Increasing returns in the life history of Columbian ground squirrels

1. We examined positive associations and trade-offs of maternal and reproductive traits in a population of Columbian ground squirrels, Spermophilus columbianus .
2. Structural size, body condition, mother's personal allocation to body mass during reproduction, and timing of littering were estimated for live-trapped reproductive females that were observed during an 8-year period, and were compared to litter mass, litter size, and average pup mass using path analyses.
3. Mothers exhibited age-structured traits that influenced reproductive patterns. Yearling mothers were significantly smaller, bred later, and had smaller litters than older females. Mothers that gained more body mass during reproduction and older mothers in good body condition that were structurally large had larger litters.
4. Early seasonal timing of littering was an important positive influence on successful reproduction by older mothers only in early breeding seasons and in years when conditions for reproduction were good for all females.
5. The number of offspring that survived to 1 year of age was most strongly associated with litter mass and litter size; date of breeding was of secondary influence, with earlier litters exhibiting greater success.
6. In general, mothers that gained the most in body mass during reproduction were concurrently more successful in weaning large litters (perhaps due to better quality of foraging habitat).
7. In addition to expected reproductive trade-offs, reproduction by Columbian ground squirrels exhibited positive associations of life-history traits that may reflect evolutionary increasing returns.     

Heterogeneity in individual quality overrides costs of reproduction in female reindeer

Reproductive allocation at one age is predicted to reduce the probability of surviving to the next year or to lead to a decrease in future reproduction. This prediction assumes that reproduction involves fitness costs. However, few empirical studies have assessed whether such costs may vary with the age at primiparity or might be overridden by heterogeneities in individual quality. We used data from 35 years’ monitoring of individually marked semi-domestic reindeer females to investigate fitness costs of reproduction. Using multi-state statistical models, we compared age-specific survival and reproduction among four reproductive states (never reproduced, experienced non-breeders, reproduced but did not wean offspring, and reproduced and weaned offspring) and among contrasted age at primiparity. We assessed whether reproductive costs occurred, resulting in a trade-off between current reproduction and future reproduction or survival, and whether early maturation was costly or rather reflected differences in individual quality of survival and reproduction capabilities. We did not find any evidence for fitness costs of reproduction in female reindeer. We found no cost of gestation and lactation in terms of future reproduction and survival. Conversely, successful breeders had higher survival and subsequent reproductive success than experienced non-breeders and unsuccessful breeders, independently of the age at primiparity. Moreover, it was beneficial to mature earlier, especially for females that successfully weaned their first offspring. Successful females at early primiparity remained successful throughout their life, clearly supporting the existence of marked among-female differences in quality. The weaning success peaked for multiparous females and was lower for first-time breeders, indicating a positive effect of experience on reproductive performance. Our findings emphasize an overwhelming importance of individual quality and experience to account for observed variation in survival and reproductive patterns of female reindeer that override trade-offs between current reproduction and future performance, at least in the absence of harsh winters.     

Female red squirrels fit Williams' hypothesis of increasing reproductive effort with increasing age

Williams predicted that reproductive effort should increase as individuals age and their reproductive value declines. This simple prediction has proven difficult to test because conventional measures of energy expenditure on reproduction may not be a true reflection of reproductive effort. We investigated age-specific variation in female reproductive effort in a stable population of North American red squirrels where energy expenditure on reproduction is likely to reflect actual reproductive effort. We used seven measures of reproductive effort spanning conception to offspring weaning. We found that females completed growth by age 3 and that reproductive value decreased after this age likely because of reproductive and survival senescence. We therefore, predicted that reproductive effort would increase from age 3 onwards. The probability of breeding, litter mass at weaning, and likelihood of territory bequeathal were all lower for 1- and 2-year-old females than for females older than 3 years, the age at which growth is completed. That growing females are faced with additional energetic requirements might account for their lower allocation to reproduction as compared with older females. The probability of attempting a second reproduction within the same breeding season and the propensity to bequeath the territory to juveniles increased from 3 years of age onwards, indicating an increase in reproductive effort with age. We think this increase in reproductive effort is an adaptive response of females to declining reproductive values when ageing, thereby supporting Williams' prediction.     

