Long-term, individual-level studies can provide valuable insights into the effects of climate and landscape change on the ecology and population dynamics of wild animals. However, many such studies lack environmental data collected at the spatial and temporal resolutions needed to determine how populations respond to changing conditions. In these cases, the retrospective use of satellite-derived data can provide a way to recover past environmental information. Using a 27-year dataset of an Australian insectivorous passerine, the superb fairy-wren Malurus cyaneus, we assessed how climate variation influences vegetation productivity and, indirectly, superb fairy-wren life history traits through potential changes in trophic interactions. Specifically, we combined long-term, individual-level monitoring of superb fairy-wrens and local weather records with Landsat satellite imagery, from which we derived measures of vegetation productivity using the Normalised Difference Vegetation Index (NDVI) as a proxy for food availability through arthropod abundance. We found a complex set of associations between NDVI and different components of weather, when considering both concurrent and lagged effects. Our analyses of the causes of seasonal variation in superb fairy-wren life history traits demonstrated that NDVI was associated with: (i) temporal variation in breeding success, with years with high spring and summer NDVI values having relatively high average breeding success; and (ii) spatial variation in adult mortality in autumn and winter, with superb fairy-wren territories with low autumn-winter NDVI values having higher average mortality rates. Notably, autumn-winter NDVI values were found to have remained relatively consistent over time, indicating that vegetation productivity cannot explain recently observed increases in adult autumn-winter mortality. Our study illustrates the potential of using long-term Landsat satellite imagery to investigate whether associations between animal life history traits and climate are mediated by vegetation productivity and to what extent temporal trends are influenced by climate change.

Population declines in common bird species abound, but it is unclear if these declines presage extinction. The consequences of climate change are particularly elusive, as there are typically both positive and negative effects on the seasonal- and life-cycles. We examine climate sensitivities of a population of a common and loved Australian bird, the superb fairy-wren Malurus cyaneus, with three decades of year-round fine-scale measurement of reproduction, survival and immigration. Population growth has been affected by many aspects of climate, but declines are most strongly linked to low fecundity in dry springs and reduced survival during unusually warm winters and preceding hot summers. Our comprehensive integrated population model is effective at summarizing these effects and predicting recent population volatility, suggesting it will be useful for assessing future population fluctuations and viability. A high risk of population extinction in the next 30-40 years is predicted. Accumulation of many small effects of climate change causes this decline. Hence a common species of Least Concern is in imminent danger, with virtually no time or way to prevent it. Such predicaments can only be detected in very well-studied species, and superb fairy-wrens may be the canary in the coal mine.

1. Nest predation is frequently the primary cause of early-life mortality in wild avian populations, generating selection for optimising the timing of reproduction to reduce predation risk. Investigating temporal patterns of nest predation is therefore necessary for understanding the intricate relationships between birds and their predators.2. In this study, we considered the role of temporal variation in nest predation in a wild population of cooperatively breeding superb fairy-wrens Malurus cyaneus in southeastern Australia, using data collected from nearly 4000 nests over a 27-year period (1994 to 2020). In this species, more than half of all nest attempts end in failure, mostly due to nest predation, with females sometimes initiating as many as ten clutches over their long breeding season.3. We analysed temporal variation in daily nest predation risk over three temporal scales in relation to: (i) the age of the young within the nest; (ii) the timing of nesting within the breeding season; and (iii) differences between years. For each of these temporal scales, we considered predation during the overall nesting period and for three specific stages of development: (i) the incubation stage (1 to 13 days from the onset of incubation); (ii) the early nestling stage (1 to 5 days post-hatching); and (iii) the late nestling stage (6 to 11 days post-hatching). 4. We found that the average daily risk of predation was lowest during the incubation stage (0.016 ± 0.124 SD), intermediate during the early nesting stage (0.025 ± 0.158 SD) and highest during the late nestling stage (0.066 ± 0.248 SD). Predation increased with the age of the clutch during the incubation stage and with the age of the brood during the early nestling stage, but there was no further increase during the late nestling stage. 5. Throughout the breeding season, daily nest predation rates varied quadratically, with a peak approximately mid-season. There was no evidence that these within-season trends differed between years, and we also found little evidence of any longer-term directional change in daily nest predation rates over the study period. Neither within nor between-year variation in nest predation was related to changes in nest density (i.e., the proportion of active nests at a given time). Instead, within-season patterns closely mirrored the breeding behaviour of pied currawongs Strepera graculina, a large corvid-like passerine that is a common predator of superb fairy-wren eggs and nestlings in our study area. 6. In addition to the temporal variation, we found higher daily rates of nest predation for females assisted by fewer helpers, for younger females, and for nests built at lower heights. However, the significance and magnitude of these effects varied across the different development stages. Furthermore, we found mixed effects of clutch and brood size. Our results therefore indicate a close association between temporal patterns of nest predation in superb fairy-wrens and this seemingly important avian nest predator.

