Decision rules for egg-color-based rejection by two cavity-nesting hosts of the brown-headed cowbird.

Hosts of obligate avian brood parasites often evolve defense mechanisms to avoid rearing unrelated young. One common defense is egg rejection, for which hosts often rely on eggshell color. Most research has assumed that hosts respond to perceived color differences between their own eggs and parasite eggs regardless of the particular color; however, recent experiments have found that many hosts respond more strongly to brown foreign eggs than to equally dissimilar blue eggs. Yet, none of these prior studies tested a brown-egg-laying species and, with only one exception, all were conducted in open nests where light levels are considered sufficient for effective color-based egg discrimination. Here, we explored how two cavity-nesting hosts of the parasitic brown-headed cowbird (Molothrus ater) - the blue-egg-laying eastern bluebird (Sialia sialis) and the brown-egg-laying house wren (Troglodytes aedon) - respond to experimental eggs painted six distinct colors ranging from blue to brown. Rejection responses of both hosts were best predicted by perceived differences in color between the model egg and their own eggs. Specifically, we found that house wrens preferentially rejected eggs bluer than their own eggs. However, although we found that bluebirds relied on perceived differences in color for their egg rejection decisions, further tests are needed to determine whether they preferentially rejected brown eggs or simply responded to absolute perceived differences in color. These findings demonstrate that these cavity-nesting birds treat perceived color differences in distinct ways, which has important implications on the coevolutionary arms races and the interpretation of avian-perceived color differences.

Sex-specific effects of hatching order on nestling baseline corticosterone in a wild songbird.

Variation in nestling growth and survival is often influenced by hatching order, with first-hatched offspring having an advantage over later-hatched younger siblings. In house wrens (Troglodytes aedon), this effect of hatching order is especially evident in asynchronously hatched broods and can lead to sex-specific differences in the size and condition of nestlings. Females appear to allocate the sex of their offspring across the laying order to capitalize on these differences. We hypothesized that levels of circulating corticosterone, the primary metabolic hormone in birds, mediates these sex-specific effects in nestlings. We predicted that: i) baseline levels of corticosterone in nestlings should vary along the hatching order, ii) effects of hatching order on baseline corticosterone should be sex specific, and iii) any sex-specificity of hatching order on baseline corticosterone could be contingent on the degree of hatching synchrony. We tested these predictions in a study in which we measured baseline corticosterone in first- and last-hatched nestlings in synchronously and asynchronously hatching broods. To assess whether any differences in nestling baseline corticosterone levels could be attributed to pre-natal maternal effects, the post-natal environment, or both, we conducted two additional studies in which we measured i) yolk corticosterone in first- and last-laid eggs and ii) baseline corticosterone in nestlings that were cross-fostered to create simulated 'asynchronously' hatched broods. There was a significant interaction between sex and relative hatching order in their effects on nestling baseline corticosterone, but no effect of hatching synchrony. Corticosterone levels remained relatively constant across the hatching order in males but decreased in females. There was a significant effect of laying order on yolk corticosterone, with first-laid eggs containing significantly higher levels of yolk corticosterone than last-laid eggs. Cross-fostering of nestlings at different points of development had no significant effect on nestling corticosterone levels. These results indicate that sex-dependent differences in corticosterone levels across the hatching order may arise, at least in part, from embryonic exposure to maternally derived corticosterone, whereas the post-natal rearing environment plays, at best, a minimal role in determining nestling baseline corticosterone levels.

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

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

Condition-Dependent Begging Elicits Increased Parental Investment in a Wild Bird Population.

