Increased Breeding Success Research Articles

First Record of White-Tailed Deer Depredating Saltmarsh Sparrow Nests

Ammospiza caudacuta (Saltmarsh Sparrow) is a salt marsh-nesting obligate species of greatest conservation concern due to decreasing populations and probable future threats, such as climate change-induced sea-level rise. Low reproductive success is a limiting factor to population growth for the species due to flooding, habitat loss, and predation. To further understand the effects of predation on nesting Saltmarsh Sparrows, we deployed camera traps at monitored nests in Jacob's Point salt marsh, Warren, RI, during the 2022 breeding period. In 2 instances of observed nest failure, camera traps captured the apparent depredation of eggs by Odocoileus virginianus (White-tailed Deer). We believe these to be the first documented cases of Saltmarsh Sparrow nest predation by White-tailed Deer. This finding has implications for the conservation of Saltmarsh Sparrows and for future management strategies aimed at increasing breeding success.

A comparison of the breeding biology of White-throated Kingfisher Halcyon smyrnensis Linnaeus, 1758 in plains and hilly areas of Bangladesh

The breeding biology of White-throated Kingfisher Halcyon smyrnensis was studied in plains and hilly areas from September 2008 to August 2011. Four villages under Savar upazilla were selected for plains, and Chittagong University Campus, Chattagram for the hilly area. The breeding season started in February in hills and April on plain. Mean (SD) time required to build a new nest was 11.3 (3.9) days in plains and 15.3 (0.57) days in hills. Clutch size was 3–4 in hills and 3–7 in plains. Mean egg parameters (length, width, and weight) and mean egg volume and surface area were similar in both areas. The mean incubation period on plains was 16.4 (1.2) days, in hills 14.1 (0.7) days. On plains fledging success was 52%, compared to 57% in hills. Theft by local inhabitants was a major reason for fledgling loss in plains, hence increased public awareness may reduce nestling mortality and increase breeding success.

Conservation measures for the Caspian Tern Hydroprogne caspia at the largest colony in Sweden

We present conservation actions during 2007–2020 as part of the national Caspian Tern Hydroprogne caspia species action plan at Stenarna in the Björn archipelago, Uppland, the largest colony in Sweden. We applied a combination of monitoring, research, and management measures conducted within an adaptive approach framework, using both established and novel techniques. The implementation of conservation measures led to increased breeding success, from 0 fledglings per pair in 2007 to 1.3 in 2020. A surveillance video camera installed in 2009 aided in monitoring efforts and also revealed predation by Herring Gull Larus argentatus and White-tailed Eagle Haliaeetus albicilla, leading to species-specific control strategies. Management of the island and surrounding archipelago, including hunting of invasive American mink Mustela vison, vegetation removal, and habitat restoration after a severe storm, have also been instrumental to the success of the project. Implementation of projects such as this have the potential to improve conditions for continued viability of endangered species in a changing world and are likely to be useful to other conservation practitioners.

Genomic analysis reveals a polygenic architecture of antler morphology in wild red deer (Cervus elaphus).

Sexually selected traits show large variation and rapid evolution across the animal kingdom, yet genetic variation often persists within populations despite apparent directional selection. A key step in solving this long-standing paradox is to determine the genetic architecture of sexually selected traits to understand evolutionary drivers and constraints at the genomic level. Antlers are a form of sexual weaponry in male red deer (Cervus elaphus). On the island of Rum, Scotland, males with larger antlers have increased breeding success, yet there has been no evidence of any response to selection at the genetic level. To try and understand the mechanisms underlying this observation, we investigate the genetic architecture of ten antler traits and their principal components using genomic data from >38,000 SNPs. We estimate the heritabilities and genetic correlations of the antler traits using a genomic relatedness approach. We then use genome-wide association and haplotype-based regional heritability to identify regions of the genome underlying antler morphology, and an empirical Bayes approach to estimate the underlying distributions of allele effect sizes. We show that antler morphology is highly repeatable over an individual's lifetime, heritable and has a polygenic architecture and that almost all antler traits are positively genetically correlated with some loci identified as having pleiotropic effects. Our findings suggest that a large mutational target and genetic covariances among antler traits, in part maintained by pleiotropy, are likely to contribute to the maintenance of genetic variation in antler morphology in this population.