Allocation from capital and income sources to reproduction shift from first to second clutch in the flesh fly, Sarcophaga crassipalpis

Understanding how resources are allocated between survival and reproduction is fundamental to the study of the evolution of life histories. Reproductive resources can come from two intrinsic resource pools, stored reserves (capital) acquired before reproduction or income acquired during reproduction. The variety of reproductive strategies in insects is remarkable and reproductive allocation encompasses the complete range of allocation strategies from pure capital breeders to pure income breeders. However, most organisms probably use a blend of capital and income and this blend is likely dynamic, changing between reproductive bouts in response to internal and external conditions. We used stable isotopes to quantify the allocation of capital and income resources to reproduction in the flesh fly, Sarcopha crassipalpis and assessed how allocation patterns change over multiple bouts of reproduction. Sarcophaga crassipalpis shifts from a slight investment of capital in the first clutch to an almost pure income breeder in the second clutch. We discuss the relationship between activity and allocation, and the potential for this system to understand how allocation patterns change in response to environmental stress.     

Effect of breeding performance on the distribution and activity budgets of a predominantly resident population of black‐browed albatrosses

Pelagic seabirds breeding at high latitudes generally split their annual cycle between reproduction, migration, and wintering. During the breeding season, they are constrained in their foraging range due to reproduction while during winter months, and they often undertake long‐distance migrations. Black‐browed albatrosses (Thalassarche melanophris) nesting in the Falkland archipelago remain within 700 km from their breeding colonies all year‐round and can therefore be considered as resident. Accordingly, at‐sea activity patterns are expected to be adjusted to the absence of migration. Likewise, breeding performance is expected to affect foraging, flying, and floating activities, as failed individuals are relieved from reproduction earlier than successful ones. Using geolocators coupled with a saltwater immersion sensor, we detailed the spatial distribution and temporal dynamics of at‐sea activity budgets of successful and failed breeding black‐browed albatrosses nesting in New Island, Falklands archipelago, over the breeding and subsequent nonbreeding season. The 90% monthly kernel distribution of failed and successful breeders suggested no spatial segregation. Both groups followed the same dynamics of foraging effort both during daylight and darkness all year, except during chick‐rearing, when successful breeders foraged more intensively. Failed and successful breeders started decreasing flying activities during daylight at the same time, 2–3 weeks after hatching period, but failed breeders reached their maximum floating activity during late chick‐rearing, 2 months before successful breeders. Moon cycle had a significant effect on activity budgets during darkness, with individuals generally more active during full moon. Our results highlight that successful breeders buffer potential reproductive costs during the nonbreeding season, and this provides a better understanding of how individuals adjust their spatial distribution and activity budgets according to their breeding performance in absence of migration.     

Territory occupancy and breeding success of Peregrine Falcons Falco peregrinus at various stages of population recovery

Organochlorine pesticides disrupted reproduction and killed many raptorial birds, and contributed to population declines during the 1940s to 1970s. We sought to discern whether and to what extent territory occupancy and breeding success changed from the pesticide era to recent years in a resident population of Peregrine Falcons Falco peregrinus in southern Scotland using long‐term (1964–2015) field data and multi‐state, multi‐season occupancy models. Peregrine territories that were occupied with successful reproduction in one year were much more likely to be occupied and experience reproductive success in the following year, compared with those that were unoccupied or occupied by unsuccessful breeders in the previous year. Probability of territory occupancy differed between territories in the eastern and western parts of the study area, and varied over time. The probability of occupancy of territories that were unoccupied and those that were occupied with successful reproduction during the previous breeding season generally increased over time, whereas the probability of occupancy of territories that were occupied after failed reproduction decreased. The probability of reproductive success (conditional on occupancy) in territories that were occupied during the previous breeding season increased over time. Specifically, for territories that had been successful in the previous year, the probability of occupancy as well as reproductive success increased steadily over time; these probabilities were substantially higher in recent years than earlier, when the population was still exposed to direct or residual effects of organochlorine pesticides. These results are consistent with the hypothesis that progressive reduction, followed by a complete ban, in the use of organochlorine pesticides improved reproductive success of Peregrines in southern Scotland. Differences in the temporal pattern of probability of reproductive success between south‐eastern and south‐western Scotland suggest that the effect of organochlorine pesticides on Peregrine reproductive success and/or the recovery from pesticide effects varied geographically and was possibly affected by other factors such as persecution.     

