A Physiological Signature of the Cost of Reproduction Associated with Parental Care

Individual quality overwrites carry-over effects across the annual cycle of a long-distance migrant

In seasonal environments, the fitness of animals depends upon the successful integration of life-history stages throughout their annual cycle. Failing to do so can lead to negative carry-over effects where individuals are transitioning into the next season in different states, consequently affecting their future performance. However, carry-over effects can be masked by individual quality when individuals vary in their efficiency at acquiring resources year after year (i.e. 'quality'), leading to cross-seasonal consistency in individual performance. Here we investigated the relative importance of carry-over effects and individual quality in determining cross-seasonal interactions and consequences for breeding success over the full annual cycle of a migratory seabird (black-legged kittiwake Rissa tridactyla). We monitored the reproduction and annual movement of kittiwakes over 13 years using geolocators to estimate their breeding success, distribution and winter energy expenditure. We combined this with an experimental approach (clutch removal experiment, 2 years) to manipulate the reproductive effort irrespective of individual quality. Piecewise path analyses showed that successful breeders reproduced earlier and were more likely to breed successfully again the following year. This positive interaction among consecutive breeding stages disappeared after controlling for individual quality, suggesting that quality was dominant in determining seasonal interactions. Moreover, controlling experimentally for individual quality revealed underlying carry-over effects that were otherwise masked by quality, with breeding costs paid in higher energy expenditure and delayed onset of reproduction. We highlight the need to combine an experimental approach along with long-term data while assessing apparent carry-over effects in wild animals, and their potential impact on fitness and population demography.

Sex-specific effects of dietary restriction on physiological variables in Japanese quails

Nutritional limitation is a common phenomenon in nature that leads to trade-offs among processes competing for limited resources. These trade-offs are mediated by changes in physiological traits such as growth factors and circulating lipids. However, studies addressing the sex-specific effect of nutritional deficiency on these physiological variables are limited in birds. We used dietary restriction to mimic the depletion of resources to various degrees and investigated sex-specific effects on circulating levels of insulin-like growth factor 1 (IGF-1) and triglycerides in Japanese quails (Coturnix japonica) subjected to ad libitum, 20%, 30% or 40% restriction of their daily requirement, for 2 weeks. We also explored the association of both physiological variables with body mass and egg production. While dietary restriction showed no effects on circulating IGF-1, this hormone exhibited a marked sexual difference, with females having 64.7% higher IGF-1 levels than males. Dietary restriction significantly reduced plasma triglyceride levels in both sexes. Females showed more than six-fold higher triglyceride levels than males. Triglyceride levels were positively associated with body mass in females while showed not association in males. Overall, our findings revealed sex-specific expression of physiological variables under dietary restriction conditions, which coincide with body size.

Individual Optimization of Reproductive Investment and the Cost of Incubation in a Wild Songbird

AbstractDespite avid interest in life history trade-offs and the costs of reproduction, evidence that increased parental allocation reduces subsequent breeding productivity is mixed. This uncertainty may be attributable to environmental heterogeneity in space and time, necessitating experiments across a range of ecological contexts. Over three breeding seasons, we cross-fostered clutches between nests to manipulate incubation duration in a wild population of Carolina wrens, a species in which only females incubate, to test for a cost of incubation on current and future reproduction. Prolonged incubation affected maternal productivity in a manner dependent on the current environment and initial investment in eggs, suggesting that incubation is optimized according to other components of reproduction and individual quality. Effects of incubation duration on foster nestling condition varied between years, being costly in one, beneficial in another, and neutral in the third. The proportion of young fledged, females' probability of breeding again within seasons, and subsequent clutch sizes all declined with increasing incubation effort-effects that became more pronounced as seasons progressed. Therefore, costs of incubation were almost entirely dependent on maternal quality and environmental variation, illustrating the importance of conducting experiments across a range of environmental settings for understanding the costs of reproduction and evolution of life histories.

