Costs and benefits of parental care in eastern kingbirds

We manipulated brood sizes of eastern kingbirds (Tyrannus tyrannus)to measure the costs and benefits of parental care and to testwhether kingbirds showed evidence of individual optimizationof reproductive effort. We found that the number of feedingtrips (trips/h) increased and that per capita feeding rates(trips/nestling/h) declined as brood size increased. The declinein per capita feeding rates was mostly due to high feeding rateto broods of one: parents made roughly equal number of tripsto feed each nestling in broods of two to five. Nonetheless,nestling mass declined with brood size, probably because largebroods were fed more small prey. Nestling condition (mass adjustedfor structural size) differed only between broods of one andfive. After controlling for effects of brood size, feeding rateshad no supplementary influence on either nestling size or condition,but productivity and feeding rate were positively and significantlyrelated. Adult male condition did not vary with brood size,manipulated brood size, or total feeding rate, but declinedas the pair's per capita feeding rates increased. In addition,males that returned to breed were in better condition beforeleaving for migration than those that failed to return. Femalecondition tended to decline, and the probability of returningto breed dropped when broods were enlarged. However, femalecondition was independent of the probability of returning. Ourresults show that high feeding rates were costly, but that theycarried benefits (greater productivity). Some evidence for individualoptimization of reproductive effort existed: variability innestling and adult female condition were better explained bychanges in brood size than by the actual number of young inthe nest. However, most evidence supported the alternative thatincreased brood size was equally costly for all birds     

Reproductive effort and T-lymphocyte cell-mediated immunocompetence in female pied flycatchers Ficedula hypoleuca

The physiological mechanism underlying the cost of reproduction may consist of immunodepression caused by increased parental effort. Here, we report effects of experimental manipulation of clutch size on T-lymphocyte cell-mediated immune response in female pied flycatchers, Ficedula hypoleuca. Parents with reduced broods provisioned at lower rates than those caring for control and enlarged broods three days after hatching. Parents caring for enlarged broods provisioned nests at higher rates 13 days after chick hatching than those feeding control and reduced broods. Females with enlarged broods weighed less than females with control or reduced broods. No effect of experimental treatment on nestling mass and size was found. The response to the injection of phytohaemagglutinin in the wing-web of females decreased with increasing brood size and with increasing provisioning rate when the chicks were three days old, when controlling for the negative effect of female mass on response. The T-lymphocyte cell-mediated response decreased from the reduced to the control, and from this to the enlarged group, when controlling for female mass. This effect of experimental manipulation of clutch size was significant and consistent with a trade-off between maternal effort and immunocompetence.     

Brood size modifications affect plumage bacterial assemblages of European starlings

During reproduction, birds face trade-offs between time and energy devoted to parental effort and traits associated with self-maintenance. We manipulated brood sizes to investigate the effects of such trade-offs on feather bacterial densities and the structure of bacterial assemblages on feathers in adult European starlings, Sturnus vulgaris, and in vitro feather degradation. As predicted by a trade-off between parental effort and self-maintenance, we found that birds with enlarged broods had more free-living bacteria on their feathers than birds with reduced broods. Furthermore, we found a significant interaction between brood manipulation and original brood size on free-living bacterial densities suggesting that the trade-off is mediated by the adults' initial reproductive investment. In contrast, brood size manipulations had no significant effect on densities of attached bacteria. Using ribosomal intergenic spacer analysis (RISA), we demonstrated that brood manipulations significantly modified the structure (band pattern) of feather-degrading bacterial assemblages, but had no significant effect on their richness (number of bands) or the in vitro feather degradation. In vitro feather degradation varied in relation to the premanipulation brood size and positively with the richness of the feather degrading bacterial community. Besides brood manipulation effect, we found that ecological factors and individual traits, such as the age, the nest location or the capture date, shaped bacterial assemblages and feather degradation capacities.     