The coevolution of parental supply and offspring demand has long been thought to involve offspring need driving begging and parental care, leaving other hypotheses underexplored. In a population of wild birds, we experimentally tested whether begging serves as a negatively condition-dependent signal of need or a positively condition-dependent signal of quality. Across multiple years, we supplemented nestling house wrens with food shortly after hatching and simultaneously manipulated corticosterone levels to simulate the hunger-induced increase in glucocorticoids thought to mediate begging. This allowed us to also test whether begging is simply a proximate signal of hunger. Days after supplementation ended, food-supplemented nestlings were in better condition than nonsupplemented nestlings and begged for food at an increased rate; their parents, in turn, increased provisioning to a greater extent than parents of nonsupplemented young, as begging positively predicted provisioning. Food-supplemented nestlings therefore attained above-average condition, which predicted their recruitment as breeding adults in the local population. Glucocorticoids increased begging in the short term, but this transient effect depended on satiety. Thus, glucocorticoids promoted begging as a proximate response to hunger, whereas the longer-term changes in nestling condition, begging, and food provisioning suggest that begging ultimately signals offspring quality to elicit increased investment, thereby enhancing offspring survival.

Size of nest-cavity entrance influences male attractiveness and paternal provisioning in house wrens.

In altricial birds, parental provisioning is plastic and can respond to a variety of environmental stimuli. In this study, we manipulated the size of entrances into artificial nest cavities (i.e., nestboxes) in a population of house wrens (Troglodytes aedon) as a means of manipulating a male's sexual attractiveness, and examined changes in parental provisioning. Nest cavities with large entrances are less desirable as nesting sites, and the males at these sites are less attractive to females. Therefore, we predicted that males at boxes that had large entrances would invest more in parental care (i.e., those that succeeded in finding a mate would provision their offspring at a higher rate) than males at nestboxes with small entrances. As predicted, males provisioned their offspring with food at the highest rates at nestboxes with enlarged entrances, and male provisioning effort positively predicted the number of fledglings they produced per egg. Males at these boxes provisioned more than their mates and more than females and males at nestboxes with small entrances. At nestboxes with small entrances, males provisioned at the same rate as females, and female provisioning did not differ significantly between treatments, on average. Male and female provisioning rates were negatively correlated, such that the increase in provisioning by males at nestboxes with enlarged entrances did not enhance nestling condition, likely because food delivery by females declined with increased provisioning by males. However, the amount of time females spent providing warmth for their ectothermic young increased with increases in male provisioning, suggesting that levels of male parental care altered the mode, not necessarily the extent, of care provided by females. These findings suggest that male provisioning is related to sexual attractiveness, and that sexual conflict over biparental care may not be as simple as the assessment of food provisioning might otherwise suggest.

Spring temperatures influence selection on breeding date and the potential for phenological mismatch in a migratory bird.

Climate change has affected the seasonal phenology of a variety of taxa, including that of migratory birds and their critical food resources. However, whether climate-induced changes in breeding phenology affect individual fitness, and how these changes might therefore influence selection on breeding date remain unresolved. Here, we use a 36-yr dataset from a long-term, individual-based study of House Wrens (Troglodytes aedon) to test whether the timing of avian breeding seasons is associated with annual changes in temperature, which have increased to a small but significant extent locally since the onset of the study in 1980. Increasing temperature was associated with an advancement of breeding date in the population, as the onset of breeding within years was closely associated with daily spring temperatures. Warmer springs were also associated with a reduced incubation period, but reduced incubation periods were associated with a prolonged duration of nestling provisioning. Nest productivity, in terms of fledgling production, was not associated with temperature, but wetter springs reduced fledging success. Most years were characterized by selection for earlier breeding, but cool and wet years resulted in stabilizing selection on breeding date. Our results indicate that climate change and increasing spring temperatures can affect suites of life-history traits, including selection on breeding date. Increasing temperatures may favor earlier breeding, but the extent to which the phenology of populations might advance may be constrained by reductions in fitness associated with early breeding during cool, wet years. Variability in climatic conditions will, therefore, shape the extent to which seasonal organisms can respond to changes in their environment.

Elevated corticosterone during egg production elicits increased maternal investment and promotes nestling growth in a wild songbird.