Phenotypic plasticity in mass loss during chick rearing in the European starling (Sturnus vulgaris)

It has long been recognized that mass loss during breeding could be adaptive (e.g., by ameliorating the costs of increased parental activity). However, many studies still commonly interpret mass loss as evidence of "stress" or a cost of reproduction (i.e., a negative effect of high workload during chick provisioning). Despite several studies reporting evidence in support of both hypotheses, the ecological and/or life-history contexts under which mass loss may be viewed as a "cost" or an adaptive strategy are still unclear. Here, we used a long-term dataset from a breeding population of European starlings (Sturnus vulgaris) to investigate natural annual and individual variation in body mass and mass loss and to test whether mass loss during chick rearing represents a phenotypically plastic trait that varies predictably in relation to ecological context and individual quality. While there was significant annual variation in incubation mass, chick-rearing mass, and mass change, there were no systematic relationships between mass loss and current breeding success or future fecundity and survival. In addition, we found no evidence of intra-annual repeatability of mass loss between first and second broods ( = .00) but moderate interannual repeatability of mass loss (R = .61) during first broods, suggesting differences in mass loss under different selective pressures. However, we found no covariation between residual intra-individual variation in mass loss for first broods and other reproductive or life-history traits. We therefore found no support for the idea that mass loss reflects "reproductive stress" in our system: there were no negative relationships between mass loss and either current or future reproduction and survival (local return rate). Our results are consistent with mass loss being an individually plastic trait, with individuals using mass loss to "level the playing field" and individually optimize reproductive effort and fitness within their specific ecological context and relative to their individual quality for a given breeding attempt.

Cooperative breeding alters physiological and behavioral responses to habitat fragmentation

Animals respond to habitat alteration with changes in their behavior and physiology. These changes determine individual performance and thus precede changes in population size. They are therefore hypothesized to provide important insights into how animals cope with environmental change. Here, we investigated physiological and behavioral responses of a cooperatively breeding bird, the placid greenbul (Phyllastrephus placidus), in a severely fragmented tropical biodiversity hotspot and combined these data with remotely sensed (LiDAR) environmental data. We found that individuals had increased glucocorticoid hormone levels when breeding in territories with low native canopy cover or located within small fragments. However, when breeding with the help of subordinates, breeders in low quality territories had similar glucocorticoid levels as those in higher quality territories. Our study shows that sociality may impact how well animals cope with environmental change and contributes to our understanding of the role of glucocorticoid physiology and behavior in response to anthropogenic change.

Mitochondrial metabolism in blood more reliably predicts whole-animal energy needs compared to other tissues

Understanding energy metabolism in free-ranging animals is crucial for ecological studies. In birds, red blood cells (RBCs) offer a minimally invasive method to estimate metabolic rate (MR). In this study with European starlings Sturnus vulgaris, we examined how RBC oxygen consumption relates to oxygen use in key tissues (brain, liver, heart, and pectoral muscle) and versus the whole organism measured at basal levels. The pectoral muscle accounted for 34%-42% of organismal MR, while the heart and liver, despite their high mass-specific metabolic rate, each contributed 2.5%-3.0% to organismal MR. Despite its low contribution to organismal MR (0.03%-0.04%), RBC MR best predicted organismal MR (r = 0.70). Oxygen consumption of the brain and pectoralis was also associated with whole-organism MR, unlike that of heart and liver. Overall, our findings demonstrate that the metabolism of a systemic tissue like blood is a superior proxy for organismal energy metabolism than that of other tissues.

Under pressure-exploring partner changes, physiological responses and telomere dynamics in northern gannets across varying breeding conditions

Background

Life history theory predicts trade-offs between reproduction and survival in species like the northern gannet (Morus bassanus). During breeding, demanding foraging conditions lead them to expand their foraging range and diversify their diet, increasing the risk of reproductive failure. Changing partners may enhance breeding success but lead to more physiological costs.

Methods

To investigate the physiological costs of reproduction upon partner changes, we measured and compared 21 biomarkers related to telomere dynamics, oxidative stress, inflammation, hematology, nutritional status, and muscle damage. We used a longitudinal approach with gannets (n = 38) over three contrasting years (2017, 2018 and 2019).

Results

Our results suggest that annual breeding conditions exert a greater influence on physiological changes than partnership status. Individuals that changed partner experienced greater short-term stress than retained partners. This transient increase in stress was marked by short-term increases in oxidative lipid damage, lower antioxidant capacity, signs of inflammation, and greater weight loss than individuals that retained partners. During favorable conditions, individuals that changed mates had stabilized telomere length, decreased antioxidant capacity, glucose concentration, and muscle damage, along with increased oxygen transport capacity. Conversely, unfavorable breeding conditions led to increased telomere attrition, stabilized antioxidant capacity, decreased inflammation susceptibility, diminished oxygen transport capacity, and increased muscle damage. In the cases where partners were retained, distinct physiological changes were observed depending on the year's conditions, yet the telomere dynamics remained consistent across both partnership status categories. During the favorable year, there was an increase in unsaturated fatty acids and oxygen transport capacity in the blood, coupled with a reduction in inflammation potential and protein catabolism. In contrast, during the unfavorable year in the retained mates, we observed an increase in oxidative DNA damage, antioxidant capacity, weight loss, but a decrease in inflammation susceptibility as observed in changed mates.