Cascading costs of reproduction in female house wrens induced to lay larger clutches

In many species, females produce fewer offspring than they are capable of rearing, possibly because increases in current reproductive effort come at the expense of a female's own survival and future reproduction. To test this, we induced female house wrens (Troglodytes aedon) to lay more eggs than they normally would and assessed the potential costs of increasing cumulative investment in the three main components of the avian breeding cycle – egg laying, incubation and nestling provisioning. Females with increased clutch sizes reared more offspring in the first brood than controls, but fledged a lower proportion of nestlings. Moreover, nestlings of experimental females were lighter than those of control females as brood size and prefledging mass were negatively correlated. In second broods of the season, when females were not manipulated, experimental females laid the same number of eggs as controls, but experienced an intraseasonal cost through reduced hatchling survival and a lower number of young fledged. Offspring of control and experimental females were equally likely to recruit to the breeding population, although control females produced more recruits per egg laid. The reproductive success of recruits from broods of experimental and control females did not differ. The manipulation also induced interseasonal costs to future reproduction, as experimental females had lower fecundity than controls when breeding at least 2 years after having their reproductive effort experimentally increased. Finally, females producing the modal clutch size of seven eggs in their first broods had the highest lifetime number of fledglings.     

Responses by Central‐Place Foragers to Manipulations of Brood Size: Parent Flickers Respond to Proximate Cues but do not Increase Work Rate

Manipulations of brood size measure the willingness or ability of parents to invest in offspring and different reproductive roles may lead to differences in feeding effort between the sexes. Parental investment in birds is usually assessed by quantifying feeding rates, but this provides an incomplete picture of parental effort because it fails to account for how parents collect food on the landscape. We studied northern flickers (Colaptes auratus), a woodpecker in which males provide the majority of parental care and used a repeated measures design and short‐term (24 h) brood enlargements (N = 35) and reductions (N = 27) to assess effects of treatment on feeding rates to nestlings and parental foraging behaviour. Parents of enlarged broods did not significantly increase feeding rate, resulting in a decline in nestling mass. Parents of reduced broods decreased their feeding rates by 84%, but increased per capita feeding rates, resulting in nestling mass gain. The variation in feeding rates to enlarged broods was not influenced by feather corticosterone, body condition, feather re‐growth rate or mass change between the incubation and nestling periods. Foraging pattern on the landscape remained the same during the enlarged treatment for both sexes. We conclude that flickers respond to proximate cues in brood demands, but do not increase feeding rates to enlarged broods, at least in the short term. A literature review suggested that this lack of response is atypical for short‐lived species. We hypothesize that parents in species with large home ranges and long nestling periods face energetic limitations that constrain their ability to respond to enlarged broods. We encourage future studies to assess foraging behaviour on the landscape to document important trade‐offs for parents such as predation risk and energy expenditure while feeding offspring.     

Reproductive effort reduces long-term immune function in breeding tree swallows (Tachycineta bicolor)

We examined whether strategies of reproductive allocation may reduce long-term immunocompetence through the effects of manipulated effort on secondary or acquired immunity. We tested whether increased reproductive effort leads to reduced immune function and survival by manipulating brood size in tree swallows (Tachycineta bicolor) and exposing breeding females to a primary and secondary exposure of sheep red blood cells to elicit a humoral immune response. Females raising enlarged broods produced fewer secondary antibodies than did females raising control or reduced broods. Most importantly, individuals with high secondary responses were more likely to survive to breed 3 years after brood manipulations, suggesting that differences in disease susceptibility may be caused by trade-offs in reproductive allocation. We also found that individual quality, measured by clutch initiation date, mediated the effects of brood manipulations, with higher-quality birds showing a greater ability to deal with increases in effort.     

Brood size and body condition in the House Sparrow Passer domesticus: the influence of brooding behaviour

In many bird species, females undergo a marked decline in body condition during the first days of the nestling period. This decline may be because brooding young chicks reduces the time available for foraging. Alternatively, it might be viewed as an adaptive way to reduce flight costs when the food demand of the brood is highest. To test these hypotheses we modified the brooding commitment of House Sparrows Passer domesticus by manipulating brood size to see if changes in time spent brooding affects adult body condition. During the nestling period, females provided on average three times as much brooding as males. Reduced broods received 14% more brooding than large broods and time spent brooding declined with brood size and chick age according to an exponential decay function. Male body condition was unaffected by brood size and remained stable throughout the reproductive period. Body condition of females with enlarged broods decreased gradually during the nestling period, whereas that of females tending reduced broods dropped abruptly and significantly upon hatching. This resulted in females with reduced broods having lower body condition during the first half of the nestling period than those with enlarged broods. The sharp drop in body condition of females with reduced broods coincided with the period that brooding was most intensive. Indeed, female body condition at the end of the nestling period was negatively correlated with the proportion of time they spent brooding during the first half of the nestling period. Thus, the probable lower homeothermic capacities of reduced broods implies a higher brooding commitment for female House Sparrows that, in turn, may reduce their opportunity to forage and consequently also their body condition.     

