Geographical variation in reproductive ageing patterns and life‐history strategy of a short‐lived passerine bird

We investigated differences in ageing patterns in three measures of breeding performance in populations of barn swallows Hirundo rustica L. from Spain and Denmark differing in breeding latitude and hence migration distance and duration of the breeding season. We found differences in ageing patterns between populations. Generally, young (i.e. yearling) and old females (i.e. ≥ 5 years of age) laid their first eggs later and produced smaller clutches than middle‐aged females (i.e. 2–4 years of age) in both populations. The southernmost population (i.e. Spanish) showing the shorter migratory distance experienced a greater within‐individual increase in timing of breeding and clutch size in early life and a greater within‐individual decrease in laying date but not in clutch size during senescence compared with the northernmost population (i.e. Danish). We also found that the number of fledglings produced annually was related to the age of the two members of the breeding pairs with pairs composed of young and old females performing less well than breeding pairs composed of middle‐aged females. We did not find reproductive senescence for the age of the male while controlling for the age of the female on the number of fledglings produced annually by the breeding pair. Differential survival between individuals did not explain age effects on laying date or annual clutch size in neither population. However, the increase in the number of fledglings produced annually with age was partly explained by the disappearance of poor‐quality members of the pairs, mainly poor‐quality males. Age‐related breeding success (i.e. number of fledglings) was similar for barn swallows from Spain and Denmark. Therefore, the study of ageing patterns and life‐history strategies in free‐ranging animals from more than a single population can throw new light on life‐history theory, population dynamics and evolutionary studies of senescence.     

Age and terminal reproductive attempt influence laying date in the thorn‐tailed rayadito

Age‐specific variation in reproductive effort can affect population dynamics, and is a key component of the evolution of reproductive tactics. Late‐life declines are a typical feature of variation in reproduction. However, the cause of these declines, and thus their implications for the evolution of life‐history tactics, may differ. Some prior studies have shown late‐life reproductive declines to be tied to chronological age, whereas other studies have found declines associated with terminal reproduction irrespective of chronological age. We investigated the extent to which declines in late life reproduction are related to chronological age, terminal reproductive attempt or a combination of both in the thorn‐tailed rayadito Aphrastura spinicauda, a small passerine bird that inhabits the temperate forest of South America. To this end we used long‐term data (10 years) obtained on reproductive success (laying date, clutch size and nestling weight) of females in a Chilean population. Neither chronological age nor terminal reproductive attempt explained variation in clutch size or nestling weight, however we observed that during the terminal reproductive attempt older females tended to lay later in the breeding season and younger females laid early in the breeding season, but this was not the case when the reproductive attempt was not the last. These results suggests that both age‐dependent and age‐independent effects influence reproductive output and therefore that the combined effects of age and physiological condition may be more relevant than previously thought.     

Maturation increases early reproductive investment in Adélie Penguins Pygoscelis adeliae

Correlations between female investment in egg production and age, breeding experience and laying date have been reported in several seabird species. In general, clutch and egg sizes increase with female age and breeding experience but decrease with laying date. Positive correlations of clutch and egg size with age and breeding experience can be caused by an increase in reproductive investment with maturation or they may be an artefact of lower survival rates for individuals with poor-quality phenotypes. Negative correlations of clutch and egg size with laying date might signal an adaptive reduction in egg production or be due in part to variation among individuals. We examined the interactions of female age, breeding experience, laying date and clutch and egg size in Adélie Penguins Pygoscelis adeliae . Breeding experience strongly affected clutch size with 87.3% of all one-egg clutches laid by first-time breeders. In addition, increasing age had a positive influence on egg size and was associated with earlier laying dates. However, there was little evidence to suggest that either clutch or egg sizes are influenced by laying date. Laying dates and clutch and egg sizes did not affect a female's probability of returning to breed in the following year, indicating that increased investment is a product of maturation and not of the loss of poor-quality breeders from the population. Our results suggest that as female Adélie Penguins gain foraging and breeding experience they are able to initiate breeding earlier, to lay complete clutches of two eggs and to lay larger eggs.     

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.     