Glucocorticoids circulating in breeding birds during egg production accumulate within eggs, and may provide a potent form of maternal effect on offspring phenotype. However, whether these steroids affect offspring development remains unclear. Here, we employed a non-invasive technique that experimentally elevated the maternal transfer of corticosterone to eggs in a wild population of house wrens. Feeding corticosterone-injected mealworms to free-living females prior to and during egg production increased the number of eggs that females produced and increased corticosterone concentrations in egg yolks. This treatment also resulted in an increase in the amount of yolk allocated to eggs. Offspring hatching from these eggs begged for food at a higher rate than control offspring and eventually attained increased prefledging body condition, a trait predictive of their probability of recruitment as breeding adults in the study population. Our results indicate that an increase in maternal glucocorticoids within the physiological range can enhance maternal investment and offspring development.

Immune activation generates corticosterone-mediated terminal reproductive investment in a wild bird.

Despite classical expectations of a trade-off between immune activity and reproduction, an emergent view suggests that individuals experiencing activation of their immune system actually increase reproductive effort and allocation to offspring as a form of terminal investment in response to reduced survival probability. However, the components and mechanisms of increased parental investment following immunostimulation are currently unknown. We hypothesize that increased glucocorticoid production following immunostimulation modulates the increase in reproductive effort that constitutes terminal investment. We activated the immune system of breeding female house wrens (Troglodytes aedon) with an immunogen and cross-fostered the eggs that they subsequently produced to separate prenatal and postnatal components of maternal investment. Cross-fostering revealed an increase in both pre- and postnatal allocation from immunostimulated females, which was confirmed by quantification of egg constituents and maternal provisioning behavior. The increase in maternal provisioning was mediated, at least in part, by increased corticosterone in these females. Offspring immune responsiveness was also enhanced through transgenerational immune priming via the egg. Thus, our results indicate that maternal immunostimulation induces transgenerational effects on offspring through both pre- and postnatal parental effects and support an important role for corticosterone in mediating parental investment.

Persistent sex-by-environment effects on offspring fitness and sex-ratio adjustment in a wild bird population.

A major component of sex-allocation theory, the Trivers-Willard model (TWM), posits that sons and daughters are differentially affected by variation in the rearing environment. In many species, the amount of parental care received is expected to have differing effects on the fitness of males and females. When this occurs, the TWM predicts that selection should favour adjustment of the offspring sex ratio in relation to the expected fitness return from offspring. However, evidence for sex-by-environment effects is mixed, and little is known about the adaptive significance of producing either sex. Here, we test whether offspring sex ratios vary according to predictions of the TWM in the house wren (Troglodytes aedon, Vieillot). We also test the assumption of a sex-by-environment effect on offspring using two experiments, one in which we manipulated age differences among nestlings within broods, and another in which we held nestling age constant but manipulated brood size. As predicted, females with high investment ability overproduced sons relative to those with lower ability. Males were also overproduced early within breeding seasons. In our experiments, the body mass of sons was more strongly affected by the sibling-competitive environment and resource availability than that of daughters: males grew heavier than females when reared in good conditions but were lighter than females when in poor conditions. Parents rearing broods with 1:1 sex ratios were more productive than parents rearing broods biased more strongly towards sons or daughters, suggesting that selection favours the production of mixed-sex broods. However, differences in the condition of offspring as neonates persisted to adulthood, and their reproductive success as adults varied with the body mass of sons, but not daughters, prior to independence from parental care. Thus, selection should favour slight but predictable variations in the sex ratio in relation to the quality of offspring that parents are able to produce. Offspring sex interacts with the neonatal environment to influence offspring fitness, thus favouring sex-ratio adjustment by parents. However, increased sensitivity of males to environmental conditions, such as sibling rivalry and resource availability, reduces the fitness returns from highly male-biased broods.

Mass-based condition measures and their relationship with fitness: in what condition is condition?

Mass or body-size measures of 'condition' are of central importance to the study of ecology and evolution, and it is often assumed that differences in condition measures are positively and linearly related to fitness. Using examples drawn from ecological studies, we show that indices of condition frequently are unlikely to be related to fitness in a linear fashion. Researchers need to be more explicit in acknowledging the limitations of mass-based condition measures and accept that, under some circumstances, they may not relate to fitness as traditionally assumed. Any relationship between a particular condition measure and fitness should first be empirically validated before condition is used as a proxy for fitness. In the absence of such evidence, researchers should explicitly acknowledge that assuming such a relationship may be unrealistic.