Discussion

Our study shows that behavioral flexibility such as mate switching can help seabirds cope with the challenges of food scarcity during reproduction, but these coping strategies may have a negative impact on physiological status at the individual level. In addition, the marked reduction in telomere length observed during harsh conditions, coupled with the stabilization of telomere length in favorable conditions, highlights the long-term physiological impact of annual breeding conditions on seabirds. These findings underscore the effect on their potential survival and fitness, emphasizing that the influence of annual breeding conditions is greater than that of partnership status.

Oxidative physiology of two small and highly migratory Arctic seabirds: Arctic terns (Sterna paradisaea) and long-tailed jaegers (Stercorarius longicaudus)

Arctic ecosystems are changing rapidly. The tundra supports nesting migratory seabirds that spend most of their year over the ocean. Migrations are demanding, but it is unclear how physiological capability may equip organisms to respond to their changing environments. For two migratory seabird species nesting in Alaska, USA, the Arctic tern (n = 10) and the long-tailed jaeger (n = 8), we compared oxidative physiology and aerobic capacity measured during incubation and we recorded individual movement paths using electronic tracking tags. Within species, we hypothesized that individuals with longer-distance migrations would show higher oxidative stress and display better aerobic capacity than shorter-distance migrants. We examined blood parameters relative to subsequent fall migration in jaegers and relative to previous spring migration in terns. We present the first measurements of oxidative stress in these species and the first migratory movements of long-tailed jaegers in the Pacific Ocean. Arctic terns displayed positive correlation of oxidative variables, or better integration than jaegers. Relative to physiological sampling, pre-breeding northward migration data were available for terns and post-breeding southward data were available for jaegers. Terns reached a farther maximum distance from the colony than jaegers (16 199 ± 275 km versus 10 947 ± 950 km) and rate of travel northward (447 ± 41.8 km/day) was positively correlated with hematocrit, but we found no other relationships. In jaegers, there were no relationships between individuals' physiology and southward rate of travel (193 ± 52.3 km/day) or migratory distance. While it is not clear whether the much longer migrations of the terns is related to their better integration, or to another factor, our results spark hypotheses that could be evaluated through a controlled phylogenetic study. Species with better integration may be less susceptible to environmental factors that increase oxidative stress, including thermal challenges or changes in prey distribution as the Arctic climate changes rapidly.

Riskiness of Movement Lifestyle Varies Inversely with Adult Survival Probability among Species

AbstractWhy do species differ in their movement lifestyles? Animals that spend more time sitting motionless and acquire food using less conspicuous movements can be more vigilant and less obvious to predators. More active animals that use food types and sites that require more conspicuous behaviors increase vulnerability to predators. Life history theory predicts that aversiveness to mortality risk evolves inversely to adult survival probability. Consequently, we postulated that long-lived species evolved inconspicuous movement lifestyles, whereas shorter-lived species use more conspicuous movement lifestyles. We tested this hypothesis by quantifying the movement lifestyles of nine tropical songbird species. Use of conspicuous movement and foraging behaviors, such as flying and hovering, was greatest in shorter-lived species and decreased with increasing adult survival probability across species. Similarly, foraging speed decreased with increasing adult survival based on a meta-analysis of 64 songbird species. Faster and conspicuous movement lifestyles of shorter-lived species likely increase food acquisition rates, which fits with faster life history strategies that include more feeding trips for young and faster growth. Similarly, slow movement lifestyles of long-lived species fit with the reduced food needs of slower life history strategies. Movement lifestyles may have evolved as an integrated component of the slow-fast life history continuum.

