main responsibility for nestling feeding have to forage further away from the nest, and based on optimal-foraging theory, they should then on average bring larger prey to their nest. Received: 4 April 1999 / Received in revised form: 12 October 1999 / Accepted: 23 October 1999     

Effectiveness of territorial polygyny and alternative mating strategies in northern fur seals, <Emphasis Type="Italic">Callorhinus ursinus</Emphasis>

We conducted a 6-year longitudinal behavioral and genetic investigation of a highly polygynous pinniped, the northern fur seal (Callorhinus ursinus), to determine the contribution of terrestrial polygyny to male fertilization success and to assess the occurrence of alternative mating strategies. Genetic samples from 37 adult males, 50 adult females, and 85 pups were collected and genotyped using five polymorphic microsatellite loci. Pup paternity was assigned using Cervus 2.0 at 99% confidence level. Paternity of 83 pups (98%) was assigned to terrestrial males who held territories or stayed temporarily in the study area during the breeding season when fertilization occurred. For 56 pups of which attendance records of their mothers were available, paternity of 45 pups (80%) was assigned to the associate males in whose territory their mothers stayed during the perioestrus period. In addition to defending breeding territories, territorial males have often been observed attempting to forcibly abduct adult females from adjacent territories (female stealing): We observed a total of 95 such cases, in which the stealers had significantly fewer females than the territorial males from whose territories they stole females. Our results indicate that terrestrial resource-defense polygyny is the major mating system in this species and that nonassociated paternity occurs mostly as a result of alternative mating strategies of less successful males. Male northern fur seals thus appear to adopt conditional alternative strategies that depend on their current social status to maximize their life-time reproductive success.     

Polygyny in the great reed warbler,Acrocephalus arundinaceus: a possible case of deception

Summary Polygyny was studied for 1 year in a great reed warbler population breeding in southern Germany. Data on various parameters of male and territory characteristics were collected and subjected to multivariate statistical analysis in order to assess their relative importance in female choice. Differences in territory characteristics appeared to be more important (Tab. 1), but there was a strong correlation between male and territory characteristics (Fig. 2, 3). Although the evolutionary advantages of polygyny to males are obvious (Fig. 5), those to females are far from clear. Models based upon the polygyny threshold and sexy son hypotheses predict that females should gain evolutionary advantage in either the short or long-term. Our data did not confirm such predictions, and secondary females showed greatly reduced breeding success (Fig. 4). If females were able to assess the status of mated males, they should instead select unmated males. Great reed warblers are a migrant species with a short breeding cycle in which the male plays relatively little part. They defend large territories in reed beds where visibility is reduced. These factors may permit males to practice a form of deception, by moving and attracting a second female who has little chance to assess that he is already mated.     

Moisture as a determinant of habitat quality for a nonbreeding neotropical migratory songbird

Identifying the determinants of habitat quality for a species is essential for understanding how populations are limited and regulated. Spatiotemporal variation in moisture and its influence on food availability may drive patterns of habitat occupancy and demographic outcomes. Nonbreeding migratory birds in the neotropics occupy a range of habitat types that vary with respect to moisture. Using carbon isotopes and a satellite-derived measure of habitat moisture, we identified a moisture gradient across home ranges of radio-tracked Northern Waterthrush (Seiurus noveboracensis). We used this gradient to classify habitat types and to examine whether habitat moisture correlates with overwinter mass change and spring departure schedules of Northern Waterthrush over the late-winter dry season in the tropics. The two independent indicators of moisture revealed similar gradients that were directly proportional to body mass change as the dry season progressed. Birds occupying drier habitats declined in body mass over the study period, while those occupying wetter habitats increased in body mass. Regardless of habitat, birds lost an average of 7.6% of their mass at night, and mass recovery during the day trended lower in dry compared with wet habitats. This suggests that daily incremental shortfalls in mass recovery can lead to considerable season-long declines in body mass. These patterns resulted in consequences for the premigratory period, with birds occupying drier habitats having a delayed rate of fat deposition compared with those in wet habitats. Taken together with the finding that males, which are significantly larger than females, are also in better condition than females regardless of habitat suggests that high-quality habitats may be limited and that there may be competition for them. The habitat-linked variation in performance we observed suggests that habitat limitation could impact individual and population-level processes both during and in subsequent periods of the annual cycle. The linkage between moisture and habitat quality for a migratory bird indicates that the availability of high-quality habitats is dynamic due to variation in precipitation among seasons and years. Understanding this link is critical for ascertaining the impact of future climate change, particularly in the Caribbean basin, where a much drier future is predicted.     

Territory takeover and sexually selected infanticide in tropical house wrens

Summary Tropical house wrens in lowland central Panama are, with rare exception, permanently monogamous. During a 3-year demographic study of 54–56 pairs of these brids, both males and females committed infanticide in association with takeover of breeding territories. New males and new females replaced previously identified (color-banded) parents before the termination of parental care in 13% of 328 breeding attempts. Replacement was accomplished by physical takeover in at least some if not all cases. Total nest failure at the egg, nestling, or fledgling stage occurred in 73% of the male replacements and 70% of the female replacements. Nest predation was excluded as a possible cause of nest failure. 76% of male replacements and 50% of female replacements bred in the same season. Infanticidal replacement males bred sooner than did non-infanticidal males arriving at similar stages of the nesting cycle. Contexts in which infanticide occurred included takeovers by first-year males and females of territories and mates, re-entry into the breeding population by males that had recently lost their mates, acquisition of neighboring females by bigamous males, and the elimination of one female of a bigamous male by the other. Sexually selected infanticide in these birds is promoted by a lengthy breeding season and extremely limited breeding opportunities for individuals not already part of the breeding population. Mate limitation appears to be more important than habitat or nestsite limitation in restricting new breeding opportunities once the breeding season has begun.     