Age-specific patterns of reproduction in White-tailed and Willow Ptarmigan Lagopus leucurus and L. lagopus

Although many studies report a difference in reproductive success between old and young birds, little is known about how, why and when productivity changes as individuals age. We examined age-dependent reproduction in two bird species that inhabit harsh tundra environments: White-tailed Ptarmigan Lagopus leucurus in alpine areas and Willow Ptarmigan Lagopus lagopus in subarctic Canada. We evaluated reproductive performance in the light of three hypotheses: constraint, restraint and selection. Using cross-sectional and longitudinal data, we observed significant age effects in seven of the eight life history and behavioural traits examined for the two species. However, the pattern of age effects variedconsiderably across life history stages; younger birds generally had smaller clutches, later laying dates and poorer spring body condition, but the nesting success did not vary with age. Brood-rearing and renesting abilities were greater for older parents. The oldest age class of White-tailed Ptarmigan showed reproductive senescence for laying date and clutch size but fledged such a large proportion of the brood that they had the highest overall production of any class. It thus appears that parental experience can compensate for reduced physical ability to produce eggs. Annual mortality rates for breeding females were U-shaped for White-tailed Ptarmigan, with higher rates for young and old birds, but mortality did not change with age in Willow Ptarmigan. Overall, the two species differed in the presence of age dependence for only two traits (renesting ability and annual survival). Age-dependent effects were generally greater for White-tailed Ptarmigan than for Willow Ptarmigan. The patterns of mortality and fecundity we observed in ptarmigan provide general support for the constraint hypothesis of reproductive performance. By examining discrete stages of reproduction, we identified the life history stages where age effects occur and propose proximate mechanisms responsible for these effects.     

A pre‐breeding immune challenge delays reproduction in the female dark‐eyed junco Junco hyemalis

Precise timing of life‐history transitions in predictably changing environments is hypothesized to aid in individual survival and reproductive success, by appropriately matching an animal's physiology and behavior with prevailing environmental conditions. Therefore, it is imperative for individuals to time energetically costly life‐history stages (i.e. reproduction) so they overlap with seasonal peaks in food abundance and quality. Female lifetime reproductive fitness is affected by several factors that influence energy balance, including arrival date, timing of egg production, and energetic condition. Therefore, any extra energetic costs during reproduction may negatively affect timing of egg production, and ultimately a female's fitness. For example, mounting an immunological response elicits a high energetic cost, and this transfer of resources towards cell and immune system maintenance could have direct negative effects on reproductive timing. In order to determine whether an immune challenge delays onset of breeding (i.e. egg production), we administered either a humoral immune challenge (keyhole limpet hemocyanin (KLH)) (treatment) or physiological saline (control) to free‐living female dark‐eyed juncos Junco hyemalis in the period immediately prior to egg‐laying (～4 weeks). We found that KLH‐injected females artificially delayed clutch initiation when compared to control females. These data help to refine our understanding of how free‐living birds allocate resources between reproduction and self‐maintenance processes during the critical pre‐laying period of the annual cycle.     

Effects of a food supplementation experiment on reproductive investment and a post-mating sexually selected trait in magpies Pica pica

Food availability is an important factor affecting breeding success in birds. Food supplementation experiments in birds have in general focused on the effects on reproductive success in terms of female investment (laying date, clutch size, egg size), however, it is also known that the estimation of mate quality based on sexually selected signals influences female reproductive investment. In the particular case of magpies, females use nest size, a post-mating sexually selected signal, to assess male's likelihood to invest in reproduction, and accordingly adjust reproductive investment (clutch size). Then, the possible effects of food supplementation on female reproductive investment could be mediated by other variables related to parental quality, such as nest size in magpies. In the present study, we explore if higher food availability in a magpie territory affected both male sexually selected traits (i.e. nest size) and female reproductive investment (laying date, egg size, clutch size). We performed a food supplementation experiment in which we experimentally increased food availability in several magpie territories, keeping others as controls. In food-supplemented territories, males built significantly larger nests and females significantly increased egg size by 4.1% compared to control females. Results suggest that the continuous provisioning of protein rich food allowed magpie females to increase egg size. However, laying date and clutch size did not differ between control and food-supplemented magpie pairs. Food availability also affected the relationship between female reproductive investment and nest size. In control territories, females decreased their egg size in response to a larger nest, whereas a tendency for the opposite relationship was revealed in food-supplemented territories. We discuss the possibility that magpie females adopt different strategies for reproductive investment according to food availability.     

Pre-incubation feeding activities and energy budgets of Snow Geese: can food on the breeding grounds influence fecundity?