Females with Increased Costs Maintain Reproductive Output: A Field Experiment in a Common Songbird

Reproduction and self-maintenance are energetically costly activities involved in classic life history trade-offs. However, few studies have measured the responses of wild organisms to simultaneous changes in reproductive and self-maintenance costs, which may have interactive effects. In free-living female Barn Swallows (Hirundo rustica), we simultaneously manipulated reproductive costs (by adding or removing two nestlings) and self-maintenance costs (by attaching a ∼1 g weight in the form of a GPS tag to half of our study birds) and measured mass, immune status, blood glucose, feather growth, and reproductive output (likelihood of a second clutch, number of eggs, and time between clutches). GPS tags allowed us to analyze how movement range size affected response to brood size manipulation. Tagging altered females' immune function as evidenced by an elevated heterophil to lymphocyte (H:L) ratio, but all females were equally likely to lay more eggs. There was no evidence of interactive effects of the tagging and brood size treatment. Range size was highly variable, and birds with large ranges grew feathers more slowly, but analyzing the effect of brood size manipulation while accounting for variation in range size did not result in any physiological response. Our results support the theoretical prediction that short-lived vertebrates do face a trade-off between reproduction and self-maintenance and, when faced with increased costs, tend to preserve investment in reproduction at the expense of parental condition. This experiment also helps us to understand how movement patterns may be relevant to life history trade-offs in wild birds.

Reduced habitat quality increases intrinsic but not ecological costs of reproduction

Although the costs of reproduction are predicted to vary with the quality of the breeding habitat thereby affecting population dynamics and life‐history trade‐offs, empirical evidence for this pattern remains sparse and equivocal. Costs of reproduction can operate through immediate ecological mechanisms or through delayed intrinsic mechanisms. Ignoring these separate pathways might hinder the identification of costs and the understanding of their consequences. We experimentally investigated the survival costs of reproduction for adult little owls (Athene noctua) within a gradient of habitat quality. We supplemented food to nestlings, thereby relieving the parents’ effort for brood provisioning. We used radio‐tracking and Bayesian multistate modeling based on marked recapture and dead recovery to estimate survival rates of adult little owls across the year as a function of food supplementation and habitat characteristics. Food supplementation to nestlings during the breeding season increased parental survival not only during the breeding season but also during the rest of the year. Thus, the low survival of parents of unfed broods likely represents both, strong ecological and strong intrinsic costs of reproduction. However, while immediate ecological costs occurred also in high‐quality habitats, intrinsic costs carrying over to the post‐breeding period occurred only in low‐quality habitats. Our results suggest that immediate costs resulting from ecological mechanisms such as predation, are high also in territories of high habitat quality. Long‐term costs resulting from intrinsic trade‐offs, however, are only paid in low‐quality habitats. Consequently, differential effects of habitat quality on immediate ecological and delayed intrinsic mechanisms can mask the increase of costs of reproduction in low‐quality breeding habitats. Intrinsic costs may represent an underrated mechanism of habitat quality affecting adult survival rate thereby considerably accelerating population decline in degrading habitats. This study therefore highlights the need for a long‐term perspective to fully assess the costs of reproduction and the role of habitat quality in modifying these costs.

Correlated evolution of parental care with dichromatism, colors, and patterns in anurans

Parental care is widespread and has fitness benefits. But caregiving parents incur costs including higher predation, and this may lead to selection for body colors or patterns that help mitigate the risks of caring. The evolution of coloration, including sexual dichromatism, however, can be driven by other factors, such as sexual selection. Therefore, examining the associations between parental care and color patterns may provide key insights into evolutionary patterns and selection pressures for parental care. Our comparative analysis of 988 anuran species reveals that dichromatic species are less likely to provide parental care, irrespective of the caregiving sex, and are more likely to breed in aquatic habitats. We then examined whether dorsal colors and patterns that enhance crypticity or function as aposematic signals are associated with the caregiving sex, and the modality of care (transport or stationary). Only caregiving males are more likely to have dorsal Stripes, but none of the colors (Green-Brown, Red, Yellow, Blue-Black) and other patterns (Plain, Bands, Spots, Mottled-Patches) were associated with caregiving females or the modality of care. Overall, sexual dichromatism, breeding ecology, and parental care are associated, but the evolution of caregiving behavior does not appear to influence the myriad colors and patterns characteristic of anurans globally.

Varying aerobic capacity in relation to breeding stage and reproductive success in giant petrels (Macronectes spp.)

