Female settlement in marsh wrens: is it affected by other females?

Summary The influence of resident females on the settling patterns of subsequent females was examined for marsh wrens (Cistothorus palustris). Proportionally more females responded to a playback of a female intruder during the pre-laying stage than during the laying and incubation stages. The strength of the response was also greatest during pre-laying and laying stages (Table 1). Female wrens did not settle randomly with respect to the presence of other females, but rather settled with bachelor males more often than predicted by chance (Table 3). Female settlement within territories was more asynchronous than settlement between territories. An attempt to fit these data to two models of female settlement suggested that prospecting females were not displaced by residents, but rather avoided settling with mated males (Table 3). Females within territories were also more dispersed than predicted by chance, a pattern that may be enforced through aggression on the part of the resident female. Staggered settlement reduces the overlap in nestling stages of harem mates and so may be a strategy to reduce competition for resources at this critical stage.     

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     

Badge size in the house sparrow Passer domesticus

Summary I studied female mate choice in house sparrows Passer domesticus in relation to the size of the black throat patch of males (badge size), which is a signal of dominance status, and to territory quality. Males with large badges obtained a mate earlier in spring than males with small badges. Males which remained unmated during the entire breeding season had smaller badges than mated males, even when controlling for the effect of other morphological traits and age. Largebadged males had territories with more breeding sites than small-badged males. Territories defended by males with large badges primarily had hole nest sites, which were safe from predators, and nestlings rarely fell from hole nests. Females were implanted with estradiol to induce copulation solicitation displays. Females responded more strongly to a male taxidermic mount and song than to song alone, and they responded more strongly and frequently to male mounts with large badges. Female house sparrows chose mates on the basis of male badges and perhaps on the basis of the quality of the nesting territories offered.     

Female aggressive response and hormonal correlates—an intrusion experiment in a free-living passerine

The optimal mating system is rarely the same for males and females—whereas males usually benefit from attracting additional females to the territory, this could incur costs for the resident female. Females should therefore prevent prospecting females from settling on the territory. We studied the male and female behavioral and hormonal responses to simulated female territorial intrusions in free-living bluethroats during the pre-laying period. In the study population, polygyny occurs with potential fitness costs for the resident female. We recorded different aspects of aggressive behavior before and after presentation of a live female decoy and playback of female song. These behaviors were compared with a set of intrusions using a male decoy. At the end of a trial, the birds were captured, and blood samples were analyzed for androstenedione, testosterone, estradiol and corticosterone. During the pre-intrusion period, none of the females were observed. Females generally responded strongly to the female decoy by increased flight rate, vocalizations, and by conspicuous perching. Nearly half of the males displayed to the female decoy but never while the resident female was present. We suggest that resident female aggression in bluethroats prevents courtship by her mate and signals her mating status to the female intruder. Female aggression could therefore prevent additional females to settle on the territory and shape the mating system. Females that responded with song had higher levels of estradiol. These findings suggest that estradiol may support aggression in breeding female birds.     

The effect of female mating status on male offspring traits

In haplodiploid insects, males develop from unfertilized eggs; consequently, unmated females can reproduce. In a patchy, highly structured population, where brothers compete for mates and the reproductive return through sons is lower, females should minimize the number of male offspring. Consequently, unmated females are likely to have a reduced fitness compared to mated females. Here, we tested the oviposition behaviour of the haplodiploid beetle Coccotrypes dactyliperda. In this species, the unmated female can mate with her son to produce daughters. We predicted that unmated females could increase their fitness by (1) producing only few and small sons sufficient for mother–son mating and (2) dispersing to a patch occupied by conspecific females in order to increase their or their sons’ chance of mating. We demonstrate that (1) unmated females are common (23 % of all females), (2) they oviposit more frequently than mated females in occupied patches, (3) unmated females oviposit more eggs than mated females—this is in spite of the trade-offs, evident in this study, between the number of sons and the number of the mother’s future offspring after mating, (4) unmated females have a higher proportion of dispersing sons, and (5) sons of unmated females are smaller than sons of mated females. We conclude that the incidence of unmated females in the structured populations of C. dactyliperda is explained by plasticity in their oviposition behaviour. We discuss conditions where a high incidence of unmated females can persist as a successful strategy in structured populations.     