The potential contribution of early spring feeding conditions in the Arctic to clutch size variation was examined in a population of Lesser Snow Geese Anser caerulescens caerulescens. Behavioural observations were combined with energetic analyses of food material to derive an estimate of the energy budgets of pre-laying and laying females. Food intake of females between arrival on the breeding grounds and incubation was considerable; estimated energy gains in this period were in the same magnitude as the cost of one or several eggs. The pre-laying period spent on the breeding grounds can thus be energetically beneficial rather than costly. Accumulation of resources for reproduction in Snow Geese is a continual process including the breeding grounds, and nutrient limitation after arrival in the Arctic cannot sufficiently explain the environmental component of clutch size variation. The timing of migration and follicle development is such that clutch size decisions are sometimes made during the late stages of migration and some-times after arrival. In the latter case food conditions on the breeding grounds may greatly influence clutch size; in the former case they may still influence readjustments of clutch size after the initial decision. The universal negative correlation between clutch size and laying date in Snow Geese can be explained by negative fitness consequences of late hatching, which outweigh the benefits of delayed laying and further nutrient accumulation. Food shortage on the breeding grounds may sometimes be a secondary factor contributing to seasonal clutch size decline.     

Increased survival and breeding performance of double breeders in little penguins Eudyptula minor,New Zealand: evidence for individual bird quality?

The little penguin Eudyptula minor is unique among penguin species in being able to fledge chicks from two clutches in one breeding season. Pairs laying two clutches in a given season make a higher reproductive investment, and may be rewarded by a higher reproductive success as they may raise twice as many chicks as pairs laying one clutch. The higher effort made by pairs laying two clutches could correlate negatively with survival, future reproductive performance or offspring survival, indicating a cost of reproduction. Conversely, a positive relationship between the number of clutches produced in a given breeding season and survival, future reproductive performance or offspring survival would indicate that birds laying two clutches belonged to a category of birds with higher fitness, compared to birds laying only one clutch in the season. In this study we used a long‐term data set taken from an increasing population of little penguins in Otago, SE New Zealand. We modelled the relationship between the number of clutches laid in a breeding season and survival probability, reproductive performance in the next breeding season and first year survival of offspring using capture‐recapture modelling.
Birds laying two clutches produced 1.7 times more fledglings during a breeding season than pairs laying one clutch. We found that birds laying two clutches had a higher probability of breeding in the following breeding season, a higher probability of laying two clutches in the following breeding season and a higher survival probability. There was no overall difference in post‐fledging survival between the young of birds producing one clutch and the young of birds producing two clutches. However, the survival of young of single clutch breeders declined with laying date, whereas the young of double clutch breeders had the same survival rate irrespective of laying date. For a subset of data with birds of known age, we found evidence that the probability of laying two clutches increased with age. However, there were also indications for differences among birds in the tendency to lay two clutches that could not be attributed to age. We tentatively interpret our results as evidence of quality difference among little penguin breeders.     

No effect of partner age and lifespan on female age‐specific reproductive performance in blue tits

Studies of age‐specific reproductive performance are fundamental to our understanding of population dynamics and the evolution of life‐history strategies. In species with bi‐parental care, reproductive ageing trajectories of either parent may be influenced by their partner's age, but this has rarely been investigated. We investigated within‐individual age‐specific performance (laying date and number of eggs laid) in wild female blue tits Cyanistes caeruleus and evaluated how the age and longevity of their male partner indirectly influenced the females’ reproductive performance. Females showed clear age‐dependence in both laying date and number of eggs laid. We found that female reproductive performance improved in early life, before showing a decline. Longer‐lived females had an earlier laying date throughout their lives than shorter‐lived females, but there was no difference in number of eggs laid between longer‐ and shorter‐lived females. Within breeding pairs, the female's (age‐specific) reproductive performance was not dependent on the age and longevity of the male partner. We conclude that the age and quality of the male partner may be of little importance for traits that are under direct female control.     

Age-related change in breeding performance in early life is associated with an increase in competence in the migratory barn swallow Hirundo rustica