Temperature and density influence survival in a rapidly declining migratory shorebird

Migratory species face geographically dispersed pressures over the course of their annual cycles. Designing effective conservation strategies for these species requires a detailed understanding of how these different pressures affect demographic rates throughout the annual cycle. As a long-lived species, population trends in the rapidly declining Eurasian curlew Numenius arquata are likely to be highly sensitive to impacts on adult survival. We combine data from multiple sources to analyse survival rates of overlapping populations that breed and/or winter in the United Kingdom. Our analyses demonstrate that curlew survival rates are reduced by cold weather and at high density; however, overall survival rates are high and have increased in recent years. Current population declines are, therefore, likely to be driven by low productivity. As such, efforts to stabilise and reverse declines should focus on increasing breeding success from current estimated levels of 0.25 chicks nest−1 to 0.43 chicks nest−1. In addition to increasing productivity, effective conservation strategies will need to maintain high levels of survival, which requires an improved understanding of population connectivity and demographic variation throughout the annual cycle.

Breeding dynamics of overabundant koala (

Abstract Context The koala (Phascolarctos cinereus) sometimes occurs as an overabundant folivore in south-eastern Australia, where high-density populations have caused defoliation of preferred food trees, threatening habitat. Kangaroo Island, South Australia and Budj Bim National Park, Victoria, are two regions where such eruptive population dynamics have arisen. One way of mitigating their damage is through management via fertility control. Aims This paper examines the outcomes of fertility control (surgical sterilisation and treatment with levonorgestrel implants) on breeding dynamics at the population level, using data obtained from two separate koala management programs conducted by State Government wildlife agencies, South Australia Department for Environment and Water and Parks Victoria. Methods The relationships between female body condition, reproductive status, fertility control, age and population density were examined using linear mixed effects models. Population density (koalas/ha) was estimated using annual census data and modelled in a Bayesian framework. Key results Body condition was a key influence on breeding success in female koalas, with sexual maturity being defined by both age and body condition, whereas the effect of reproductive experience was minimal. It is likely that reduced densities have led to decreased intraspecific competition for food resources and territory, leading to increased breeding success at one management site (BBNP) and by allowing females to begin breeding at a lower overall body condition. The reduction in densities to sustainable levels at both management sites, despite differing fertility-control methods, supports the use of the less invasive and more cost-effective levonorgestrel implants as the preferred fertility-control method. Conclusions In addition to contraceptive effects, broad-scale fertility control may have resulted in a compensatory higher breeding success in the untreated population, possibly in response to decreasing densities, increased resource availability or behavioural responses to population management. Implications Although compensatory breeding mechanisms can reduce the effectiveness of fertility control at the population level, ongoing fertility-control management can still be successful at reducing overabundant koala populations to sustainable levels with significant long-term commitment, provided that annual control targets are continuously met.

The effects of inbreeding on covering success, gestation length and foal sex ratio in Australian thoroughbred horses

BackgroundHorses produce only one foal from an eleven-month gestation period, making the maintenance of high reproductive rates essential. Genetic bottlenecks and inbreeding can increase the frequency of deleterious variants, resulting in reduced reproductive levels in a population. In this study we examined the influence of inbreeding levels on foaling rate, gestation length and secondary sex ratio in Australian Thoroughbred mares. We also investigated the genetic change in these traits throughout the history of the breed. Phenotypic data were obtained from 27,262 breeding records of Thoroughbred mares provided by three Australian stud farms. Inbreeding was estimated using the pedigree of each individual dating back to the foundation of the breed in the eighteenth century.ResultsWhile both gestation length and foaling rate were heritable, no measurable effect of inbreeding on either trait was found. However, we did find that the genetic value for both traits had decreased within recent generations. A number of environmental factors also had significant effects on foaling rate and gestation length. Secondary sex ratio had only an extremely small paternal heritable effect and was not susceptible to environmental influences.ConclusionsIn contrast to racing performance, inbreeding had no measurable effect on foaling rate or gestation length in Australian Thoroughbred horses. This could be because the level of inbreeding in the population examined is not high enough to show a discernible effect on reproductive traits. Populations that experience higher levels of inbreeding due to use of artificial reproductive technologies or extremely small population sizes may show a more pronounced reduction in natural foaling rate or gestation length. It is also possible that the intensive management techniques used in the Thoroughbred population masks any negative effects of inbreeding. The decrease in the genetic value of foaling rate is likely to be because horses with unfavourable genetic potential have not yet been selected out of the population. The change in genetic value of gestation length may be due to selective breeding favouring horses with shorter pregnancies. We also found that prioritising the mating of older mares, and avoiding out of season mating could lead to an increased breeding success.