Avian malaria is associated with increased reproductive investment in the blue tit

Haemosporidians causing avian malaria are very common parasites among bird species. Their negative effects have been repeatedly reported in terms of deterioration in survival prospects or reproductive success. However, a positive association between blood parasites and avian fitness has also been reported. Here, we studied a relationship between presence of malaria parasites and reproductive performance of the host, a hole‐breeding passerine – the blue tit Cyanistes caeruleus. Since the malaria parasites might affect their hosts differently depending on environmental conditions, we performed brood size manipulation experiment to differentiate parental reproductive effort and study the potential interaction between infection status and brood rearing conditions on reproductive performance. We found individuals infected with malaria parasites to breed later in the season in comparison with uninfected birds, but no differences were detected in clutch size. Interestingly, infected parents produced heavier and larger offspring with stronger reaction to phytohemagglutinin. More importantly, we found a significant interaction between infection status and brood size manipulation in offspring tarsus length and reaction to phytohemagglutinin: presence of parasites had stronger positive effect among birds caring for experimentally enlarged broods. Our results might be interpreted either in the light of the parasite‐mediated selection or terminal investment hypothesis.     

Sex-specific costs of reproduction in Eastern Bluebirds Sialia sialis

In species with bi-parental care, individuals must partition energy between parental effort and mating effort. Typically, female songbirds invest more than males in reproductive activities such as egg-laying and incubation, but males invest more in secondary sexual traits used in attracting mates. Animals that breed more than once within a season must also allocate time and energy between first and subsequent breeding attempts and between current and future breeding seasons. To investigate strategies of reproductive investment by males and females and the consequences of such strategies, we manipulated the size of broods of Eastern Bluebirds Sialia sialis . Pairs with enlarged first broods were less likely to produce a second clutch or took longer to initiate one than pairs with reduced broods. After rearing enlarged broods, females were less likely than males to survive to the following year. Although plumage coloration is a sexually selected trait in Eastern Bluebirds that is influenced by nutritional stress, we did not detect an effect of brood-size manipulation on female coloration. Past research, however, demonstrates that, in males, plumage colour is negatively affected by increasing brood size. We suggest that there are sex-specific strategies of reproductive investment in Eastern Bluebirds, and that researchers should incorporate measures of residual reproductive value in studies of life-history evolution.     

Twofold cost of reproduction: an increase in parental effort leads to higher malarial parasitaemia and to a decrease in resistance to oxidative stress

Parental effort is usually associated with high metabolism that could lead to an increase in the production of reactive oxidative species giving rise to oxidative stress. Since many antioxidants involved in the resistance to oxidative stress can also enhance immune function, an increase in parental effort may diminish the level of antioxidants otherwise involved in parasite resistance. In the present study, we performed brood size manipulation in a population of great tits (Parus major) to create different levels of parental effort. We measured resistance to oxidative stress and used a newly developed quantitative PCR assay to quantify malarial parasitaemia. We found that males with an enlarged brood had significantly higher level of malarial parasites and lower red blood cell resistance to free radicals than males rearing control and reduced broods. Brood size manipulation did not affect female parasitaemia, although females with an enlarged brood had lower red blood cell resistance than females with control and reduced broods. However, for both sexes, there was no relationship between the level of parasitaemia and resistance to oxidative stress, suggesting a twofold cost of reproduction. Our results thus suggest the presence of two proximate and independent mechanisms for the well-documented trade-off between current reproductive effort and parental survival.     