Reproduction, and parental care in particular, are among the most energy-demanding activities within the annual cycle of adult birds. Parents that cannot meet the metabolic demands and other physiological costs of raising offspring may opt to abandon chicks in favour of self-maintenance and future reproduction. Recent work examining reproductive trade-offs in birds revealed an important role of oxygen carrying capacity in mediating variation in parental effort. This study explores the aerobic factors underlying the success or failure of parental care in two closely-related petrel species during their breeding season on Bird Island, South Georgia: northern giant petrels (Macronectes halli) and southern giant petrels (M. giganteus). Failed breeders of both sexes and species had significantly lower hematocrit levels (by 5.48 ± 0.64%) than successful breeders, and reticulocyte counts also tended to be lower in failed males, consistent with the hypothesis that parental care and workload depend on aerobic capacity. We discuss these results in relation to differences in the foraging ecology of both species and sexes.

Physiological adjustments to high foraging effort negatively affect fecundity but not final reproductive output in captive zebra finches

Foraging at elevated rates to provision offspring is thought to be an energetically costly activity and it has been suggested that there are physiological costs associated with the high workload involved. However, for the most part, evidence for costs of increased foraging and/or reproductive effort is weak. Furthermore, despite some experimental evidence demonstrating negative effects of increased foraging and parental effort, the physiological mechanisms underlying costs associated with high workload remain poorly understood. To examine how high workload affects haematology, oxidative stress and reproductive output, we experimentally manipulated foraging effort in captive zebra finches, Taeniopygia guttata, using a previously described technique, and allowed individuals to breed first in low foraging effort conditions and then in high foraging effort conditions. We found that birds upregulated haematocrit and haemoglobin concentration in response to training. Birds subjected to increased workload during reproduction had lower fecundity, although final reproductive output was not significantly different than that of controls. Offspring of parents subjected to high workload during reproduction also had higher oxidative stress when they were 90 days of age. Total antioxidant capacity and reactive oxygen metabolites of birds responded differently in the two breeding attempts, but we did detect an overall increase in oxidative stress in response to training in either attempt, which could explain the lower fecundity observed in birds subjected to increased workload during reproduction.

Sex-specific energy management strategies in response to training for increased foraging effort prior to reproduction in captive zebra finches

Free-living animals often engage in behaviour that involves high rates of workload and results in high daily energy expenditure (DEE), such as reproduction. However, the evidence for elevated DEE accompanying reproduction remains equivocal. In fact, many studies have found no difference in DEE between reproducing and non-reproducing females. One of the hypotheses explaining the lack of difference is the concept of an 'energetic ceiling'. However, it is unclear whether the lack of increase in energy expenditure is due to the existence of an energetic ceiling and/or compensation by males during parental care. To investigate whether an energetic ceiling exists, we experimentally manipulated foraging effort in captive zebra finches, Taeniopygia guttata, creating two groups with high and low foraging efforts followed by both groups breeding in the low foraging effort common garden condition. DEE was measured in both sexes throughout the experiment. We show sex-specific energy management strategies in response to training for increased foraging effort prior to reproduction. Specifically, males and females responded differently to the high foraging effort treatment and subsequently to chick rearing in terms of energy expenditure. Our results also suggest that there is an energetic ceiling in females and that energetic costs incurred prior to reproduction can be carried over into subsequent stages of reproduction in a sex-specific manner.

Female birds monitor the activity of their mates while brooding nest-bound young

In addition to food and protection, altricial young in many species are ectothermic and require that endothermic parents provide warmth to foster growth, yet only one parent-typically the female-broods these young to keep them warm. When this occurs, reduced provisioning by males obliges females to forage instead of providing warmth for offspring, favoring the temporal mapping of male activities. We assessed this in a wild house wren population while experimentally feeding nestlings to control offspring satiety. While brooding, females look out from the nest to inspect their surroundings, and we hypothesized that this helps to determine if their mate is nearby and likely to deliver food to the brood (males pass food to brooding females, which pass the food to nestlings). Females looked out from the nest less often when their partner was singing nearby and when his singing and provisioning were temporally linked, signaling his impending food delivery. Females also left to forage less often when their mate was nearby and likely to deliver food. Nestling begging did not affect these behaviors. Females looking out from the nest more often also provisioned at a higher rate and were more likely to divorce and find a new mate prior to nesting again within seasons, as expected if females switch mates when a male fails to meet expectations. Our results suggest anticipatory effects generated by male behavior and that brooding females temporally map male activity to inform decisions about whether to continue brooding or to leave the nest to forage.