Genetic and environmental variation in condition, cutaneous immunity, and haematocrit in house wrens.

BACKGROUND: Life-history studies of wild bird populations often focus on the relationship between an individual's condition and its capacity to mount an immune response, as measured by a commonly-employed assay of cutaneous immunity, the PHA skin test. In addition, haematocrit, the packed cell volume in relation to total blood volume, is often measured as an indicator of physiological performance. A multi-year study of a wild population of house wrens has recently revealed that those exhibiting the highest condition and strongest PHA responses as nestlings are most likely to be recruited to the breeding population and to breed through two years of age; in contrast, intermediate haematocrit values result in the highest recruitment to the population. Selection theory would predict, therefore, that most of the underlying genetic variation in these traits should be exhausted resulting in low heritability, although such traits may also exhibit low heritability because of increased residual variance. Here, we examine the genetic and environmental variation in condition, cutaneous immunity, and haematocrit using an animal model based on a pedigree of approximately 2,800 house wrens. RESULTS: Environmental effects played a paramount role in shaping the expression of the fitness-related traits measured in this wild population, but two of them, condition and haematocrit, retained significant heritable variation. Condition was also positively correlated with both the PHA response and haematocrit, but in the absence of any significant genetic correlations, it appears that this covariance arises through parallel effects of the environment acting on this suite of traits. CONCLUSIONS: The maintenance of genetic variation in different measures of condition appears to be a pervasive feature of wild bird populations, in contradiction of conventional selection theory. A major challenge in future studies will be to explain how such variation persists in the face of the directional selection acting on condition in house wrens and other species.

Neonatal body condition, immune responsiveness, and hematocrit predict longevity in a wild bird population.

Measures of body condition, immune function, and hematological health are widely used in ecological studies of vertebrate populations, predicated on the assumption that these traits are linked to fitness. However, compelling evidence that these traits actually predict long-term survival and reproductive success among individuals in the wild is lacking. Here, we show that body condition (i.e., size-adjusted body mass) and cutaneous immune responsiveness to phytohaemagglutinin (PHA) injection among neonates positively predict recruitment and subsequent longevity in a wild, migratory population of house wrens (Troglodytes aedon). However, neonates with intermediate hematocrit had the highest recruitment and longevity. Neonates with the highest PHA responsiveness and intermediate hematocrit prior to independence eventually produced the most offspring during their lifetime breeding on the study site. Importantly, the effects of PHA responsiveness and hematocrit were revealed while controlling for variation in body condition, sex, and environmental variation. Thus, our data demonstrate that body condition, cutaneous immune responsiveness, and hematocrit as a neonate are associated with individual fitness. Although hematocrit's effect is more complex than traditionally thought, our results suggest a previously underappreciated role for this trait in influencing survival in the wild.

INTERACTIVE EFFECTS OF INCREASED NESTBOX TEMPERATURE AND VITAMIN E ON NESTLING GROWTH ARE ATTENUATED BY PLASTICITY IN FEMALE INCUBATION EFFORT.

In recent years, temperatures have increased globally, and nestlings of many bird species are likely regularly exposed to increased temperatures both pre- and postnatally. Even small increases in nest temperature during incubation affect offspring growth and survival in a variety of species, one cause of which is thought to be increased production of prooxidants in embryos and nestlings. Defences marshalled in response to this oxidative stress could, in turn, result in trade-offs that lead to reduced survival or growth. If so, any downstream negative effects on nestlings of increased ambient temperatures during incubation could be counteracted by increasing their antioxidant intake. We predicted, therefore, that dietary supplements of an antioxidant would reduce or eliminate any detrimental effects on nestling growth and survival of experimentally increased nest temperature during the incubation period. We employed a split-brood design in which we increased nest temperature of entire clutches and, after hatching, provided dietary supplements of the antioxidant vitamin E to half of the nestlings within broods. We also recorded female incubation and provisioning behaviour to control for the possibility that heating nests might also influence maternal behaviour. There was a significant interaction between nestbox heating treatment and vitamin E treatment in their effect on nestling mass, a trait that is positively correlated with survival and future reproductive success in the study population. Vitamin E supplementation promoted increased nestling mass in heated nests, whereas it had the opposite effect in control nests, but these effects were weak. Heating significantly affected female incubation behaviour, with females in heated nestboxes investing less in incubation than those in unheated boxes. These results suggest that within at least some range of expected increased ambient temperatures during the 21st century, effects of climate change on nestling bird development can be mitigated by adjustments in female incubation behaviour.