1. We investigated age-related changes in two reproductive traits (laying date and annual fecundity) in barn swallows Hirundo rustica L. using a mixed model approach to di-stinguish among between- and within-individual changes in breeding performance with age. 2. We tested predictions of age-related improvements of competence (i.e. constraint hypothesis) and age-related progressive disappearance of poor-quality breeders (i.e. selection hypothesis) to explain age-related increase in breeding performance in early life. 3. Reproductive success increased in early life, reaching a plateau at middle age (e.g. at 3 years of age) and decreasing at older age (> 4 years). Age-related changes in breeding success were due mainly to an effect of female age. 4. Age of both female and male affected timing of reproduction. Final linear mixed effect models (LME) for laying date included main and quadratic terms for female and male age, suggesting a deterioration in reproductive performance at older age for both males and females. 5. We found evidence supporting the constraints hypothesis that increases in competence within individuals, with ageing being the most probable cause of the observed increase in breeding performance with age in early life. Two mechanisms were implicated: (1) advance in male arrival date with age provided middle-aged males with better access to mates. Yearling males arrived later to the breeding grounds and therefore had limited access to high-quality mates. (2) Breeding pairs maintaining bonds for 2 consecutive years (experienced pairs) had higher fecundity than newly formed inexperienced breeding pairs. 6. There was no support for the selection hypothesis because breeding performance was not correlated with life span. 7. We found a within-individual deterioration in breeding and migratory performance (arrival date) in the oldest age-classes consistent with senescence in these reproductive and migratory traits.     

Constraints on, and determinants of, the annual number of breeding attempts in the multi-brooded Black Redstart Phoenicurus ochruros

With reference to models predicting patterns of reproduction in multi-brooded species, I analysed some of the factors potentially affecting the number of breeding attempts per season made by female Black Redstarts Phoenicurus ochruros in the central Swiss Alps. The maximum number of successful broods per female per season was three, the median two. The proportion of females initiating only a single clutch per season varied from 16 to 58% over 10 years. Variation in the frequency of single brooding between years was positively related to the date of the onset of breeding. Egg-laying started after a temperature-related threshold value was crossed in April. Females frequently re-nested before fledglings of the previous brood were independent. Short interbrood intervals were associated with triple brooding. The interbrood interval shortened during the breeding season. Intra-individual variation in the number of breeding attempts per season was partitioned according to female age, laying date of the season's first egg (standardized in relation to the median date of first laying within a year) and the occurrence of breeding failures before the ultimate nesting attempt. However, mortality during the season often terminated breeding early, in particular among novice breeder females. Seasonal reproductive success increased linearly with each additional breeding attempt. The productivity of a breeding attempt was independent of the time in the season. Increased reproductive effort affected neither current or future survival nor reproduction of females. The reproductive patterns and trade-offs in multi-brooded Black Redstarts contrast in many aspects with those found in single-brooded species. They are in accordance with models predicting that multi-brooded species are selected to start breeding as early as possible, continue breeding for as long as conditions are suitable, and save time by overlapping broods.     

Individual heterogeneity in fitness in a long‐lived herbivore

Heterogeneity in the intrinsic quality and nutritional condition of individuals affects reproductive success and consequently fitness. Black brant (Branta bernicla nigricans) are long‐lived, migratory, specialist herbivores. Long migratory pathways and short summer breeding seasons constrain the time and energy available for reproduction, thus magnifying life‐history trade‐offs. These constraints, combined with long lifespans and trade‐offs between current and future reproductive value, provide a model system to examine the role of individual heterogeneity in driving life‐history strategies and individual heterogeneity in fitness. We used hierarchical Bayesian models to examine reproductive trade‐offs, modeling the relationships between within‐year measures of reproductive energy allocation and among‐year demographic rates of individual females breeding on the Yukon‐Kuskokwim Delta, Alaska, using capture–recapture and reproductive data from 1988 to 2014. We generally found that annual survival tended to be buffered against variation in reproductive investment, while breeding probability varied considerably over the range of clutch size‐laying date combinations. We provide evidence for relationships between breeding probability and clutch size, breeding probability and nest initiation date, and an interaction between clutch size and initiation date. Average lifetime clutch size also had a weak positive relationship with apparent survival probability. Our results support the use of demographic buffering strategies for black brant. These results also indirectly suggest associations among environmental conditions during growth, fitness, and energy allocation, highlighting the effects of early growth conditions on individual heterogeneity, and subsequently, lifetime reproductive investment.     