Parasitism by an invasive nest fly reduces future reproduction in Galápagos mockingbirds

Organisms allocate limited resources to competing activities such as reproduction, growth, and defense against parasites and predators. The introduction of a novel parasite may create new life history trade-offs. As hosts increase their investment in self-maintenance or defense, the cost of parasitism may carry over to other aspects of host biology. Here, in an experimental field study, we document delayed effects of an introduced nest parasite, Philornis downsi, on reproduction of Galápagos mockingbirds (Mimus parvulus). Parasitism of first nests reduced both the number and size of chicks that parents hatched when they re-nested several weeks later. The delayed effect of P. downsi on future reproduction may have been mediated by behavioral shifts by the parents to avoid or resist parasitism. Our results demonstrate that effects of parasitism can persist even after immediate exposure ends. We draw attention to the potential implications that introduced parasites have for host reproductive strategies.

Oxidative status and telomere length are related to somatic and physiological maturation in chicks of European starlings (Sturnus vulgaris)

Telomere length can be considered as an indicator of an organism's somatic state, long telomeres reflecting higher energy investment in self-maintenance. Early-life is a period of intense investment in somatic growth and in physiological maturation but how this is reflected in telomere length remains unclear. Using European starling chicks we tested: (i) how telomere length measured at asymptotic mass is related to proxies of somatic growth and physiological maturity in 17-day-old nestlings; (ii) how telomere length measured at 17 days then predicts the changes in somatic and physiological maturity occurring in fledglings (between 17 and 21 days); (iii) how growth and telomere length co-vary when chicks are under experimentally good (fed) growth conditions. Depending on environmental conditions, our data suggest links between somatic growth, physiological maturation and body maintenance parameters (positive with oxidative stress and negative with telomere length) in nestlings. Telomere length measured at day 17 predicted a subsequent change in physiological maturation variables observed in fledglings, but only in second-brood chicks: chicks with shorter telomeres had a higher pre-fledging rate of increase in haematocrit and haemoglobin content and a greater decrease in reticulocyte count. Finally, food supplementation of chicks did not change telomere length compared with that in control siblings. Our results suggest that physiological maturation prior to fledging may occur at the expense of telomere length but only when environmental conditions are sub-optimal.

Reproduction and maternal care increase oxidative stress in a mouthbrooding cichlid fish

Investment in reproduction and postzygotic parental care is an energetically costly yet fundamental aspect of the life-history strategies in many species. Recently, oxidative stress has received attention as a potential mediator in the trade-off between reproduction, growth, and survival. During activities that increase metabolic activity, such as providing offspring care, an overproduction of reactive oxygen species can occur that cannot be counteracted by antioxidants, leading to oxidative stress and tissue damage. Here, we investigated the oxidative costs of reproduction and maternal care over the course of the reproductive cycle in a mouthbrooding cichlid fish within socially stable and unstable environments. We manipulated social stability by disrupting the habitat in socially unstable tanks. We expected to see an increase in the burden of maternal care within unstable environments due to increased male harassment of females as a byproduct of increased male–male aggression. We found that brooding females have higher levels of oxidative stress than nonbrooding females and oxidative stress fluctuates throughout the reproductive cycle. These fluctuations were driven by a spike in reactive oxygen metabolites at the beginning of brood care followed by an increase in antioxidant defense. Surprisingly, the link between reproduction and oxidative stress was not different between females from stable or unstable environments. Our study illustrates a more complete picture of the physiological costs of reproduction and parental care throughout different stages of care rather than a simplistic end-point observation of how reproduction and parental care affect an individual.

Senescence and costs of reproduction in the life history of a small precocial species

Species following a fast life history are expected to express fitness costs mainly as increased mortality, while slow-lived species should suffer fertility costs. Because observational studies have limited power to disentangle intrinsic and extrinsic factors influencing senescence, we manipulated reproductive effort experimentally in the cavy (Cavia aperea) which produces extremely precocial young. We created two experimental groups: One was allowed continuous reproduction (CR) and the other intermittent reproduction (IR) by removing males at regular intervals. We predicted that the CR females should senesce (and die) earlier and produce either fewer and/or smaller, slower growing offspring per litter than those of the IR group. CR females had 16% more litters during three years than IR females. CR females increased mass and body condition more steeply and both remained higher until the experiment ended. Female survival showed no group difference. Reproductive senescence in litter size, litter mass, and reproductive effort (litter mass/maternal mass) began after about 600 days and was slightly stronger in CR than IR females. Litter size, litter mass, and offspring survival declined with maternal age and were influenced by seasonality. IR females decreased reproductive effort less during cold seasons and only at higher age than CR females. Nevertheless, offspring winter mortality was higher in IR females. Our results show small costs of reproduction despite high reproductive effort, suggesting that under ad libitum food conditions costs depend largely on internal regulation of allocation decisions.