Climate change may influence animal population dynamics through reproduction and mortality. However, attributing changes in mortality to specific climate variables is challenging because the exact time of death is usually unknown in the wild. Here, we investigated climate effects on adult mortality in Australian superb fairy-wrens (Malurus cyaneus). Over a 27-year period, mortality outside the breeding season nearly doubled. This nonbreeding season mortality increased with lower minimum (night-time) and higher maximum (day-time) winter temperatures and with higher summer heat wave intensity. Fine-scale analysis showed that higher mortality in a given week was associated with higher maxima 2 weeks prior and lower minima in the current fortnight, indicating costs of temperature drops. Increases in summer heat waves and in winter maximum temperatures collectively explained 62.6% of the increase in mortality over the study period. Our results suggest that warming climate in both summer and winter can adversely affect survival, with potentially substantial population consequences.

1. Reproductive performance in birds can be affected by both social environment and small-scale environmental heterogeneity via food abundance, availability of nesting sites, and predation risk. However, to date, the best studies of effects of microhabitat variation on avian populations have been on northern hemisphere passerines using nestboxes, where birds have limited control over nest sites and have a comparatively simple social structure. 2. Here we utilise a multi-decade dataset on the superb fairy-wren, Malurus cyaneus, a southern hemisphere passerine with facultative cooperative breeding. We monitored territory characteristics, nest locations and breeding success, and used GIS to relate these to social organisation and a survey of vegetation characteristics throughout the study area.3. There was a long-term nearly two-fold decline in population density over the study period (1994-2015). This was associated with a corresponding decline in the mean number of helpers per group, and hence in the extent of cooperative breeding: in the first four years of the study (1994-1997), 56% of groups had at least one helper, but in the final four years (2012-2015), this was reduced to 28%. Mean territory size also increased (from 0.74ha in the first four years to 1ha in the final four years) such that on average, years with lower numbers of helpers per territory had larger territory sizes. However, helper number was positively correlated with territory size within years.4. Reproductive performance was related to microhabitat heterogeneity: fledgling production was lower and nest predation higher in territories with dense midstorey vegetation, possibly because avian predators using visual information to detect nests can conceal themselves from nesting birds. Predation during the nesting phase decreased over time, indicating that the population decline was not driven by increased predation.5. The causes of overall population decline remain to be determined, however our analyses have uncovered both microspatial patterns in nesting behaviour of birds, and temporal changes in population density and social group dynamics. From a methodological perspective, the study demonstrates the utility of GIS methods for investigating fine-scale habitat dynamics over time.

Abstract In wild bird populations, the structure of vegetation around nest‐sites can influence the risk of predation of dependent offspring, generating selection for nest‐sites with vegetation characteristics associated with lower predation rates. However, vegetation structure can be difficult to quantify objectively in the field, which might explain why there remains a general lack of understanding of which characteristics are most important in determining predation rates. Airborne laser scanning (ALS) offers a powerful means of measuring vegetation structure at unprecedented resolution. Here, we combined ALS with 11 years of breeding data from a wild population of superb fairy‐wrens Malurus cyaneus in southeastern Australia, a species which nests relatively close to the ground and has high rates of nest and fledgling predation. We derived structural measurements of understorey (0–8 m) vegetation from a contiguous grid of 30 × 30 m resolution cells across our c. 65 hectares study area. We found that cells with nests (nest‐cells) differed in their understorey vegetation structure characteristics compared to unused cells, primarily in having denser vegetation in the lowest layer of the understorey (0–2 m; the ‘groundstorey’ layer). The average height of understorey vegetation was also lower in cells with nests than in those without nests. However, relationships between understorey vegetation structure characteristics and breeding performance were mixed. Nest success rates decreased with higher volumes of groundstorey vegetation, as did fledgling survival rates, though only in nest‐cells with lower height vegetation. Our results indicate that ALS can identify vegetation characteristics relevant for superb fairy‐wren nest‐site selection, but that nesting preferences are not beneficial under current predation pressures. The study illustrates the potential for using ALS to investigate how ecological conditions affect behaviour and life‐histories in wild animal populations.