Intrapopulation variation in reproduction by female eastern kingbirds Tyrannus tyrannus: the impacts of age, individual performance, and breeding site

I used data from a 13-year study of eastern kingbirds Tyrannus tyrannus from central New York, USA, to evaluate the relative impact of female age and body size on reproduction. I also calculated repeatabilities of reproductive traits for both females and the sites where they bred in an attempt to evaluate the relative contribution of each to intrapopulation variation in reproduction. Female age had a strong influence on timing of breeding (breeding date advanced by one day for each year of life), but was not a significant source of variation for clutch size, egg mass, number of young to hatch or fledge, or total seasonal production. Repeatabilities of breeding date for females and sites were both significant (0.284 and 0.181, respectively), but the only other significant repeatabilities were for female clutch size (0.282) and female egg mass (0.746). Among-year repeatabilities of breeding date for females who bred at two or more sites over their lifetime were as high as those for females that were site faithful. Thus, breeding date was probably affected independently by the female and site. No measure of productivity exhibited a repeatable pattern in comparisons made among females or sites. All reproductive traits were entered as dependent variables in a series of stepwise multiple regression analyses in an attempt to identify female properties (size, lifespan and condition) that might be linked proximately to differences in breeding statistics. I found that (a) large birds tended to breed the earliest, (b) clutch size was independent of female size, condition and lifespan, (c) female body size and egg size were correlated positively, but (d) production of young was independent of all measured female properties. Reproduction appears to be linked more closely to the female than to the site. Body size accounts for a portion of the repeatable portion of breeding date and egg mass, but most of the intrapopulation variation in these and other traits remained unexplained.     

Early reproductive investment,senescence and lifetime reproductive success in female Asian elephants

The evolutionary theory of senescence posits that as the probability of extrinsic mortality increases with age, selection should favour early‐life over late‐life reproduction. Studies on natural vertebrate populations show early reproduction may impair later‐life performance, but the consequences for lifetime fitness have rarely been determined, and little is known of whether similar patterns apply to mammals which typically live for several decades. We used a longitudinal dataset on Asian elephants (Elephas maximus) to investigate associations between early‐life reproduction and female age‐specific survival, fecundity and offspring survival to independence, as well as lifetime breeding success (lifetime number of calves produced). Females showed low fecundity following sexual maturity, followed by a rapid increase to a peak at age 19 and a subsequent decline. High early life reproductive output (before the peak of performance) was positively associated with subsequent age‐specific fecundity and offspring survival, but significantly impaired a female's own later‐life survival. Despite the negative effects of early reproduction on late‐life survival, early reproduction is under positive selection through a positive association with lifetime breeding success. Our results suggest a trade‐off between early reproduction and later survival which is maintained by strong selection for high early fecundity, and thus support the prediction from life history theory that high investment in reproductive success in early life is favoured by selection through lifetime fitness despite costs to later‐life survival. That maternal survival in elephants depends on previous reproductive investment also has implications for the success of (semi‐)captive breeding programmes of this endangered species.     

Effects of food abundance, density and climate change on reproduction in the sparrowhawk Accipiter nisus

The reproductive success of predators depends on abiotic environmental conditions, food abundance and population density, and food abundance, density and their interactions may respond to changes in climatic conditions. Timing of reproduction by five of the eight numerically most common prey of the sparrowhawk Accipiter nisus advanced significantly since 1971, during a period of temperature increase. There was no evidence that mean laying date or any other reproductive parameter of sparrowhawks changed consistently during the study period 1977–1997. Laying date advanced and percentage of unsuccessful female sparrowhawks decreased with beech mast in the current year, an index of food abundance for avian prey. Mean laying date of sparrowhawks was advanced in warmer springs, and although mean clutch size was not larger in warm than in cold springs, mean brood size of successful pairs and breeding success increased in such springs, showing that sparrowhawks enjoyed a fitness gain when reproducing early. The timing of sparrowhawk reproduction with respect to the peak in abundance of fledgling prey increased, from a good match between mean timing of fledging by prey and maximum demand for food by the predator in 1977, to reproduction occurring later than the peak in fledging prey availability in 1997. The size of the breeding population of sparrowhawks was not predicted by mean spring temperature, the size of the breeding population the previous year or beech mast crop. The size of the post-breeding population was predicted by size of the breeding and post-breeding population the previous year and by the proportion of unsuccessful females the current year. These findings imply that sparrowhawks did not respond to change in climate, although climate changed the timing of reproduction by the main prey species.     