The adaptive value of energy storage and capital breeding in seasonal environments

Timing of reproduction in a seasonal cycle is a life history trait with important fitness consequences. Capital breeders produce offspring from stored resources and, by decoupling feeding and reproduction, may bend the constraints caused by seasonality in food or predation. Income breeders, on the other hand, produce offspring from concurrent food intake, with the disadvantage of less flexibility, but with high efficiency and no inventory costs of carrying stores. Here, we assess relative profitability of capital and income breeding in herbivorous zooplankton inhabiting seasonal, high-latitude environments. We apply a state-dependent life history model where reproductive values are used to optimise energy allocation and diapause strategies over the year. Three environmental scenarios were modelled: an early, an intermediate, and a late feeding season. We found that capital breeding was most important in the early season. Capital breeding ranged from 7–9% of the eggs produced but, because of the high reproductive value of early eggs, capital breeding ranged from 9–30% when measured in terms of reproductive value. The main benefit of capital breeding was reproduction prior to the feeding season – when the reproductive value of an egg peaked. In addition, capital breeding was also used to increase egg production rates at times of income breeding. For individuals born late in the season the model predicted a two-year cycle instead of the typical annual life cycle. These individuals could then reap the benefits of early reproduction and capital breeding in their second year instead of income breeding late in the first year. We emphasize the importance of evaluating reproductive strategies such as capital and income breeding from a complete life cycle perspective. In particular, knowing the seasonality in offspring fitness is essential to appreciate evolutionary and population-level consequences of capital breeding.     

Senescence and carryover effects of reproductive performance influence migration,condition, and breeding propensity in a small shorebird

Breeding propensity, the probability that an animal will attempt to breed each year, is perhaps the least understood demographic process influencing annual fecundity. Breeding propensity is ecologically complex, as associations among a variety of intrinsic and extrinsic factors may interact to affect an animal's breeding decisions. Individuals that opt not to breed can be more difficult to detect than breeders, which can (1) lead to difficulty in estimation of breeding propensity, and (2) bias other demographic parameters. We studied the effects of sex, age, and population reproductive success on the survival and breeding propensity of a migratory shorebird, the piping plover (Charadrius melodus ), nesting on the Missouri River. We used a robust design Barker model to estimate true survival and breeding propensity and found survival decreased as birds aged and did so more quickly for males than females. Monthly survival during the breeding season was lower than during migration or the nonbreeding season. Males were less likely to skip breeding (range: 1–17%) than females (range: 3–26%; β_sex = ?0.21, 95% CI: ?0.38 to ?0.21), and both sexes were less likely to return to the breeding grounds following a year of high reproductive success. Birds that returned in a year following relatively high population‐wide reproductive output were in poorer condition than following a year with lower reproductive output. Younger adult birds and females were more likely to migrate from the breeding area earlier than older birds and males; however, all birds stayed on the breeding grounds longer when nest survival was low, presumably because of renesting attempts. Piping plovers used a variety of environmental and demographic cues to inform their reproduction, employing strategies that could maximize fitness on average. Our results support the “disposable soma” theory of aging and follow with predictions from life history theory, exhibiting the intimate connections among the core ecological concepts of senescence, carryover effects, and life history.     

Reproductive timing and reliance on hoarded capital resources by lactating red squirrels