Physiological predictors of reproductive performance in the European Starling (Sturnus vulgaris)

Background

It is widely assumed that variation in fitness components has a physiological basis that might underlie selection on trade-offs, but the mechanisms driving decreased survival and future fecundity remain elusive. Here, we assessed whether physiological variables are related to workload ability or immediate fitness consequences and if they mediate future survival or reproductive success. We used data on 13 physiological variables measured in 93 female European starlings (Sturnus vulgaris) at two breeding stages (incubation, chick-rearing), for first-and second-broods over two years (152 observations).

Results

There was little co-variation among the physiological variables, either in incubating or chick-rearing birds, but some systematic physiological differences between the two stages. Chick-rearing birds had lower hematocrit and plasma creatine kinase but higher hemoglobin, triglyceride and uric acid levels. Only plasma corticosterone was repeatable between incubation and chick-rearing. We assessed relationships between incubation or chick-rearing physiology and measures of workload, current productivity, future fecundity or survival in a univariate manner, and found very few significant relationships. Thus, we next explored the utility of multivariate analysis (principal components analysis, Mahalanobis distance) to account for potentially complex physiological integration, but still found no clear associations.

Conclusions

This implies either that a) birds maintained physiological variables within a homeostatic range that did not affect their performance, b) there are relatively few links between physiology and performance, or, more likely, c) that the complexity of these relationships exceeds our ability to measure it. Variability in ecological context may complicate the relationship between physiology and behavior. We thus urge caution regarding the over-interpretation of isolated significant findings, based on single traits in single years, in the literature.

Electronic supplementary material

The online version of this article (10.1186/s12983-018-0288-3) contains supplementary material, which is available to authorized users.

Enzymatic antioxidants but not baseline glucocorticoids mediate the reproduction-survival trade-off in a wild bird

The trade-off between reproductive investment and survival is central to life-history theory, but the relative importance and the complex interactions among the physiological mechanisms mediating it are still debated. Here we experimentally tested whether baseline glucocorticoid hormones, the redox system or their interaction mediate reproductive investment-survival trade-offs in wild great tits (Parus major). We increased the workload of parental males by clipping three feathers on each wing, and 5 days later determined effects on baseline corticosterone concentrations (Cort), redox state (reactive oxygen metabolites, protein carbonyls, glutathione peroxidase [GPx], total non-enzymatic antioxidants), body mass, body condition, reproductive success and survival. Feather-clipping did not affect fledgling numbers, chick body condition, nest provisioning rates or survival compared with controls. However, feather-clipped males lost mass and increased both Cort and GPx concentrations. Within feather-clipped individuals, GPx increases were positively associated with reproductive investment (i.e. male nest provisioning). Furthermore, within all individuals, males that increased GPx suffered reduced survival rates. Baseline Cort increases were related to mass loss but not to redox state, nest provisioning or male survival. Our findings provide experimental evidence that changes in the redox system are associated with the trade-off between reproductive investment and survival, while baseline Cort may support this trade-off indirectly through a link with body condition. These results also emphasize that plastic changes in individuals, rather than static levels of physiological signals, may mediate life-history trade-offs.

Immune function and blood parasite infections impact stopover ecology in passerine birds

Stopovers play a crucial role for the success of migrating animals and are key to optimal migration theory. Variation in refuelling rates, stopover duration and departure decisions among individuals has been related to several external factors. The physiological mechanisms shaping stopover ecology are, however, less well understood. Here, we explore how immune function and blood parasite infections relate to several aspects of stopover behaviour in autumn migrating short- and long-distance migrating songbirds. We blood sampled individuals of six species and used an automated radio-telemetry system in the stopover area to subsequently quantify stopover duration, 'bush-level' activity patterns (~ 0.1-30 m), landscape movements (~ 30-6000 m), departure direction and departure time. We show that complement activity, the acute phase protein haptoglobin and blood parasite infections were related to prolonged stopover duration. Complement activity (i.e., lysis) and total immunoglobulins were negatively correlated with bush-level activity patterns. The differences partly depended on whether birds were long-distance or short-distance migrants. Birds infected with avian malaria-like parasites showed longer landscape movements during the stopover than uninfected individuals, and birds with double blood parasite infections departed more than 2.5 h later after sunset/sunrise suggesting shorter flight bouts. We conclude that variation in baseline immune function and blood parasite infection status affects stopover ecology and helps explain individual variation in stopover behaviour. These differences affect overall migration speed, and thus can have significant impact on migration success and induce carry-over effects on other annual-cycle stages. Immune function and blood parasites should, therefore, be considered as important factors when applying optimal migration theory.