Remotely sensed forest understory density and nest predator occurrence interact to predict suitable breeding habitat and the occurrence of a resident boreal bird species.

Habitat suitability models (HSM) based on remotely sensed data are useful tools in conservation work. However, they typically use species occurrence data rather than robust demographic variables, and their predictive power is rarely evaluated. These shortcomings can result in misleading guidance for conservation. Here, we develop and evaluate a HSM based on correlates of long‐term breeding success of an open nest building boreal forest bird, the Siberian jay. In our study site in northern Sweden, nest failure of this permanent resident species is driven mainly by visually hunting corvids that are associated with human settlements. Parents rely on understory nesting cover as protection against these predators. Accordingly, our HSM includes a light detection and ranging (LiDAR) based metric of understory density around the nest and the distance of the nest to the closest human settlement to predict breeding success. It reveals that a high understory density 15–80 m around nests is associated with increased breeding success in territories close to settlements (<1.5 km). Farther away from human settlements breeding success is highest at nest sites with a more open understory providing a favorable warmer microclimate. We validated this HSM by comparing the predicted breeding success with landscape‐wide census data on Siberian jay occurrence. The correlation between breeding success and occurrence was strong up to 40 km around the study site. However, the HSM appears to overestimate breeding success in regions with a milder climate and therefore higher corvid numbers. Our findings suggest that maintaining patches of small diameter trees may provide a cost‐effective way to restore the breeding habitat for Siberian jays up to 1.5 km from human settlements. This distance is expected to increase in the warmer, southern, and coastal range of the Siberian jay where the presence of other corvids is to a lesser extent restricted to settlements.

Diversionary feeding of red fox in spring increased productivity of forest grouse in southeast Norway

In Eurasia, forest grouse have been declining throughout most of their geographical ranges. Presumably, poor recruitment due to high predation of nests and chicks is one important causal factor. In a southeastern Norwegian forested landscape (Fjella), we provided diversionary food to predators – directed mainly at the red fox Vulpes vulpes – during the nesting and early brood season of capercaillie Tetrao urogallus and black grouse T. tetrix in three nearby areas. In Eidsberg, where populations were censused during 33 years (1985–2017), food was provided during the last 22 years. In two other areas, a six-year experimental program was conducted (2003–2008) by providing food in one area for three years, then switching feeding to the other area for three years. During May and June, 10 kg of food – mainly moose offal and ungulate carcasses – was deposited at feeding stations once a week. In Eidsberg, black grouse breeding success increased by 43% after feeding was initiated, mainly due to larger brood sizes. In capercaillie, overall breeding success tended to increase, solely due to more females successfully rearing chicks. In the experimental areas, feeding increased breeding success an estimated 57% in black grouse. In capercaillie there was also a tendency for a positive effect, but sample sizes were too small for statistical inference. No increases were detected in adult birds. However, in capercaillie, increased breeding success after feeding led to a significant skew in sex ratio favouring males. A similar tendency in black grouse suggested source–sink dynamics and a net loss of young females during natal dispersal. We conclude that diversionary feeding of foxes in spring and early summer might be a feasible management tool to increase the reproductive output in local grouse populations, but that it needs to be implemented on a larger scale in order to improve breeding numbers.

Applying science to pressing conservation needs for penguins.

More than half of the world's 18 penguin species are declining. We, the Steering Committee of the International Union for Conservation of Nature Species Survival Commission Penguin Specialist Group, determined that the penguin species in most critical need of conservation action are African penguin (Spheniscus demersus), Galápagos penguin (Spheniscus mendiculus), and Yellow‐eyed penguin (Megadyptes antipodes). Due to small or rapidly declining populations, these species require immediate scientific collaboration and policy intervention. We also used a pairwise‐ranking approach to prioritize research and conservation needs for all penguins. Among the 12 cross‐taxa research areas we identified, we ranked quantifying population trends, estimating demographic rates, forecasting environmental patterns of change, and improving the knowledge of fisheries interactions as the highest priorities. The highest ranked conservation needs were to enhance marine spatial planning, improve stakeholder engagement, and develop disaster‐management and species‐specific action plans. We concurred that, to improve the translation of science into effective conservation for penguins, the scientific community and funding bodies must recognize the importance of and support long‐term research; research on and conservation of penguins must expand its focus to include the nonbreeding season and juvenile stage; marine reserves must be designed at ecologically appropriate spatial and temporal scales; and communication between scientists and decision makers must be improved with the help of individual scientists and interdisciplinary working groups.