Successful reproduction in a seasonal environment can be accomplished with resources that are stored before use (“capital resources”) or resources that are used immediately (“income resources”). Research examining capital versus income resource usage during reproduction has primarily focused on assigning species to positions along a capital–income gradient. Here, we examine the causes and reproductive consequences of among and within-year variation in hoarded capital versus income resource usage by female North American red squirrels (Tamiasciurus hudsonicus) during mid-lactation in a highly seasonal environment. Among years, the proportion of feeding events that were on capital resources (PROPCAP) averaged 39 % during the yearly median mid-lactation periods, but ranged widely between 2 and 100 %. In years with earlier parturition dates, females primarily used hoarded capital resources during mid-lactation, whereas in years with later parturition dates, females primarily used income resources during mid-lactation. Within years, PROPCAP during mid-lactation tended to be greater in early-breeding females than in late-breeding females. Rates of water flux in females during mid-lactation provided further evidence that late-breeding females used more water-rich income resources. The proportion of litters that were partially or completely lost, and the litter mass that lactating females supported, was not influenced by the large among-year differences in hoarded capital resource usage. Red squirrels appear to delay reproduction following years with low cone production to time peak reproductive demands to be late enough to be supported by income resources that only become available later in the season. In conclusion, our results offer a rare example of the capacity of a food-hoarding mammal to support reproduction exploiting a wide range of capital and income resources.     

Evaluation of reproductive costs for Weddell seals in Erebus Bay, Antarctica

1. Organisms balance current reproduction against future survival and reproduction, which results in life-history trade-offs. These trade-offs are also known as reproductive costs and may represent significant factors shaping life-history strategy for many species. 2. Using multistate mark-resight models and 26 years of mark-resight data (1979-2004), we estimated the costs of reproduction to survival and reproductive probabilities for Weddell seals in Erebus Bay, Antarctica and evaluated whether this species either conformed to the 'prudent parent' reproductive strategy predicted by life-history theory for long-lived mammals or alternatively, incurred costs to survival in order to reproduce in a variable environment (flexible-strategy hypothesis). 3. Results strongly supported the presence of reproductive costs to survival (mean annual survival probability was 0.91 for breeders vs. 0.94 for nonbreeders), a notable difference for a long-lived mammal, demonstrating that investment in reproduction does result in a cost to survival for Weddell seals, contrary to the prudent parent hypothesis. 4. Reproductive costs to subsequent reproductive probabilities were also present for first-time breeders (mean probability of breeding the next year was 31.3% lower for first-time breeders than for experienced breeders), thus supporting our prediction of the influence of breeding experience. 5. We detected substantial annual variation in survival and breeding probabilities. Breeding probabilities were negatively influenced by summer sea-ice extent, whereas weak evidence suggested that survival probabilities were affected more by winter sea-ice extent, and the direction of this effect was negative. However, a model with annual variation unrelated to any of our climate or sea-ice covariates performed best, indicating that further study will be needed to determine the appropriate mechanism or combination of mechanisms underlying this annual variation.     

Costs of reproduction and terminal investment by females in a semelparous marsupial

Evolutionary explanations for life history diversity are based on the idea of costs of reproduction, particularly on the concept of a trade-off between age-specific reproduction and parental survival, and between expenditure on current and future offspring. Such trade-offs are often difficult to detect in population studies of wild mammals. Terminal investment theory predicts that reproductive effort by older parents should increase, because individual offspring become more valuable to parents as the conflict between current versus potential future offspring declines with age. In order to demonstrate this phenomenon in females, there must be an increase in maternal expenditure on offspring with age, imposing a fitness cost on the mother. Clear evidence of both the expenditure and fitness cost components has rarely been found. In this study, we quantify costs of reproduction throughout the lifespan of female antechinuses. Antechinuses are nocturnal, insectivorous, forest-dwelling small (20-40 g) marsupials, which nest in tree hollows. They have a single synchronized mating season of around three weeks, which occurs on predictable dates each year in a population. Females produce only one litter per year. Unlike almost all other mammals, all males, and in the smaller species, most females are semelparous. We show that increased allocation to current reproduction reduces maternal survival, and that offspring growth and survival in the first breeding season is traded-off with performance of the second litter in iteroparous females. In iteroparous females, increased allocation to second litters is associated with severe weight loss in late lactation and post-lactation death of mothers, but increased offspring growth in late lactation and survival to weaning. These findings are consistent with terminal investment. Iteroparity did not increase lifetime reproductive success, indicating that terminal investment in the first breeding season at the expense of maternal survival (i.e. semelparity) is likely to be advantageous for females.     