Photoperiodic control of reproduction and molt in the kestrel, Falco tinnunculus

An adaptive decline in average clutch size with progressive date of laying is characteristic of most bird species with a single clutch of variable size per year. The effect of photoperiod on timing of laying, clutch size, and subsequent molt was investigated in kestrel pairs breeding and raising their young in captivity. In natural daylight (nLD), clutch size, under ad libitum feeding, showed the same decline with date as in nature. Birds breeding later also started molt later and molted faster (more feathers simultaneously), so that all birds completed molt more or less at the same time. Constant long days (LD 17.5:6.5 and LD 13:11) from December 1 onward advanced both reproduction and molt. The LD 17.5:6.5 group developed the reproductive system faster, had a shorter courtship period, and laid eggs earlier than the LD 13:11 group. In both photoperiods there was a decrease in clutch size with progressive laying date, similar to that in nLD. Molt started in both groups at about the end of the laying period and slowed down in the longer photoperiod, especially in males. Plasma luteinizing hormone (LH) changes in the two photoperiods were different for males and females. Males showed the expected slower LH response in the shorter photoperiod, but the initial LH response by females was the same in both photoperiods. Data on LH, reproductive behavior, and body mass suggest that females have a wider annual reproductive window than males. Data on time of laying and number of eggs suggest that clutch size in the kestrel is determined by laying date itself, following an endogenous rhythm that is phase-locked to the reproductive cycle.     

Intra-seasonal decline of clutch size in Lesser Snow Geese

Summary Different mechanisms proposed to explain the intra-seasonal decline in clutch size of Lesser Snow Geese (Anser caerulescens caerulescens) were tested at the La Perouse Bay colony, Manitoba, Canada. Ovary examination of females collected after laying revealed that the actual number of eggs produced per female decreased over the laying period. This finding eliminates nest-parasitism, partial clutch predation and renesting attempts as sufficient explanations for seasonal clutch size decline. Follicular atresia induced by reserve depletion was also rejected since its occurrence was similar among early and late nesters. The decline in clutch size was observed within age-classes, and therefore age effects on clutch size and laying date per se do not account for the observed relationship. Clutch size and laying date were respectively positively and negatively correlated with the amount of nutrient reserves in females at the onset of laying, and also covaried within individuals observed breeding in several seasons. Laying date repeatability was estimated at 0.22. It is postulated that the seasonal decline in clutch size results mainly from a positive feed-back of female's nutrient reserves on the hormones controlling ovary development. Hatching synchrony or shortness of the breeding season may be the ultimate factors responsible for the intra-seasonal clutch size decline in Lesser Snow Geese.     

Effects of diet and host access on fecundity and lifespan in two fruit fly species with different life‐history patterns

The reproductive ability of female tephritids can be limited and prevented by denying access to host plants and restricting the dietary precursors of vitellogenesis. The mechanisms underlying the delayed egg production in each case are initiated by different physiological processes that are anticipated to have dissimilar effects on lifespan and reproductive ability later in life. The egg‐laying abilities of laboratory‐reared females of the Mediterranean fruit fly (Ceratitis capitata Wiedmann) and melon fly (Bactrocera cucurbitae Coquillett) from Hawaii are delayed or suppressed by limiting access to host fruits and dietary protein. In each case, this is expected to prevent the loss of lifespan associated with reproduction until protein or hosts are introduced. Two trends are observed in each species: first, access to protein at eclosion leads to a greater probability of survival and a higher reproductive ability than if it is delayed and, second, delayed host access reduces lifetime reproductive ability without improving life expectancy. When host access and protein availability are delayed, the rate of reproductive senescence is reduced in the medfly, whereas the rate of reproductive senescence is generally increased in the melon fly. Overall, delaying reproduction lowers the fitness of females by constraining their fecundity for the remainder of the lifespan without extending the lifespan. © 2013 The Royal Entomological Society     

Differences in size between first and replacement clutches match the seasonal decline in single clutches in Tree Swallows Tachycineta bicolor

The seasonal decline in clutch size in birds can be a response to the environmentally conditioned decrease in prospects for offspring or a consequence of a lower physical ability of late‐breeding females. To find out which of the explanations apply in Tree Swallows Tachycineta bicolor, we assessed whether replacement clutch size in this species is affected by an individual female's ability to lay a certain number of eggs. To do this, we measured the decline in clutch size as a function of laying date between first and replacement clutches in individuals that re‐nested following natural failure, and compared this with the rate of decline in clutch size with laying date for Tree Swallows that laid only a single clutch in that season. Additionally, we assessed whether the clutch size and the rate of its seasonal decline varied across years. We accounted for the truncated and under‐dispersed nature of clutch size data by using a Bayesian approach in the analysis. We found little variation in the rate of clutch size decline across years at our breeding site. Accounting for this seasonal decline in clutch size, mean clutch size was similar between single‐time breeding females and those that laid replacement clutches, implying that the number of eggs laid on the second attempt by female Tree Swallows is determined by laying date, rather than by the female's physical ability to produce a clutch of a certain size.