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. Received: 1 December 1994 / Accepted after revision: 28 October 1995     

Sex-role reversal in willow tit nest defence

Summary The roles of sexes in the nest defence behavior of monogamous willow tit (Parus montanus) were studied near Oulu, northern Finland, in 1988–1990. The relative roles of the sexes changed during the breeding cycle: females defended their nests more vigorously before hatching and males defended more after hatching. This sexual asymmetry was studied by means of the cost/benefit model of optimal parental investment (PI). Because of the monogamous breeding system, sexual differences in future benefits were not a likely explanation for the asymmetry. This was also supported by preliminary results of DNA-fingerprinting analyses. Behavioral observations — sex-role reversal, high correlation between the mates, and equal variations in male and female behavior — indicated the same. In the beginning of the breeding cycle, higher female defence was related to renesting potential. The extra costs of renesting are considerably higher for females than for males, and therefore, females appeared to benefit more than males in keeping the first clutch alive while renesting was still possible. Such a female strategy was possible in this northern population, since the time for renesting was rather short and never lasted beyond the hatching of the first nests. After this point, the male can only invest in the present clutch. By the end of the breeding cycle, increased defence of the nest by males may be a consequence of males being larger and/or in better condition than females. Therefore, in the latter half of the breeding cycle, the sexual differences in nest defence did not indicate any sexual difference in PI. The asynchronous arrival of the parents at the nest also affected individual responses, indicating that defending the nest is a dynamic process.     

Resource defense polygyny shifts to female defense polygyny over the course of the reproductive season of a Neotropical harvestman

Although studies classify the polygynous mating system of a given species into female defense polygyny (FDP) or resource defense polygyny (RDP), the boundary between these two categories is often slight. Males of some species may even shift between these two types of polygyny in response to temporal variation in social and environmental conditions. Here, we examine the mating system of the Neotropical harvestman Acutisoma proximum and, in order to assess if mate acquisition in males corresponds to FDP or RDP, we tested four contrasting predictions derived from the mating system theory. At the beginning of the reproductive season, males fight with other males for the possession of territories on the vegetation where females will later oviposit, as expected in RDP. Females present a marked preference for specific host plant species, and males establish their territories in areas where these host plants are specially abundant, which is also expected in RDP. Later in the reproductive season, males reduce their patrolling activity and focus on defending individual females that are ovipositing inside their territories, as what occurs in FDP. This is the first described case of an arachnid that exhibits a shift in mating system over the reproductive season, revealing that we should be cautious when defining the mating system of a species based on few observations concentrated in a brief period. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Secondary polygyny by inbred wingless sexuals in the dolichoderine ant Technomyrmex albipes

Summary Technomyrmex albipes makes huge polydomous colonies which consist of up to several millions of adults. In field colonies, dealate queens are rare or absent, though winged males and winged females emerge annually (synchronously) in large numbers from late may to mid June. Field and laboratory observations showed that the reproduction of established colonies was performed by wingless females inseminated by wingless males from the same colony. Dissections and morphological examinations revealed that wingless females are workers with no spermatheca and intercastes with a spermatheca. Most intercastes were inseminated, had developed ovaries, and seemed to reproduce, while workers did not seem to reproduce. Extranidal tasks were performed only by workers. Approximately half of the adult population were intercastes, and wingless males represented only a small portion of all adults, the rest being nonreproductive workers. Intercastes and wingless males were produced throughout the year except in winter. The winged females and males copulate outside the nest only after the nuptial flight and the dealate females are able to perform independent founding, but they are also eventually supplanted by intercastes. The adoption of dealate queens by an established natal colony did not seem to occur. Thus we infer that in this species the winged reproductives disperse and found new colonies, while inbred wingless reproductives allow the enlargement and budding of colonies. This species has a special trophic-flow system. There is no trophallaxis among adults, and nutrient transfer from adults to other colony members is achieved exclusively by specialized trophic eggs. All females (dealate queens, intercastes, and workers) seem to produce trophic eggs. This aphid-like life cycle, i.e., the occurrence of both winged and wingless reproductive forms, may have evolved as an adaptation supporting the development of secondary polygyny and polydomy.Offprint requests to: K. Yamauchi     

Song rate as a signal for nest site quality in blackcaps (Sylvia atricapilla)

In this study we examine male song output as a measure of nest site quality in blackcaps (Sylvia atricapilla). Song rate, breeding success, predation on nests and reaction to playbacks were investigated in individual males. Habitat features determining nest site and song post quality in terms of vegetation cover were compared between successful nests and nests that had suffered predation. We then related song rate of unmated males to habitat factors in territories and nesting sites in order to examine a possible predictor function of blackcap song for habitat quality. Several habitat features are responsible for variation in nesting success. These features also correlate with song rate of unmated males. The study indicates a potential role of song rate in the advertisement of territory quality. Furthermore, the data suggest that females use song rates rather than territory quality in mating decisions. The information females may gain about male quality in relation to territory quality are also discussed.