Resource use for reproduction depends on spring arrival time and wintering area in an arctic breeding shorebird

Resources for egg production may come from body reserves stored before breeding (“capital breeders”) or from food acquired at the breeding site (“income breeders”). Arctic migrants were long thought to be capital breeders, because they often arrive at a time when local food availability is still limited. However, later evidence suggested that arctic breeding shorebirds are primarily income breeders, or that they use a mixed strategy depending on laying date. We explored the relationship between laying date and resource use for reproduction in the pectoral sandpiper Calidris melanotos breeding in the Alaskan arctic by contrasting carbon isotope (δ¹³C) values of the local diet and of maternal plasma, cellular blood, feather and claw with those of the eggs produced. Our results revealed that early breeding females utilize resources for egg production that were acquired recently at staging areas, whereas later breeding females mostly relied on nutrients derived from local food sources. These findings suggest that the resource allocation strategy used for reproduction differs among females, and varies depending on the timing of arrival and the start of reproduction. The arrival date at the breeding ground and laying date may critically depend on non‐breeding season events such as winter habitat choice, staging areas or migration routes. By comparing maternal feather δ¹³C, claw δ¹³C and feather δD, we examined whether non‐breeding season events influenced the use of resources for egg production through variation in capture date or clutch initiation date. Female pectoral sandpipers originating from moulting areas characterized by higher (more positive) δD signatures were caught earlier and started laying earlier, and they used stored resources for reproduction. Using regional maps of δD values for precipitation in the wintering sites in South America, we compared the spatial variation in the observed feather δD signatures. This analysis indicated that female pectoral sandpipers with higher δD signatures, presumably coming from more north‐easterly wintering sites in southern America, started laying earlier and used mostly stored resources for egg production, compared to females that wintered (or at least moulted) further south. Our results thus show that winter moulting habitat is linked to breeding resource allocation strategy in this high‐arctic breeding shorebird.     

Long-term shifts in the cyclicity of outbreaks of a forest-defoliating insect

Optimisation of reproductive investment is crucial for Darwinian fitness, and detailed long-term studies are especially suited to unravel reproductive allocation strategies. Allocation strategies depend on the timing of resource acquisition, the timing of resource allocation, and trade-offs between different life-history traits. A distinction can be made between capital breeders that fuel reproduction with stored resources and income breeders that use recently acquired resources. In capital breeders, but not in income breeders, energy allocation may be decoupled from energy acquisition. Here, we tested the influence of extrinsic (weather conditions) and intrinsic (female characteristics) factors during energy storage, vitellogenesis and early gestation on reproductive investment, including litter mass, litter size, offspring mass and the litter size and offspring mass trade-off. We used data from a long-term study of the viviparous lizard, Lacerta (Zootoca) vivipara. In terms of extrinsic factors, rainfall during vitellogenesis was positively correlated with litter size and mass, but temperature did not affect reproductive investment. With respect to intrinsic factors, litter size and mass were positively correlated with current body size and postpartum body condition of the previous year, but negatively with parturition date of the previous year. Offspring mass was negatively correlated with litter size, and the strength of this trade-off decreased with the degree of individual variation in resource acquisition, which confirms theoretical predictions. The combined effects of past intrinsic factors and current weather conditions suggest that common lizards combine both recently acquired and stored resources to fuel reproduction. The effect of past energy store points out a trade-off between current and future reproduction.     

Population dynamics in a long-lived seabird: I. Impact of breeding activity on survival and breeding probability in unbanded king penguins

Understanding the trade-off between current reproductive effort, future survival and future breeding attempts is crucial for demographic analyses and life history studies. We investigated this trade-off in a population of king penguins (Aptenodytes patagonicus) marked individually with transponders using multistate capture-recapture models. This colonial seabird species has a low annual proportion of non-breeders (13%), despite a breeding cycle which lasts over 1 year. To draw inferences about the consequences of non-breeding, we tested for an effect of reproductive activity on survival and on the probability of subsequent breeding. We found that birds non-breeding in year t show the same survival rate as breeders (two-states analysis: breeding and non-breeding). However, breeders had a lower probability of breeding again the following year. This negative phenotypic correlation suggests the existence of reproductive costs affecting future breeding probability, but it might also be strengthened by late arrival for courtship in year t. A three-state analysis including breeding success revealed that failed breeders in year t have a lower probability to reproduce successfully in year t + 1 than non-breeders in year t, providing some evidence for the existence of reproductive costs. Moreover, successful breeders showed higher survival probability. This positive phenotypic correlation between current reproduction and subsequent survival supports the hypothesis of an heterogeneity in individual quality. Males breeding in year t had a lower probability to breed again in year t + 1 than females, suggesting higher reproductive costs for this sex. Such additional costs might be due to higher male parental investment in the final phase of chick-rearing, which also delays the arrival of males in year t + 1, and decreases their breeding probability. Our study is the first to explore the breeding biology and the demography of penguins without the disturbance of flipper-bands.     