Review of the management of the Asian elephant Elephas maximus EEP: current challenges and future solutions

This article reviews the current situation in the Asian Elephant Elephas maximus European Association of Zoos and Aquaria Ex situ Programme (EEP). In recent years, developments in husbandry and gained knowledge about the reproductive biology of Asian elephants have contributed to increased breeding success and resulted in a mean of 15 births per year in the last 5 years. At the time of writing, the Asian elephant EEP population contains 307 individuals: 90.217 (♂♂.♀♀). Based on the life table for 1998–2018, most demographic parameters show healthy numbers [e.g. lambda (λ) = 1·025], while the population has retained 98·44% of the gene diversity. However, this EEP is also facing multiple challenges, such as the presence of subspecies, transport barriers between some EEP participants and the societal debate about the purpose of zoos. The growing number of male elephants in the EEP population appears to be the most immediate challenge. In the short term, the authors suggest that females could be managed to conceive for the first time at 8 years of age and adhere to an interbirth interval of 7 years. This would be an attempt to decrease the reproductive rate without compromising the future reproductive potential of the population. The authors also prescribe improving facilities for elephants to allow zoos to utilize a fission–fusion housing strategy, making it possible to house the increasing number of males appropriately over the longer term.

Female mate choice significantly increases captive breeding success, and scents can be frozen to determine choice, in the stripe-faced dunnart

Devising non-invasive techniques to maintain natural behaviours and increase breeding success of captive populations is a high priority in the conservation of endangered species. Allowing animals to choose their own mates not only preserves behaviours that are essential for successful release and re-establishment of wild populations, but could increase the number and genetic quality of young and genetic viability of a population. It can also decrease stress and aggression between mates and reduce the likelihood of fighting, injuries and deaths. Despite this, mate choice often remains underused in captive breeding programs. Additionally, the use of freezing and storing scents to determine a female’s mate choice between institutions has not been examined. We examined the effects of familiarity between mates, male age, preferences of relatives, and freezing scents on female mate choice and breeding success in a colony of a small marsupial, the stripe-faced dunnart (Sminthopsis macroura).Experiments were conducted using olfactory cues from males placed in the female’s enclosure. Pairings based on mate choice resulted in significantly more pregnancies per female and per oestrus cycle (p = 0.001), and a shorter time to parturition (p = 0.017), than females paired on pedigree alone, though there was no significant difference in litter sizes (p = 0.380). Familiarity between mates did not affect female preferences (P > 0.50), but significantly decreased levels of aggression and minor injuries between mates (p = 0.042). Male age similarly did not affect a female’s preference (P > 0.18). Females paired with the choice of their sisters had high breeding success (73.7%), suggesting that mate choices can be applied to related animals. Freezing a male’s scent for up to 40 days at −20 °C did not affect female choice and may be used for enabling mate preference between captive breeding institutes. The results show that female mate choice can significantly increase breeding success in a captive marsupial breeding colony and may be a powerful tool for a variety of endangered species breeding programs.

Conserving wildlife facing mass-tourism calls for effective management

Conserving wildlife facing mass-tourism calls for effective management

Endangered Père David's deer genome provides insights into population recovering.

The Milu (Père David's deer, Elaphurus davidianus) were once widely distributed in the swamps (coastal areas to inland areas) of East Asia. The dramatic recovery of the Milu population is now deemed a classic example of how highly endangered animal species can be rescued. However, the molecular mechanisms that underpinned this population recovery remain largely unknown. Here, different approaches (genome sequencing, resequencing, and salinity analysis) were utilized to elucidate the aforementioned molecular mechanisms. The comparative genomic analyses revealed that the largest recovered Milu population carries extensive genetic diversity despite an extreme population bottleneck. And the protracted inbreeding history might have facilitated the purging of deleterious recessive alleles. Seventeen genes that are putatively related to reproduction, embryonic (fatal) development, and immune response were under high selective pressure. Besides, SCNN1A, a gene involved in controlling reabsorption of sodium in the body, was positively selected. An additional 29 genes were also observed to be positively selected, which are involved in blood pressure regulation, cardiovascular development, cholesterol regulation, glycemic control, and thyroid hormone synthesis. It is possible that these genetic adaptations were required to buffer the negative effects commonly associated with a high‐salt diet. The associated genetic adaptions are likely to have enabled increased breeding success and fetal survival. The future success of Milu population management might depend on the successful reintroduction of the animal to historically important distribution regions.