Female reactions to male absence after pairing in the pied flycatcher

Mating with an already mated pied flycatcher (Ficedula hypoleuca) male is costly for a female. Two hypotheses explain why some females still mate with already mated males. The deception hypothesis suggests that some females mate with already mated males since it is difficult to assess perfectly the mating status of a male with separate territories (polyterritoriality). The search cost hypothesis states that females are aware of male mating status but the costs of searching for an unmated male exceed costs associated with the status of secondary female. One potential cue that could disclose a male's mating status is the existence of brief visits to the primary territory by polyterritorial males. To mimic such visits I removed the male from the territory for 60 min soon after female settlement. Only few females abandoned their mates as a consequence of male removal even though they had available unmated males close by. This result is most consistent with the deception hypothesis. Females did not use occasional male absence as a cue to avoid presumably polyterritorial males and in this respect they were not perfect in assessing males.     

Territoriality and monogamy among agile gibbons (Hylobates agilis)

Summary I simulated territorial intrusions by mated pairs, solitary males and solitary females through song playback to investigate the behaviors that agile gibbons (Hylobates agilis) use to maintain territoriality and monogamy. Mated male and female gibbons approached and duetted in response to a simulated territorial intrusion by a neighboring group. Strange solitary females elicited similar approach and singing responses from mated pairs. In contrast, the simulated presence of a solitary male consistently evoked approaches by mated males only. Females and males did not contribute equally to pair movements or to answering songs; males led the majority of approaches, while females initiated all duets. I hypothesize that carrying small, vulnerable infants may inhibit females from leading approaches. These results are consistent with previous studies which suggest that female territoriality prevents male gibbons from becoming polygynous.     

How reliable are behavioral cues for assessment of male mating status in polyterritorial wood warblers,Phylloscopus sibilatrix?

It has been suggested that polyterritoriality in birds, i.e. mated males defending two spatially separated territories, deceives females into accepting polygyny against their best interest. Mated males singing on secondary territories should make it difficult for new arriving females to assess the mating status of unmated and mated males advertising to females. However, there have been objections to the deception hypothesis, suggesting that differences in territorial behavior between males of different mating status may make correct assessment possible. Polyterritorial males frequently leave their secondary territory to visit the female in the primary territory, thereby revealing their mating status to females in search of mates. The aim of this study is to investigate how reliable behavioral cues are in assessing the mating status of males in polyterritorial wood warblers. Our study shows that singing activity of mated males on secondary territories and unmated males was similar in the morning, while there was a significant difference in the afternoon. Unmated males spent, on average, more time on their territories than mated males spent on their secondary territories. However, there was considerable variation in both unmated and mated males in the frequency in which they left their territories. For instance, unmated males left their territories quite frequently early in the season compared to later. Hence, females must be able to interpret singing behavior and a male leaving his territory differently depending on time of the season and time of the day in order to correctly assess the mating status of advertising males. Estimating the increase in probability of finding an unmated male by using behavioral cues showed that females could increase the probability of finding an unmated male compared to random choice with respect to mating status. The uncertainty was, however, still considerable. The most important factor affecting the probability of finding an unmated male was the proportion of mated males on secondary territories compared to unmated males. Our study suggests that presence on a territory is not a reliable cue for the assessment of male mating status in polyterritorial wood warblers.     

Age,experience, and enemy recognition by wild song sparrows

Summary Older female Song Sparrows (Melospiza melodia) breeding on Mandarte Island, B.C., Canada, are more often parasitized by a brood parasite, the Brown-headed Cowbird (Molothrus ater), than yearling females breeding for the first time. This may be explained if older Song Sparrows behave differently than yearlings towards searching female cowbirds, and are thus more readily recognized as potential hosts. We tested this hypothesis by measuring the responses of Song Sparrows to a stuffed mount of a female cowbird in 1982, when no cowbirds frequented the island. This mount, and a control (a female junco) were presented near the nests of wild female sparrows of known age. As predicted, adult female sparrows behaved differently towards the cowbird model from yearlings (Table 1). They did not differ in their responses to the junco. Adult males also responded differently to the cowbird and junco (Table 2), but adult males did not differ significantly from yearlings in their response to either model. When birds that responded weakly as yearlings in the absence of cowbirds were retested as 2-year-olds after the recolonization of the island by cowbirds, they responded strongly to cowbird models. Two-year-old birds not exposed to cowbirds as yearlings, were parasitized at a rate intermediate between the rate for experienced adults and that for yearlings. Yearling females were parasitized less often at the beginning of the period of breeding activity by cowbirds than at the end. All these results are consistent with our hypothesis that age-selective parasitism results from differences among age classes in the mobbing responses of Song Sparrows to cowbirds. The greater response of adult sparrows to cowbirds seems non-adaptive, because it apparently results in a loss of reproductive output through selective parasitism. We suggest that this paradox can be resolved if the response to cowbirds near the nest is an instance of a more general acquired response to all potential enemies that approach a nest.     