Reproductive trade‐offs in a long‐lived bird species: condition‐dependent reproductive allocation maintains female survival and offspring quality

Life history theory is an essential framework to understand the evolution of reproductive allocation. It predicts that individuals of long‐lived species favour their own survival over current reproduction, leading individuals to refrain from reproducing under harsh conditions. Here we test this prediction in a long‐lived bird species, the Siberian jay Perisoreus infaustus. Long‐term data revealed that females rarely refrain from breeding, but lay smaller clutches in unfavourable years. Neither offspring body size, female survival nor offspring survival until the next year was influenced by annual condition, habitat quality, clutch size, female age or female phenotype. Given that many nests failed due to nest predation, the variance in the number of fledglings was higher than the variance in the number of eggs and female survival. An experimental challenge with a novel pathogen before egg laying largely replicated these patterns in two consecutive years with contrasting conditions. Challenged females refrained from breeding only in the unfavourable year, but no downstream effects were found in either year. Taken together, these findings demonstrate that condition‐dependent reproductive allocation may serve to maintain female survival and offspring quality, supporting patterns found in long‐lived mammals. We discuss avenues to develop life history theory concerning strategies to offset reproductive costs.     

Cooperative enhancement of reproductive success in white-winged choughs

Summary White-winged choughs are a cooperatively breeding species which provide parental care to their young over an entire year. I traced the reproductive success of groups of white-winged choughs from the start of one breeding season to the next over 3 years. I examined the effect of helper number on timing of breeding, the success of each effort, the number of efforts made in a season, and the final reproductive success at the end of each year. Timing of commencement of breeding varied between years but was not related to group size. Early broods were not more successful than late broods. Nest building (July–September) commenced earlier in years which had high rainfall in July; choughs rely on rainfall for supplies of mud for nest construction. Most nest failures occurred gradually and were attributed to starvation of nestlings, although some sudden failures were attributed to predation. Large groups have more young by the beginning of the following season; this is due to higher fledging success and a greater likelihood of having second broods. Disappearance of young after fledging and during the transition to independence was not dependent on group size. Only groups of seven and above produce more than one young on average over the entire year; choughs provide one of the most marked cases for helpers enhancing the reproductive success of breeders. Large groups are virtually guaranteed of reproductive success over the whole year and grow more quickly than small groups. These results highlight the need to consider the effect of helpers over the entire period of reproduction and care of young, rather than just at fledging.     

Reproductive phenology of a food-hoarding mast-seed consumer: resource- and density-dependent benefits of early breeding in red squirrels

The production of offspring by vertebrates is often timed to coincide with the annual peak in resource availability. However, capital breeders can extend the energetic benefits of a resource pulse by storing food or fat, thus relaxing the need for synchrony between energy supply and demand. Food-hoarding red squirrels (Tamiasciurus hudsonicus) breeding in the boreal forest are reliant on cones from a masting conifer for their nutrition, yet lactation is typically completed before the annual crop of cones is available for consumption such that peaks in energy supply and demand are not synchronized. We investigated the phenological response of red squirrels to annual variation in environmental conditions over a 20-year span and examined how intra- and inter-annual variation in the timing of reproduction affected offspring recruitment. Reproductive phenology was strongly affected by past resource availability with offspring born earlier in years following large cone crops, presumably because this affected the amount of capital available for reproduction. Early breeders had higher offspring survival and were more likely to renest following early litter loss when population density was high, perhaps because late-born offspring are less competitive in obtaining a territory when vacancies are limited. Early breeders were also more likely to renest after successfully weaning their first litter, but renesting predominantly occurred during mast years. Because of their increased propensity to renest and the higher survival rates of their offspring, early breeders contribute more recruits to the population but the advantage of early breeding depends on population density and resource availability.     