Early arrival at breeding grounds: Causes, costs and a trade‐off with overwintering latitude

Early arrival at breeding grounds is of prime importance for migrating birds as it is known to enhance breeding success. Adults, males and higher quality individuals typically arrive earlier, and across years, early arrival has been linked to warmer spring temperatures. However, the mechanisms and potential costs of early arrival are not well understood. To deepen the understanding of arrival date differences between individuals and years, we studied them in light of the preceding spring migration behaviour and atmospheric conditions en route. GPS and body acceleration (ACC) data were obtained for 35 adult white storks (Ciconia ciconia) over five years (2012-2016). ACC records were translated to energy expenditure estimates (overall dynamic body acceleration; ODBA) and to behavioural modes, and GPS fixes were coupled with environmental parameters. At the interindividual level (within years), early arrival was attributed primarily to departing earlier for migration and from more northern wintering sites (closer to breeding grounds), rather than to migration speed. In fact, early-departing birds flew slower, experienced weaker thermal uplifts and expended more energy during flight, but still arrived earlier, emphasizing the cost and the significance of early departure. Individuals that wintered further south arrived later at the breeding grounds but did not produce fewer fledglings, presumably due to positive carry-over effects of advantageous wintering conditions (increased precipitation, vegetation productivity and daylight time). Therefore, early arrival increased breeding success only after controlling for wintering latitude. Males arrived slightly ahead of females. Between years, late arrival was linked to colder temperatures en route through two different mechanisms: stronger headwinds causing slower migration and lower thermal uplifts resulting in longer stopovers. This study showed that distinct migratory properties underlie arrival time variation within and between years. It highlighted (a) an overlooked cost of early arrival induced by unfavourable atmospheric conditions during migration, (b) an important fitness trade-off in storks between arrival date and wintering habitat quality and (c) mechanistic explanations for the negative temperature-arrival date correlation in soaring birds. Such understanding of arrival time can facilitate forecasting migrating species responses to climate changes.

Predation and food–weather interactions drive colony collapse in a managed metapopulation of Arctic Terns (Sterna paradisaea)

Seabirds are considered bioindicators of bottom-up ecosystem processes, owing to seabirds’ dependence on marine prey. However, ground-nesting seabirds are susceptible to predation, which can limit their use as bioindicators. Machias Seal Island (MSI) supported the largest colony of Arctic Terns (Sterna paradisaea Pontoppidan, 1763) in the Gulf of Maine metapopulation, but prolonged breeding failure led ∼90% of terns to abandon the colony in 2006. We analyzed 12 years of food, weather, and predation data using logistic regression models to determine which had the strongest influence on breeding success. Food–weather interactions were important; under low rainfall, more euphausiids (northern krill, Meganyctiphanes norvegica (M. Sars, 1857)) in the diet increased breeding success, but euphausiids had a negative effect as rainfall became moderate or high. Predation by Herring Gulls (Larus argentatus Pontoppidan, 1763) increased following the cessation of lethal predator control; we identified a predation threshold of 25%, beyond which terns could not breed successfully. The collapse of MSI’s tern colony can be attributed entirely to gull predation. The breeding success of terns at MSI cannot be used as a bottom-up ecosystem bioindicator without accounting for predation. Managers of ground-nesting seabirds should consider predation and food as equally valid potential causes of population or reproductive declines.

Not much ado about something: behavioural mechanisms of pair bond separation and formation in long-term pairing rooks

ABSTRACTIn birds, long-term pairing often comes with increased breeding success, but also with strong and intricate relationships associated with social benefits, resulting in a lower likelihood of divorce. However, a relationship may wither and the temptation of new partnership opportunities can lead to divorces and re-pairing, especially in social species, like rooks, who live all year round in colonies. Here, we scrutinized the behavioural patterns leading to divorce and pair formation by adulthood in a group of captive rooks. Divorces were concomitant to the formation of new pairs, with one or both separating partners gradually switching to another relationship. Individuals switching from one pair to another were socially less active than their non-switching partner (the old and new). Once established, newly formed pairs were immediately as strong as older stable pairs. Separating pairs were characterized by high rates of agonistic behaviours. Food sharing played a role in the formation of new pairs, while sexual behaviours did not. We showed that adult rooks do not necessarily pair for life, and have the ability to strongly re-bond with another partner. This emphasizes the social flexibility of this cognitively advanced species, and sheds new light on mate choice strategies in long-term monogamous species.