Nosy neighbours: large broods attract more visitors. A field experiment in the pied flycatcher, <Emphasis Type="Italic">Ficedula hypoleuca</Emphasis>

Life is uncertain. To reduce uncertainty and make adaptive decisions, individuals need to collect information. Individuals often visit the breeding sites of their conspecifics (i.e., “prospect”), likely to assess conspecifics’ reproductive success and to use such information to identify high-quality spots for future breeding. We investigated whether visitation rate by prospectors and success of visited sites are causally linked. We manipulated the reproductive success (enlarged, reduced, and control broods) in a nest-box population of migratory pied flycatchers, Ficedula hypoleuca, in Finland. We measured the visitation rates of prospectors at 87 nest-boxes continuously from manipulation (day 3 after hatching) to fledging. 302 adult pied flycatchers prospected 9194 times on these manipulated nests (at least 78% of detected prospectors were successful breeders). While the number of visitors and visits was not influenced by the relative change in brood size we induced, the resulting absolute brood size predicted the prospecting behaviour: the larger the brood size after manipulation, the more visitors and visits a nest had. The parental provisioning rate at a nest and brood size pre-manipulation did not predict the number of visitors or visits post-manipulation. More visitors, however, inspected early than late nests and broods in good condition. Our study suggests that individuals collect social information when visiting conspecific nests during breeding and provides evidence that large broods attract more visitors than small broods. We discuss the results in light of individual decision-making by animals in their natural environments.     

Cost of reproduction: parental survival and production of recruits in the Willow Tit Parus montanus

Summary Brood sizes of the Willow Tit were altered experimentally by subtracting or adding two nestlings in 1986 and 1987 in the vicinity of Oulu, northern Finland. The manipulated broods were within the normal range observed in natural conditions. Unaltered broods were used as controls. Data from natural broods from 1978–1985 were available for comparison. When the nestlings were 13 days old they were ringed and weighed and their tarsus, wing, and tail lengths were measured. On the same day the parents were caught, weighed, and measured. In 1986 there were no differences in nestling mortality between the reduced, control, or enlarged broods; i.e. parents were able to fledge the two extra young. In 1987 starvation was most pronounced in the enlarged broods. This resulted in the number of fledglings being practically the same in each manipulation category. Especially the body weight, but also the other indices of body size, decreased as a function of the brood size category, suggesting that there may be quality differences between the young reared in different experimental groups. In 1986 there was a non-significant trend towards lower body weight of the parents attending reduced, control, or enlarged broods, in that order. In 1987 the differences were much smaller. These results were not due to size differences between the groups, so possibly the increased reproductive effort of raising extra young was responsible for the trend observed in 1986. There were no significant differences in parental survival associated with the manipulation category, although the trend in the females was consistent with the hypothesis of reproductive cost. It is possible that environmental conditions in 1986 were so favourable that the tits were not unduly stressed even when attending two extra young. Correlative data from 1978–1985 did not support the cost hypothesis either. A non-significant trend towards reduced post-fledging survival and recruitment of the young was observed with increased brood size. The average fitness value of parents, incorporating parental survival and number of recruits, showed that the success of the adults raising enlarged broods may be lower than that of others. It seems that the reproductive cost, if it exists, decreases individual fitness value by reducing the chances of recruiting descendants into the next generation. The reproductive stress may be insufficient to reduce the subsequent survival of parents. More data are however needed to confirm these results.     

The effect of elevated reproductive effort onhumoral immune function in collared flycatcher females

In order to test whether high reproductive investments impair immune function in naturally breeding collared flycatchers, we performed a brood manipulation experiment and simultaneously induced an immune response by challenging birds with a non-pathogenic antigen – sheep red blood cells (SRBC). Females rearing experimentally enlarged number of nestlings showed significantly lower level of specific anti-SRBC antibodies than control females attending unaltered broods, but only in one of the two study years. The haemoconcentration of leukocytes did not differ between the two groups in both study years. The significant difference in immunological responsiveness between control and enlarged group coincided with differences in survival probability to the next breeding season: females attending enlarged broods showed lower probability of survival than control females, but there was no relationship between the level of immune response and survival probability. Our results indicate that reproduction may indeed trade for resources with immune functions at least in terms of specific antibody production. However, as in the other studies on reproductive costs, these costs seem not always to be pronounced.     