Male song structure predicts offspring recruitment to the breeding population in a migratory bird.

Bird song is a classic example of a sexually selected trait, but much of the work relating individual song components to fitness has not accounted for song typically being composed of multiple, often-correlated components, necessitating a multivariate approach. We explored the role of sexual selection in shaping the complex male song of house wrens (Troglodytes aedon) by simultaneously relating its multiple components to fitness using multivariate selection analysis, which is widely used in insect and anuran studies but not in birds. The analysis revealed significant variation in the form and strength of selection acting on song across different selection episodes, from nest-site defense to recruitment of offspring to the breeding population. Males that sang more song typically employed in close communication sired more offspring that were subsequently recruited to the breeding population than those that sang more far-communication song. However, this relationship was not consistent across earlier selection episodes, as evidenced by non-linear selection acting on these song components in other contexts. Collectively, our results present a complex picture of multivariate selection on male song structure that would not be evident using univariate approaches and suggest possible trade-offs within and among song components at different points of the breeding season.

Sibling cooperation influences the age of nest leaving in an altricial bird.

In altricial birds, siblings raised within a nest usually leave the nest within hours of each other, despite often differing considerably in age. The youngest members of the brood are typically underdeveloped at this time and less likely than their older siblings to survive outside the nest, yet they risk abandonment if they do not fledge with their older siblings. Nest leaving is usually initiated by the older offspring, which may delay this process to provide more time for their younger siblings to mature, increasing the younger siblings' postfledging survival and their own inclusive fitness. We tested this hypothesis in a population of house wrens Troglodytes aedon and found that broods with broad age spans among siblings had longer nestling periods than broods with narrow age spans and that delayed fledging improves the survival and reproductive prospects of younger siblings, although at a potential cost to future siblings. We also manipulated age spans through cross-fostering and found that older foster nestlings postponed fledging when raised with younger broodmates, as predicted if the age of younger nestlings determines the time of fledging. Our results support kin-selection theory and demonstrate that the exact time of fledging is attributable, in part, to sib-sib interactions.

Sex-biased terminal investment in offspring induced by maternal immune challenge in the house wren (Troglodytes aedon).

The reproductive costs associated with the upregulation of immunity have been well-documented and constitute a fundamental trade-off between reproduction and self-maintenance. However, recent experimental work suggests that parents may increase their reproductive effort following immunostimulation as a form of terminal parental investment as prospects for future reproduction decline. We tested the trade-off and terminal investment hypotheses in a wild population of house wrens (Troglodytes aedon) by challenging the immune system of breeding females with lipopolysaccharide, a potent but non-lethal antigen. Immunized females showed no evidence of reproductive costs; instead, they produced offspring of higher phenotypic quality, but in a sex-specific manner. Relative to control offspring, sons of immunized females had increased body mass and their sisters exhibited higher cutaneous immune responsiveness to phytohaemagglutinin injection, constituting an adaptive strategy of sex-biased allocation by immune-challenged females to enhance the reproductive value of their offspring. Thus, our results are consistent with the terminal investment hypothesis, and suggest that maternal immunization can induce pronounced transgenerational effects on offspring phenotypes.

Avian eggshell coloration predicts shell-matrix protoporphyrin content.