Vocal interactions in common nightingales (<Emphasis Type="Italic">Luscinia megarhynchos</Emphasis>): males take it easy after pairing

Seasonal patterns of bird song have been studied intensively with a focus on individual males. However, little is known about seasonal patterns of singing during vocal interactions between males. Vocal interactions have been shown to be important in sexual selection as males may signal aspects of motivation or quality. Here, we investigated in nightingales (Luscinia megarhynchos) whether a male’s behaviour in vocal interactions at different stages of the breeding season is influenced by its mating status. We examined how males that differ in their subsequent mating success respond to a non-interactive, nocturnal playback presented during the period of mate attraction and subsequently during the egg-laying period. We found that mated males overlapped fewer songs and had a lower song rate during the egg-laying period compared to their responses during the mate-attraction period, whereas unpaired males did not vary in their responses between the two periods. Our results suggest that mating status is a key factor affecting singing behaviour in vocal interactions and that a time-specific singing pattern like song overlapping is used flexibly during vocal interactions. Because song overlapping is thought to be a signal of aggression in male–male vocal interactions, it seems that males vary the level of aggression in vocal interactions according to their mating status and to the stage in the breeding season.     

Fecundity studies on Acartia tonsa (Copepoda: Calanoida) in standardized culture

Acartia tonsa Dana longevity and egg production data were studied over a 4-year period under standardized culture conditions. Egg-laying patterns and daily rates were evaluated as bioassay tools for measuring the nutritional value of various copepod foods, as well as assessing other aspects of environmental quality. Altogether, 337 females were observed in various aspects of this work. Experiments were performed with newly matured females in loosely covered crystallizing dishes. Fresh water and a standard concentration of algae were supplied daily, and a running daily tally was kept of the eggs produced. The effects of male copepods on egg production were analyzed. Males were either removed after the first eggs were observed or they remained with the female throughout her life span. Daily egg-laying rates of this latter group of females appeared to be more stable, but abnormal embryo development occurred if the males were not removed after fertilization. Likewise, the results of females which were not mated were studied. Life spans were shorter and egg production rates were lower than those of contemporary mated females. Unmated females were subjectively observed to be less active than their mated counterparts. The effects of continuous and temporary starvation on egg-laying were also observed. Under both normal and starved conditions, males appeared to have shorter life spans than either mated or unmated females. During starvation there were no real differences between the life spans or egg productions of mated or unmated females. Non-nutritional particulate matter was evaluated as a food source. It was ingested by the females, but did nothing to prolong the egg-laying period. Finally, the three algal components of the standard ration were evaluated. Females produced equally well with a mixed diet or with Thalassiosira pseudonana (100 m³) or Chroomonas salina (140 m³), the two larger species of this diet, but when Isochrysis galbana (40 m³) was fed, females reacted as if in a starved situation.     

Duetting in the subdesert mesite Monias benschi: evidence for acoustic mate defence?

Despite numerous hypotheses proposed for the function of duets, there is currently no consensus as to why males and females should coordinate their songs in such a precise way. There is evidence indicating that duets sometimes serve in territory defence, but additional functions are rarely considered. The mate-defence hypothesis proposes that birds sing in response to their partner's song and the resulting duet repels rivals and may prevent desertion of a partner. We investigated this idea in the subdesert mesite Monias benschi using playback experiments in which we broadcast recordings of solos and duets to single birds and groups. Two predictions of the hypothesis were met: (1) the solo songs of both sexes incited aggressive responses from paired birds of the same sex; and (2) compared to solo songs, pair duets elicited weaker responses from groups and duetting pairs. However, groups responded to male duets with a vigour equal to that with which they responded to male solos. This indicated that the weaker responses of groups to pair duets compared to male solos was a function of the sex rather than number of vocalising birds. Groups responded more strongly to male solos than to either female solos or pair duets, and females' responses were generally weaker than those of males. This may reflect stronger competition among males for mates, due to a male-biased sex ratio in the population. We conclude that song serves similar functions in each sex and that duets may arise through mutual mate defence.     