Azure‐winged Magpies Cyanopica cyanus trade off reproductive success and parental care by establishing a size hierarchy among nestlings

It is common in birds that the sizes of nestlings vary greatly when multiple young are produced in one nest. However, the methods used by parents to establish size hierarchy among nestlings and their effect on parental provisioning pattern may differ between species. In the Azure‐winged Magpie Cyanopica cyanus, we explored how and why parents controlled the sizes of nestlings. Asynchronous hatching was the main cause of size hierarchy within the brood, although the laying of larger eggs later in the laying sequence reduced this effect. Parents with asynchronous broods produced more eggs and fledged more nestlings than those with synchronous broods but their brood provisioning rates, food delivery per feeding bout and feeding efficiency did not differ. We performed a cross‐fostering experiment to synchronize some asynchronous broods. Provisioning rates of asynchronous broods were lower than those of synchronized broods, but the daily growth rates and fledging body mass of their nestlings were not different. Our findings indicate that parents of asynchronous broods can achieve higher reproductive success than those of synchronous broods based on the same parental care, and the same reproductive success as those of synchronized broods based on less parental care. It appears that parent birds can better trade off reproductive success and parental care by establishing a size hierarchy among nestlings.     

Ectoparasites and reproductive trade-offs in the barn swallow (Hirundo rustica)

Parents are predicted to trade offspring number and quality against the costs of reproduction. In altricial birds, parasites can mediate these costs because intensity of parasitism may increase with parental effort. In addition, parasites may mediate a trade-off between offspring number and quality because nestlings in large broods may have reduced anti-parasite immune defence. In this study, we experimentally analysed the effect of brood size on infestation by an ectoparasitic mite in nests of barn swallows (Hirundo rustica). Nests with an enlarged brood had larger prevalence and intensity of infestation than those with a reduced brood. Importantly, each nestling in enlarged broods was exposed to a larger number of mites, even when measured on a per nestling basis, than in reduced broods. Nestlings in enlarged broods had smaller body mass and T-cell-mediated immune response compared to reduced broods. T-cell-mediated immune response and feather growth were negatively correlated with per nestling intensity of infestation in enlarged but not in reduced broods. The results suggest that nestlings in enlarged broods have depressed immunity leading to larger per nestling mite infestation. Hence, exposure to parasites of offspring and parents increases with brood size, and parasitism can thus mediate trade-offs between reproduction and number and quality of the progeny in the barn swallow.     

Do maternal food deprivation and offspring predator cues interactively affect maternal effort in fish?

The state of the environment parents are exposed to during reproduction can either facilitate or impair their ability to take care of their young. Thus, the environmental conditions experienced by parents can have a transgenerational impact on offspring phenotype and survival. Parental energetic needs and the variance in offspring predation risk have both been recognized as important factors influencing the quality and amount of parental care, but surprisingly, they are rarely manipulated simultaneously to investigate how parents adjust care to these potentially conflicting demands. In the maternally mouthbrooding cichlid Simochromis pleurospilus, we manipulated female body condition before spawning and exposure to offspring predator cues during brood care in a two‐by‐two factorial experiment. Subsequently, we measured the duration of brood care and the number and size of the released young. Furthermore, we stimulated females to take up their young by staged predator attacks and recorded the time before the young were released again. We found that food‐deprived females produced smaller young and engaged less in brood care behaviour than well‐nourished females. Final brood size and, related to this, female protective behaviour were interactively determined by nutritional state and predator exposure: well‐nourished females without a predator encounter had smaller broods than all other females and at the same time were least likely to take up their young after a simulated predator attack. We discuss several mechanisms by which predator exposure and maternal nutrition might have influenced brood and offspring size. Our results highlight the importance to investigate the selective forces on parents and offspring in combination, if we aim to understand reproductive strategies.     

Sex allocation in black-capped chickadees Poecile atricapilla

Optimal sex allocation for individuals can be predicted from a number of different hypotheses. Fisherian models of sex allocation predict equal investment in males and females up to the end of parental care and predict brood compositions based on the relative costs of producing males and females. The Trivers-Willard hypothesis predicts that individual females should alter the sex ratio of their broods based on their own condition if it has a differential impact on the lifetime reproductive success of their sons and daughters. The Charnov model of sex allocation predicts that females should alter sex allocation based on paternal attributes that may differentially benefit sons versus daughters. Because females are the heterogametic sex in birds, many recent studies have focussed on primary sex ratio biases. In black-capped chickadees Poecile atricapilla , males are larger than females suggesting they may be more costly to raise than females. Female condition affects competitive ability in contests for mates, and thus may be related to variance in fecundity. Females prefer high-ranking males as both social and extrapair partners. These observations suggest that females might vary the sex ratio of their broods based on the predictions of any of the above models. Here, we report on the results of PCR based sex determination of 1093 nestlings in 175 broods sampled from 1992 to 2001. Population-wide, we found a mean brood sex ratio of 0.525±0.016, with no significant deviation from a predicted binomial distribution. We found no effect of clutch size, female condition, hatch date, parental rank or paternity. Our results reject the idea that female black-capped chickadees systematically vary sex allocation in their broods.     