Timing of current reproduction directly affects future reproductive output in European coots

Life-history theory suggests that the variation in the seasonal timing of reproduction within populations may be explained on the basis of individual optimization. Optimal breeding times would vary between individuals as a result of trade-offs between fitness components. The existence of such trade-offs has seldom been tested empirically. We experimentally investigated the consequences of altered timing of current reproduction for future reproductive output in the European coot (Fulica atra). First clutches of different laying date were cross-fostered between nests, and parents thereby experienced a delay or an advance in the hatching date. The probability and success of a second brood, adult survival until and reproduction in the next season were then compared to the natural variation among control pairs. Among control pairs the probability of a second brood declined with the progress of season. Delayed pairs were less likely and advanced pairs were more likely to produce a second brood. These changes were quantitatively as predicted from the natural seasonal decline. The number of eggs in the second clutch was positively related to egg number in the first clutch and negatively related to laying date. Compared to the natural variation, delayed females had more and advanced females had fewer eggs in their second clutch. The size of the second brood declined with season, but there was no significant effect of delay or advance. Local adult survival was higher following a delay and reduced following an advance. The effect of the experiment on adult survival was independent of sex. Laying date and clutch size of females breeding in the next year were not affected by treatment. The study demonstrates the existence of a trade-off between increased probability of a second brood and decreased parental survival for early breeding. Timing-dependent effects of current reproduction on future reproductive output may thus play an important role in the evolution of the seasonal timing of reproduction.     

Best squirrels trade a long life for an early reproduction

Age at primiparity plays a crucial role in population dynamics and life-history evolution. Long-term data on female North American red squirrels were analysed to study the fitness consequences of delaying first reproduction. Early breeders were born earlier, had a higher breeding success and achieved a higher lifetime reproductive success than females who delayed their first reproduction, which suggests a higher quality of early breeders. However, early breeders had similar mass when tagged, and similar number of food caches available at one year of age as late breeders. Nevertheless, we found evidence of survival costs of early primiparity. Early breeders had a lower survival between one and two years of age than late breeders and a lower lifespan. Our study points out that two reproductive tactics co-occurred in this population: a tactic based on early maturity at the cost of a lower survival versus a tactic based on delayed maturity and long lifespan. High quality individuals express the most profitable tactic by breeding early whereas low quality individuals do the best of a bad job by delaying their first reproduction.     

Reduced reproductive performance associated with warmer ambient temperatures during incubation in a winter‐breeding,food‐storing passerine

Timing of reproduction can influence individual fitness whereby early breeders tend to have higher reproductive success than late breeders. However, the fitness consequences of timing of breeding may also be influenced by environmental conditions after the commencement of breeding. We tested whether ambient temperatures during the incubation and early nestling periods modulated the effect of laying date on brood size and dominant juvenile survival in gray jays (Perisoreus canadensis), a sedentary boreal species whose late winter nesting depends, in part, on caches of perishable food. Previous evidence has suggested that warmer temperatures degrade the quality of these food hoards, and we asked whether warmer ambient temperatures during the incubation and early nestling periods would be associated with smaller brood sizes and lower summer survival of dominant juveniles. We used 38 years of data from a range‐edge population of gray jays in Algonquin Provincial Park, Ontario, where the population has declined over 50% since the study began. Consistent with the “hoard‐rot” hypothesis, we found that cold temperatures during incubation were associated with larger brood sizes in later breeding attempts, but temperatures had little effect on brood size for females breeding early in the season. This is the first evidence that laying date and temperature during incubation interactively influence brood size in any bird species. We did not find evidence that ambient temperatures during the incubation period or early part of the nestling period influenced summer survival of dominant juveniles. Our findings provide evidence that warming temperatures are associated with some aspects of reduced reproductive performance in a species that is reliant on cold temperatures to store perishable food caches, some of which are later consumed during the reproductive period.