Why do female pied flycatchers mate with already mated males: deception or restricted mate sampling?

The polygyny threshold model suggests that females make an optimal choice between mated and unmated males. However; in birds in which males provide parental care, the fitness of secondary females is often lower than expected from this model. This has been explained by the deception hypothesis, which states that males hide their mating status and deceive females into polygyny. Yet there is no direct evidence that secondary females are unaware of male mating status when they settle. Alternatively, females settle with mated males as a result of mate competition and costs of searching. We used videofilming at nestboxes defended by males to study mate sampling of female pied flycatchers Ficedula hypoleuca. The females visited on average only 2.74 males (range 1–8, n = 43). Most (16 of 19) of the polygynous matings occurred because females had only visited mated males, or the unmated males visited became occupied by competitors during the sampling period. Among females that could choose between both mated and unmated males, the majority (13 of 16) settled with unmated males. These results lend little support to the deception hypothesis but are consistent with the view that females are able to detect male mating status but sometimes settle with mated males because of cost of searching. Prospecting females seemed willing and able to suffer the cost of fighting with aggressive primary females in the males' secondary territory if no alternative mating options were available. In addition to male mating status, females took male quality (plumage colour, age) into account in mate choice but the former appeared to be the more important. Correspondence to: T. Slagsvold     

Land or lover? Territorial defence and mutual mate guarding in the crimson-breasted shrike

Crimson-breasted shrikes produce duets which are used in interactions with neighbours and intruders. We examined two major hypotheses explaining duetting, the territorial defence and mate guarding hypotheses, using playback experiments. The responses of 12 pairs towards solo male, solo female and stereo duet playback were analysed using principal component analysis and ANOVA. The measures of response intensity revealed in most cases highly coordinated responses of the pair partners. Although these joint responses could suggest cooperative territorial defence or mutual mate defence, responses to the treatments differed between males and females in terms of three variables. Females increased their answer rates significantly in response to unpaired female intruders, compared to unpaired male intruders. During duet playback, females directed their responses equally at both intruding pair members. In contrast, males were equally responsive towards unpaired male intruders and intruding pairs, but in the latter case directed their responses at the male pair member. In the presence of unpaired female intruders, males increased their solo song rates, but in the presence of intruding pairs, they increased overlapping rates and duetted with their mates. These divergent responses suggest that females use duets for mate guarding against unpaired females, whereas males use solo songs for mate guarding and both solos and duets in territorial defence.     

Female tactile cues maximize paternal behavior in prairie voles

Most monogamous male mammals display parental care, but the mechanism by which they become motivated to care for infants remains unknown. We used prairie voles (Microtus ochrogaster) as a model of biparental species to test the hypothesis that physical contact with a female enhances male parental responsiveness before the birth of offspring. We tested four groups of males: mated with physical contact with their mate, mated with only distal cues from their mate, mated with no cues, and unmated with distal cues from a female. The subjects were placed in an oversized cage divided into two compartments by a perforated barrier. At mid-gestation, the bedding and females were removed, and the males’ responses toward two infants were videotaped. Although most males behaved parentally, mated males exposed to their mate’s tactile or distal cues approached the young faster and were more likely to care for both infants than unmated males which received female distal cues. Males who had physical contact with their mate showed “kyphosis” (a crouching position over infants) and contacted infants more frequently than mated males which received no cues from their mate or unmated males which received female distal cues. Furthermore, the frequency of non-social behaviors was lowest in mated males that had physical contact with their mate. The data suggest that exposure to female distal cues after mating is sufficient to increase male parental attentiveness, but female somatosensory cues affect the male's tendency to remain in physical contact with infants.     