Sexual conflict among polygynous pied flycatchers feeding young

In the polygynous pied flycatcher, Ficedula hypoleuca, reproductivesuccess of females is constrained by male food provisioningduring the nestling period. Hence, there will be conflictinginterests among the male and each of his mates as to how malefeeding effort should be shared among broods. This paper describesthree experiments designed to examine the parental behaviorof the members of a bigynous trio, i.e., the male and his twomates, in light of these conflicts. In all experiments, primaryand secondary broods were manipulated to hatch on the same dayto reduce the difference in brood-reproductive value due toage. Males divided their effort equally when the two broodswere the same size. However, males did not allocate their investmentin proportion to brood size when brood sizes differed, but investedmore heavily per young in the larger broods. This finding suggeststhat males tried to optimize the joint effort of their two mates.Males and females showed similar responses to experimental reductionin brood demands, which indicates no difference in their willingnessto invest in offspring. When one of the male’s mates wasremoved temporarily, the male increased his total feeding rateand provided proportionately more food to the "motherless" brood.Through flexible allocation of parental investment, males seemable to optimize their reproductive interests in the two broods.The only way a polygynously mated female might successfullyincrease the amount of male assistance at her nest is to makeher own brood more valuable for the male, relative to the otherbroods he might have. We discuss some ways this might be achieved.[Behav Ecol 1991;2:106–115]     

Is the small clutch size of a Corsican blue tit population optimal?

In an attempt to test predictions of the optimisation hypothesis of life history traits in birds, we estimated fitness consequences of brood size manipulations. Experiments were carried out over a period of 4 years in a Mediterranean population of blue tits Parus caeruleus which is confronted with a particular set of environmental constraints. Effects of brood size manipulation were investigated in relation to year-to-year variation in environmental conditions, especially caterpillar abundance. There was a strong variation in the effects of brood size manipulation depending on year. Most effects were on offspring quality (fledging mass, tarsus length). The absolute number of recruits did not significantly differ among categories (reduced, control, enlarged broods) but varied considerably among years. Females recruited from enlarged broods were of lower quality, started to breed later and laid fewer eggs than those recruited from control and reduced broods. Neither parental survival nor reproductive performances of adults in year n + 1 was affected by brood size manipulation in year n. Thus there was no evidence for a cost of reproduction in this population. Since the number of recruits did not depend on brood size manipulation (recruitment rates were higher in reduced broods), but recruits from reduced broods were of better quality compared with other groups, we conclude that adults lay a clutch that is larger than that which is predicted by the optimisation hypothesis. Producing more young could incur some penalties because offspring from large broods are of lower quality and less likely to recruit in the population. Two possible reasons why decision rules in this population seem to be suboptimal are discussed. Received: 10 March 1998 / Accepted: 1 July 1998     

Filial Cannibalism in the Biparental Fish Cichlasoma nigrofasciatum (Pisces: Cichlidae) in Response to Early Brood Reductions

Filial cannibalism (eating one's own offspring) may enhance a parent's lifetime reproductive success if the costs associated with this behaviour are outweighed by its benefits. An organism might limit its parental care to relatively high quality offspring and eat the others. Or, an organism capable of repeated breeding in the same season might eat the survivors of a brood that was reduced by predators, and breed again. In this study, we manipulated brood size of convict cichlids by removing 0 % (control), 33 % (ER 33) or 66 % (ER 66) of the eggs spawned. The results show that smaller broods are more likely to be cannibalized by their parent(s) than are larger broods. Furthermore, pairs with reduced broods were preparing to respawn; female gonad weights were significantly higher in the brood-reduction groups than in the control group. In a second experiment, we show that the behaviour of the parents differed significantly between experimental groups; the ER 66 group performed less parental care than the control group. Moreover, females invested more parental effort than males. The results suggest that filial cannibalism may be a tactic used by parental convict cichlids to enhance their lifetime reproductive success.