Physiology, activity and costs of parental care in birds

Parental care is assumed to be costly in that it requires sustained, high-intensity activity sufficient to cause costs of reproduction (decreased survival and future fecundity of parents). Costs of reproduction are, in turn, thought to have a physiological basis where intense activity causes a decrease in parental condition. However, attempts to identify the physiological basis of costs of reproduction have produced mixed results. Here, I argue that in birds, the central idea that parental care represents sustained, high-intensity work might be incorrect. Specifically: (a) the duration of intense activity associated with chick-rearing might be quite limited; (b) flight, the most obvious sustained, high-intensity activity, might only represent a small component of an individual's overall activity budget; (c) some (high-quality) individuals might be able to tolerate costs of intense activity, either owing to their physiological state or because they have access to more resources, without perturbation of physiological homeostasis; and (d) individuals might utilise other mechanisms to modulate costs of activity, for example, mass loss, again avoiding more substantial physiological costs. Furthermore, I highlight the important fact that life-history theory predicts that reproductive trade-offs should only be expected under food stress. Most birds breed in spring and early summer precisely because of seasonal increases in food abundance, and so it is unclear how often parents are food stressed. Consequently, I argue that there are many reasons why costs of reproduction, and any physiological signature of these costs, might be quite rare, both temporally (in different years) and among individuals.

The oxidative costs of parental care in cooperative and pair-breeding African starlings

The cost of parental care has long been thought to favor the evolution of cooperative breeding, because breeders can provide reduced parental care when aided by alloparents. Oxidative stress-the imbalance between reactive oxygen species and neutralizing antioxidants-has been proposed to mediate the cost of parental care, though results from empirical studies remain equivocal. We measured changes in oxidative status during reproduction in cooperatively breeding superb starlings (Lamprotornis superbus) to gain insight into the relationships among breeding status, parental care, and oxidative stress. We also compared the oxidative cost of reproduction in the cooperatively breeding superb starling to that in a sympatric non-cooperatively breeding species, the greater blue-eared glossy starling (L. chalybaeus), to determine whether cooperatively breeding individuals face reduced oxidative costs of parental care relative to non-cooperatively breeding individuals. Breeders and alloparents of the cooperative species did not differ in oxidative status throughout a breeding attempt. However, individuals of the non-cooperative species incurred an increase in reactive oxygen metabolites proportionally to an individual's workload during offspring care. These findings suggest that non-cooperative starlings experience an oxidative cost of parental care, whereas cooperatively breeding starlings do not. It is possible that high nest predation risk and multi-brooding in the cooperatively breeding species may have favored reduced physiological costs of parental care more strongly compared to pair-breeding starlings. Reduced physiological costs of caring for young may thus represent a direct benefit that promotes cooperative breeding.

No short-term physiological costs of offspring care in a cooperatively breeding bird

The cost of reproduction results in a life-history trade-off where investment in current reproduction via costly parental care decreases subsequent fitness. Although this trade-off is thought to occur ubiquitously across animals, there is equivocal evidence that parental care behaviours are costly. A major challenge of studying the cost of parental care has been a lack of consensus over which physiological mechanisms underlie this trade-off. Here we compare four traits believed to mediate the cost of parental care by examining whether glucocorticoids, oxidative stress, immune function, or body condition represent a cost of performing offspring care and shape subsequent fitness. We use a 4-year dataset collected in free-living cooperatively breeding superb starlings (Lamprotornis superbus), a species in which parental and alloparental care effort varies widely among individuals and across years. Our results showed that within-individual change in physiology was unrelated to investment in offspring care, and physiological state during chick-rearing did not predict the likelihood that an individual would breeding in subsequent seasons. Instead, individuals that had elevated baseline corticosterone during incubation performed more nest guarding, suggesting that this hormone may play a preparatory role for investing in offspring care. Together, our results indicate that superb starlings modify their investment in offspring care according to their physiological state during incubation, despite no evidence of a short-term physiological cost of parental or alloparental care. Thus, breeding cooperatively appears to provide individuals with the flexibility to adjust their investment in offspring care and overcome any potential costs of reproduction.