Sex ratio response to conspecifics in a parasitoid wasp: test of a prediction of local mate competition theory and alternative hypotheses

Maternal manipulation of offspring sex ratio in response to conspecifics is considered in relation to sex ratio theory using the parasitoid wasp Spalangia endius. Females produced a greater proportion of sons in response to mated but not virgin females. This is the first demonstration of a differential sex ratio response to virgin versus mated females and provides support for local mate competition theory. More recent sex ratio models that predict sex ratio responses to conspecifics, specifically constrained, perturbation, and crowding models, were not supported. An increased proportion of sons in response to another mated female occurred on the second day of oviposition but not on the first, and the day effect resulted from experience not age. When females oviposited alone after 2 days' exposure to another female, they still produced a greater proportion of sons than if they had always been alone, but only if the other female was mated, not if she was virgin. Females do not seem to assess the presence of virgin versus mated females indirectly by using a low density of males or a long latency to mate as an indicator for virgin females: neither affected offspring sex ratio. That mated females adjusted their sex ratios in response to other mated females, but not virgin females or males, may be due proximally to mated females not often encountering the latter. Virgin females and males are not located as deep in the oviposition substrate as mated females.     

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     

Infanticidal and anti-infanticidal strategies in the swallow Hirundo rustica

Summary Five definite and nine suspected cases of infanticide were documented in the swallow Hirundo rustica. Unmated males visited neighbouring nests and removed entire broods during the first days of the nestling period, preferentially from nests owned by young late-breeding individuals. Infanticide followed the disappearance of the male nest owner in 11 of the 14 cases. The unmated infanticidal male later mated with the female victim although renesting during the same season only took place in 12 of the 14 cases. The fraction of unmated males increased with colony size, and infanticide was relatively more common if unmated males were abundant. Females did not engage in extra-pair copulations in order to avoid infanticide. Intense guarding of the nest and its contents prevented infanticide. Nest guarding intensity was higher in colonial compared with solitarily breeding swallows, and guarding intensity increased with colony size. Experimental removal of male swallows during the early nestling period reduced the nest guarding intensity and increased frequency of visits from unmated males in colonies, but not among solitarily breeding swallows. Colonial nests from which the male was removed suffered from infanticide more often than solitary nests, and nests where infanticide was recorded were guarded significantly less intensely than other nests before the infanticidal incidents.     

Size assessment via a visual signal in snapping shrimp

Snapping shrimp are highly aggressive decapod crustaceans, with large, asymmetric chelae. Body size determines the outcome of both inter- and intrasexual interactions. Both the body and chela sizes of mated pairs are correlated, but the body size correlation is significantly stronger. In competitive interactions between individuals of the same sex, larger individuals usually win. Because the size of the major chela is a function of body size in both males and females, chela size could be used to assess body size early in interactions, before engaging in more high risk behaviors. To determine whether the major chela is used in size assessment, I presented shrimp with isolated chelae. Male snapping shrimp responded aggressively to isolated chelae when they were fixed open in a display posture, and the degree of aggressive response depended on the relative size of the chela. These data provide direct experimental evidence for the use of a visual signal in size assessment. Females, in contrast, responded aggressively to both the open and closed chela, and their responses did not depend on relative size. This sex difference in response may be due to differences in the value of certain resources, such as shelters, to males and females: females may be more willing to respond aggressively regardless of the apparent size of their opponent, in order to acquire more reliable information regarding size, motivation or fighting ability. Received: 30 December 1994/Accepted after revision: 14 August 1995     

Fecundity,egg weight and longevity in relation to multiple matings in females of the monarch butterfly

Summary In many insects nutrients transferred by the male at mating are later incorporated into both the eggs and soma of the mated females. Accordingly, it has been suggested that insect females can use these male-derived nutrients for somatic maintenance and enhancement of their fecundity and fitness of their offspring. I this paper we tested the validity of these predictions by studying the longevity and reproductive output of 1, 2, 3, 4 or 5 times mated female monarchs Danaus plexippus, a nectar-feeding butterfly that is long-lived and strongly polyandrous and emerges as an adult without mature eggs. Females mated five times received ejaculates that corresponded to an average of 38% of their body weight at eclosion. However, we found that the number of times females had mated had no effect on their longevity, life-time fecundity, or egg weight. Although negative evidence should always be interpreted with caution, our study suggests that male-derived nutrients are less important for female longevity and reproductive output than are larval and adult food.