Brood-parasitic cuckoos lay their eggs in the nests of other birds, abandoning parental care to their hosts. Many host species have evolved defences to escape or reduce the costs associated with cuckoo parasitism. Superb Fairy-wrens Malurus cyaneus, which are a host to several cuckoo species in Australia, can distinguish cuckoo eggs based on their size or shape, or by using indirect cues such as the timing of egg laying or the presence of an adult cuckoo near the nest. They have previously been shown to reject cuckoo eggs by methods of egg burial or abandonment. These methods are likely to be costlier than ejecting the cuckoo egg from the nest (as seen in some other host species), because of the complete failure of the reproductive attempt, in addition to the costs associated with renesting. In this study, we document with photograph and video footage an incident of a Fan-tailed Cuckoo Cacomantis flabelliformis parasitising a nest of a Superb Fairy-wren in the Australian National Botanic Gardens, Canberra. Shortly after the parasitism event occurred, we recorded the female Fairywren returning to the nest and ejecting the single Cuckoo egg. To our knowledge, this footage represents the first evidence of ejection of a cuckoo egg in this species.

The rate of adaptive evolution, the contribution of selection to genetic changes that increase mean fitness, is determined by the additive genetic variance in individual relative fitness. To date, there are few robust estimates of this parameter for natural populations, and it is therefore unclear whether adaptive evolution can play a meaningful role in short-term population dynamics. We developed and applied quantitative genetic methods to long-term datasets from 19 wild bird and mammal populations and found that, while estimates vary between populations, additive genetic variance in relative fitness is often substantial and, on average, twice that of previous estimates. We show that these rates of contemporary adaptive evolution can affect population dynamics and hence that natural selection has the potential to partly mitigate effects of current environmental change.

Why do senescence rates of fitness-related traits often vary dramatically? By considering the full aging trajectories of multiple traits, we can better understand how a species’ life history shapes the evolution of senescence within a population. Here, we examined age-related changes in sex-specific survival, reproduction, and several components of reproduction using a long-term study of a cooperatively breeding songbird, the superb fairy-wren (Malurus cyaneus). We compared aging patterns between traits by estimating standardized rates of maturation, age of onset of senescence, and rates of senescence while controlling for confounding factors reflecting individual variability in life history. We found striking differences in aging and senescence patterns between survival and reproduction as well as between reproductive traits. In both sexes, survival started to decline from maturity onward. In contrast, all reproductive traits showed improvements into early adulthood, and many showed little or no evidence of senescence. In females, despite senescence in clutch size, number of offspring surviving to independence did not decline in late life, possibly due to improvements in maternal care with age. Superb fairy-wrens have exceptionally high levels of extragroup paternity, and while male within-group reproductive success did not change with age, extragroup reproductive success showed a dramatic increase in early ages, followed by a senescent decline, suggesting that male reproductive aging is driven by sexual selection. We discuss how the superb fairy-wrens’ complex life history may contribute to the disparate aging patterns across different traits.

In cooperatively breeding species, the presence of male helpers in a group often reduces the breeding female's fidelity to her social partner, possibly because there is more than one potential sire in the group. Using a long-term study of cooperatively breeding superb fairy-wrens (Malurus cyaneus) and records of paternity in 1936 broods, we show that the effect of helpers on rates of extrapair paternity varied according to the helpers' relatedness to the breeding female. The presence of unrelated male helpers in a group increased average rates of extrapair paternity, from 57% for groups with no unrelated helpers, to 74% with one unrelated helper, to 86% with 2+ unrelated helpers. However, this increase was due in equal part to helpers within the group and males in other groups achieving increased paternity. In contrast, helpers who were sons of the breeding female did not gain paternity, nor did they affect the level of extra-group paternity (which occurred at rates of 60%, 58%, 61% in the presence of 0, 1, 2+ helper sons, respectively). There was no evidence of effects of helpers' relatedness to the female on nest productivity or nestling performance. Because the presence of helpers per se did not elevate extrapair reproduction rates, our results undermine the "constrained female hypothesis" explanation for an increase in extrapair paternity with helper number in cooperative breeders. However, they indicate that dominant males are disadvantaged by breeding in "cooperative" groups. The reasons why the presence of unrelated helpers, but not of helper-sons, results in higher rates of extra-group reproduction are not clear.