Supplementary feeding increases nestling feather corticosterone early in the breeding season in house sparrows.

Several studies on birds have proposed that a lack of invertebrate prey in urbanized areas could be the main cause for generally lower levels of breeding success compared to rural habitats. Previous work on house sparrows Passer domesticus found that supplemental feeding in urbanized areas increased breeding success but did not contribute to population growth. Here, we hypothesize that supplementary feeding allows house sparrows to achieve higher breeding success but at the cost of lower nestling quality. As abundant food supplies may permit both high‐ and low‐quality nestlings to survive, we also predict that within‐brood variation in proxies of nestling quality would be larger for supplemental food broods than for unfed broods. As proxies of nestling quality, we considered feather corticosterone (CORT f), body condition (scaled mass index, SMI), and tarsus‐based fluctuating asymmetry (FA). Our hypothesis was only partially supported as we did not find an overall effect of food supplementation on FA or SMI. Rather, food supplementation affected nestling phenotype only early in the breeding season in terms of elevated CORT f levels and a tendency for more variable within‐brood CORT f and FA. Early food supplemented nests therefore seemed to include at least some nestlings that faced increased stressors during development, possibly due to harsher environmental (e.g., related to food and temperature) conditions early in the breeding season that would increase sibling competition, especially in larger broods. The fact that CORT f was positively, rather than inversely, related to nestling SMI further suggests that factors influencing CORT f and SMI are likely operating over different periods or, alternatively, that nestlings in good nutritional condition also invest in high‐quality feathers.

Does rotational heather burning increase red grouse abundance and breeding success on moors in northern England?

Rotational burning of moorland vegetation is considered by grouse moor managers to create structurally diverse habitat that helps maximise red grouse Lagopus lagopus scotica (Lath.) abundance for shooting by increasing breeding success and adult survival. However, there is growing concern that increasing levels of burning, particularly on deep peat, are impacting upon upland ecosystems. We used linear mixed models to examine the influence of burning on heather Calluna vulgaris (L.) height and structure, and on grouse density and breeding success, at 36 moors in northern England which practiced driven grouse shooting and employed full-time gamekeepers to burn heather, but also to control generalist predators of grouse and their parasitic nematode worms. Variation in heather height was positively associated with burning extent on moors and the relationship between heather height and burning was similar on moors of deep (blanket bog) and shallow peat (heath). Grouse pre-breeding density was not related to burning extent, but breeding success and post-breeding density were positively associated with extent of burning on moors. Relationships between grouse and burning were similar on heath and blanket bog. Higher grouse breeding success and post-breeding density were likely to be associated with a more varied vegetation structure following burning. Rotational burning is associated with higher grouse breeding success and post-breeding densities beneficial for driven grouse shooting. Thus a conflict may exist between rotational burning to benefit grouse and potential impacts of burning on ecosystem services such as carbon storage and flood protection. Potential benefits of burning for increasing grouse breeding success and post-breeding density need to be considered carefully against any likely impacts on ecosystem services, particularly in areas of blanket peat.

Fathers matter: male body mass affects life-history traits in a size-dimorphic seabird.

One of the predicted consequences of climate change is a shift in body mass distributions within animal populations. Yet body mass, an important component of the physiological state of an organism, can affect key life-history traits and consequently population dynamics. Over the past decades, the wandering albatross—a pelagic seabird providing bi-parental care with marked sexual size dimorphism—has exhibited an increase in average body mass and breeding success in parallel with experiencing increasing wind speeds. To assess the impact of these changes, we examined how body mass affects five key life-history traits at the individual level: adult survival, breeding probability, breeding success, chick mass and juvenile survival. We found that male mass impacted all traits examined except breeding probability, whereas female mass affected none. Adult male survival increased with increasing mass. Increasing adult male mass increased breeding success and mass of sons but not of daughters. Juvenile male survival increased with their chick mass. These results suggest that a higher investment in sons by fathers can increase their inclusive fitness, which is not the case for daughters. Our study highlights sex-specific differences in the effect of body mass on the life history of a monogamous species with bi-parental care.