Assignment of parentage with molecular markers is most difficult when the true parents have close relatives in the adult population. Here, we present an efficient solution to that problem by extending simple exclusion approaches to parentage analysis with single nucleotide polymorphic markers (SNPs). We augmented the previously published homozygote opposite test (hot), which counts mismatches due to the offspring and candidate parent having different homozygous genotypes, with an additional test. In this case, parents homozygous for the same SNP are incompatible with heterozygous offspring (i.e., “Homozygous Identical Parents, Heterozygous Offspring are Precluded”: hiphop). We tested this approach in a cooperatively breeding bird, the superb fairy‐wren, Malurus cyaneus, where rates of extra‐pair paternity are exceptionally high, and where paternity assignment is challenging because breeding males typically have first‐order adult relatives in their neighbourhood. Combining the tests and conditioning on the maternal genotype with a set of 1376 autosomal SNPs always allowed us to distinguish a single most likely sire from his relatives, and also to identify cases where the true sire must have been unsampled. In contrast, if just the hot test was used, we failed to identify a single most‐likely sire in 2.5% of cases. Resampling enabled us to create guidelines for the number of SNPs required when first‐order relatives coexist in the mating pool. Our method, implemented in the R package hiphop, therefore provides unambiguous parentage assignments even in systems with complex social organisation. We also identified a suite of Z‐ and W‐linked SNPs that always identified sex correctly.

Age-related changes in parental phenotypes or genotypes can impact offspring fitness, but separating germline from nongermline transgenerational effects of ageing is difficult for wild populations. Further, in cooperatively breeding species, in addition to parental ages, the age of 'helpers' attending offspring may also affect juvenile performance. Using a 30-year study of a cooperative breeder with very high rates of extra-pair paternity, the superb fairy-wren (Malurus cyaneus), we investigated the effects of maternal, paternal and helper ages on three measures of offspring performance: nestling weight, juvenile survival to independence and recruitment to the breeding population. Mothers with a longer lifespan had offspring with higher juvenile survival, indicating selective disappearance, but the effect of maternal age on juvenile survival was of similar magnitude but negative. For extra-pair offspring, there was no evidence of any effect of the ages of either the genetic sire or the cuckolded 'social' father. However, for within-pair offspring, there was a positive effect of paternal age on juvenile survival, which we suggest may be driven by sexual selection. There were positive associations between the average age of helpers attending a nest and two of the three aspects of offspring performance; these effects were stronger than any of the effects of parental age. In general, the multiple associations between offspring fitness and the ages of adults around them appeared to be driven more by age-related changes in environmental effects than by age-related changes in the germline.

Climate warming has been shown to affect the timing of the onset of breeding of many bird species across the world. However, for multi‐brooded species, climate may also affect the timing of the end of the breeding season, and hence also its duration, and these effects may have consequences for fitness. We used 28 years of field data to investigate the links between climate, timing of breeding, and breeding success in a cooperatively breeding passerine, the superb fairy‐wren ( Malurus cyaneus ). This multi‐brooded species from southeastern Australia has a long breeding season and high variation in phenology between individuals. By applying a “sliding window” approach, we found that higher minimum temperatures in early spring resulted in an earlier start and a longer duration of breeding, whereas less rainfall and more heatwaves (days > 29°C) in late summer resulted in an earlier end and a shorter duration of breeding. Using a hurdle model analysis, we found that earlier start dates did not predict whether or not females produced any young in a season. However, for successful females who produced at least one young, earlier start dates were associated with higher numbers of young produced in a season. Earlier end dates were associated with a higher probability of producing at least one young, presumably because unsuccessful females kept trying when others had ceased. Despite larger scale trends in climate, climate variables in the windows relevant to this species’ phenology did not change across years, and there were no temporal trends in phenology during our study period. Our results illustrate a scenario in which higher temperatures advanced both start and end dates of individuals’ breeding seasons, but did not generate an overall temporal shift in breeding times. They also suggest that the complexity of selection pressures on breeding phenology in multi‐brooded species may have been underestimated.

Inbreeding depression plays a major role in shaping mating systems: in particular, inbreeding avoidance is often proposed as a mechanism explaining extra‐pair reproduction in socially monogamous species. This suggestion relies on assumptions that are rarely comprehensively tested: that inbreeding depression is present, that higher kinship between social partners increases infidelity, and that infidelity reduces the frequency of inbreeding. Here, we test these assumptions using 26 years of data for a cooperatively breeding, socially monogamous bird with high female infidelity, the superb fairy‐wren (Malurus cyaneus). Although inbred individuals were rare (∼6% of offspring), we found evidence of inbreeding depression in nestling mass (but not in fledgling survival). Mother–son social pairings resulted in 100% infidelity, but kinship between a social pair did not otherwise predict female infidelity. Nevertheless, extra‐pair offspring were less likely to be inbred than within‐pair offspring. Finally, the social environment (the number of helpers in a group) did not affect offspring inbreeding coefficients or inbreeding depression levels. In conclusion, despite some agreement with the assumptions that are necessary for inbreeding avoidance to drive infidelity, the apparent scarcity of inbreeding events and the observed levels of inbreeding depression seem insufficient to explain the ubiquitous infidelity in this system, beyond the mother–son mating avoidance.

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Despite extensive research on the topic, it has been difficult to reach general conclusions as to the effects of climate change on morphology in wild animals: in particular, the effects of warming temperatures have been associated with increases, decreases or stasis in body size in different populations. Here, we use a fine-scale analysis of associations between weather and offspring body size in a long-term study of a wild passerine bird, the cooperatively breeding superb fairy-wren, in south-eastern Australia to show that such variation in the direction of associations occurs even within a population. Over the past 26 years, our study population has experienced increased temperatures, increased frequency of heatwaves and reduced rainfall - but the mean body mass of chicks has not changed. Despite the apparent stasis, mass was associated with weather across the previous year, but in multiple counteracting ways. Firstly, (i) chick mass was negatively associated with extremely recent heatwaves, but there also positive associations with (ii) higher maximum temperatures and (iii) higher rainfall, both occurring in a period prior to and during the nesting period, and finally (iv) a longer-term negative association with higher maximum temperatures following the previous breeding season. Our results illustrate how a morphological trait may be affected by both short- and long-term effects of the same weather variable at multiple times of the year and that these effects may act in different directions. We also show that climate within the relevant time windows may not be changing in the same way, such that overall long-term temporal trends in body size may be minimal. Such complexity means that analytical approaches that search for a single 'best' window for one particular weather variable may miss other relevant information, and is also likely to make analyses of phenotypic plasticity and prediction of longer-term population dynamics difficult.

1. In structured populations, phenotypic change can result from changes throughout an individual's lifetime (phenotypic plasticity, age-related changes), selection and changes in population composition (environment- or density-driven fluctuations in age-structure). 2. The contribution of population dynamics to phenotypic change has often been ignored. However, for understanding trait dynamics, it is important to identify both the individual- and population-level mechanisms responsible for trait change, because they potentially reinforce or counteract each other. 3. We use 22 years of field data to investigate the dynamics of a sexually selected phenological trait, the timing of nuptial moult in superb fairy-wrens Malurus cyaneus. 4. We show that trait expression is both climate- and age-dependent, but that phenotypic plasticity in response to climate variability also varies with age. Old males can acquire nuptial plumage very early after high rainfall, but 1- to 2-year-olds cannot. However, males of all ages that defer moult to later in the year acquire nuptial plumage earlier when conditions are warmer. 5. The underlying mechanism appears to be that old males may risk moulting in the most challenging period of the year: in autumn, when drought restricts food abundance and during the cold winter. By contrast, young males always moult during the spring transition to benign - warmer and generally wetter - conditions. Temperature changes dominate this transition that heralds the breeding season, thereby causing both young and late-moulting older birds to be temperature sensitive. 6. Climate and age also affect trait dynamics via a population dynamical pathway. The same high rainfall that triggers early moulting in old males concurrently increases offspring recruitment and thereby reduces the average age of males in the population. Consequently, effects of rainfall on trait dynamics through phenotypic plasticity of old males are dampened by synchronous rejuvenation of the age-structure. 7. A long-term trend towards drier environments prompted phenotypic change because of plasticity, but this was masked by climate-driven demographic change (causing apparent stasis). This suggests a novel explanation for why trait change may fail to reflect the observed pattern of directional selection or phenotypic plasticity.

Female superb fairy-wrens Malurus cyaneus initiate extragroup fertilizations by forays to the territory of preferred males, just before sunrise, 2-4 days before egg laying. Over a prolonged breeding season, males advertise their availability to foraying females by singing during the dawn chorus. Here, we show that 1) males commence dawn advertisement at the same time of the year regardless of their quality or status; 2) subordinate males advertise by singing in close proximity to the dominant, or by using the dominant's song perch, despite inevitable punishment; 3) low-quality dominants and their helpers sing from the boundary of their own territory, which increases their proximity to attractive neighboring dominants; 4) each spatial cluster of males use a common dialect of a song that is implicated in extragroup choice, despite the ability of individual males to sing several dialects; and 5) there is leakage of paternity to lower-quality helpers and neighbors as a result of their "satellite" behavior. Collectively, these data suggest that Wagner's hidden lek hypothesis (Wagner RA, 1998. Hidden leks: sexual selection and the clustering of avian territories. In: Parker PG, Burley NT, editors. Avian reproductive tactics: female and male perspectives. Ornithological Monographs No. 49. Allen Press. p. 123-145) can be extended to birds that defend year-round all-purpose territories and that mating induced by parasitic behavior of low-quality satellites can be one explanation for polyandry in birds.

1. Subordinate helpers in cooperative societies may gain both immediate and future benefits, including paternity and territorial inheritance. However, if such opportunities correlate with rank in the queue, it is unclear why such queues should be stable. 2. In cooperatively breeding superb fairy-wrens Malurus cyaneus, only males are generally philopatric, and form stable hierarchical queues for the dominant position. 3. Male opportunities for reproduction are influenced both by their dominance status within the group, and their relatedness to the breeding female. For young queuing subordinates, the breeding female is typically their mother. Because of incest avoidance, reproduction is possible only through extra-group mating, even if the dominant position is achieved while the mother is still on the territory. If the mother dies while the helper is still a subordinate, he can seek matings both outside the group, and with the unrelated replacement female within the group. Finally, males can achieve the dominant position and pair with an unrelated female by inheritance, dispersal to a neighbouring vacancy, or by forming a liaison with an immigrant subordinate female that causes fission of the natal territory. 4. On average males spent more time living with unrelated females than with their mother. Subordinate males gained no survival advantages when living with their mother rather than an unrelated female, contrary to the prediction that parents facilitate the survival of their offspring. 5. Dominants and subordinates also had similar survival. Mortality accelerated over time, probably because older males invest more in extra-group courtship display. 6. Fairy-wren queues are likely to be stable because older birds are superior, and because extra-pair mating provides direct benefits to subordinates.

1. Correlational studies of reproductive success are plagued by difficulty over the direction of causation. For example, improved reproductive success with age can result from increased experience or reproductive effort, or selection against low-quality phenotypes that survive poorly. An association between supernumeraries and reproductive success in cooperative breeders can arise either because supernumeraries boost productivity, or productive territories accumulate supernumeraries. 2. Paired comparisons of parents sampled with and without supernumeraries have recently been widely applied to quantify help. However, Dickinson & Hatchwell (2004) have argued that this approach is flawed. They conjectured that those groups that gain supernumeraries are a biased superior sample of those that initially lack supernumeraries, while groups that lose supernumeraries will be a sample of inferior cooperative groups. They predict that these biased comparisons will underestimate the effect of help. 3. This conjecture has neither been explored theoretically, nor empirically tested. We use data from a 19-year study of the superb fairy-wren Malurus cyaneus to examine the conjecture and derive predictors of annual reproductive success in this species. 4. We introduce statistical models of reproductive success based on a zero-inflated Poisson link function to identify three strong correlates of reproductive success: high spring rainfall, progress from the first to later years of life, and acquisition of supernumeraries. 5. First year females that died after breeding and those that survived to breed again had similar productivity. As female productivity improves with age, increased reproductive skill or effort is implicated rather than selection against inferior phenotypes. 6. We argue that the Dickinson-Hatchwell conjecture does not constrain paired comparisons in M. cyaneus. The dominant male and breeding female gain no immediate fecundity advantage from supernumeraries. 7. Effects on the future survival of dominants are even more difficult, as while helpers could enhance survival of dominants, a territory that facilitates survival should also accumulate philopatric supernumeraries. Males, the philopatric sex, did not survive better on territories with supernumeraries. However, females, the dispersive sex, had higher survival as the number of supernumeraries increased, because helpers allowed them to reduce the costs of reproduction. These data exacerbate the paradox posed by previously reported costs that supernumeraries impose on dominant males.