Avian eggshell pigmentation may provide information about a female's physiological condition, in particular her state of oxidative balance. Previously we found that female house wrens (Troglodytes aedon Vieillot, 1809) with lighter, less-maculated, and redder ground-colored shells were older and produced heavier offspring than females laying darker, browner eggs. The strong pro-oxidant protoporphyrin is responsible for this species' eggshell pigmentation, so differences in pigmentary coloration may be related to eggshell protoporphyrin content and reflect female oxidative balance and condition during egg-formation. Therefore, we tested the assumption that egg-surface coloration is related to the amount of protoporphyrin in the shell matrix. We analyzed digital photographs of eggs to determine maculation coverage as a measure of the overall ground coloration of the egg and its red-, green-, and blue-channel pixel values. Pigments were then extracted from these same eggs and analyzed using high-performance liquid chromatography. There was a strong, positive relationship between eggshell redness and protoporphyrin content of eggshells, but no relationship between percent maculation and protoporphyrin content. Thus, when older, larger females deposit more protoporphyrin in their eggshells, this may reflect a tolerance for high levels of circulating protoporphyrin or an effective mechanism for off-loading protoporphyrin into the eggshell matrix.

Adaptive sex allocation in relation to hatching synchrony and offspring quality in house wrens.

Increased variance in the reproductive success of males relative to females favors mothers that optimally allocate sons and daughters to maximize their fitness return. In altricial songbirds, one influence on the fitness prospects of offspring arises through the order in which nestlings hatch from their eggs, which affects individual mass and size before nest leaving. In house wrens (Troglodytes aedon), the influence of hatching order depends on the degree of hatching synchrony, with greater variation in nestling mass and size within broods hatching asynchronously than in those hatching synchronously. Early-hatching nestlings in asynchronous broods were heavier and larger than their later-hatching siblings and nestlings in synchronous broods. The effect of hatching order was also sex specific, as the mass of males in asynchronous broods was more strongly influenced by hatching order than the mass of females, with increased variation in the mass of males relative to that of females. As predicted, mothers hatching their eggs asynchronously biased first-laid, first-hatching eggs toward sons and late-laid, late-hatching eggs toward daughters, whereas females hatching their eggs synchronously distributed the sexes randomly among the eggs of their clutch. We conclude that females allocate the sex of their offspring among the eggs of their clutch in a manner that maximizes their own fitness.

Experimentally increased in ovo testosterone leads to increased plasma bactericidal activity and decreased cutaneous immune response in nestling house wrens.

Maternally derived testosterone in the eggs of birds may benefit nestlings by increasing various aspects of their growth, condition and behavioral development, but these benefits may come at a cost, including suppression of immune responsiveness. Experiments on a variety of species in which in ovo levels of testosterone have been experimentally increased have produced mixed results; some have found increased growth and suppressed immune function of nestlings whereas others have found the opposite. In an attempt to clarify the relationship between in ovo testosterone and nestling size, mass, health state and immune responsiveness, we experimentally increased levels of testosterone in the eggs of house wrens (Troglodytes aedon). We simultaneously determined the size, mass, hematocrit (a measure of health state), cutaneous immune response to phytohaemagglutinin and plasma bactericidal activity of nestlings near the time of fledging. We predicted that nestlings hatching from testosterone-injected eggs would exhibit lower immune responsiveness, but achieve greater mass, size and condition, than nestlings hatching from vehicle-injected control eggs. Instead, we found that nestlings hatching from testosterone-injected eggs had a weaker cutaneous immune response but greater bactericidal activity than those hatching from control eggs. They did not, however, differ significantly in mass, size or hematocrit from controls. These results suggest that experimentally increased in ovo testosterone induced a trade-off between bactericidal activity and the cutaneous immune response. The opposite responses by two different measures of immune function to experimentally increased in ovo testosterone underscore the importance of including multiple immune assays when investigating the potential for trade-offs with the immune system and other physiological functions.

Evidence for heterozygote instability in microsatellite loci in house wrens.

Microsatellite loci have high mutation rates and high levels of allelic variation, but the factors influencing their mutation rate are not well understood. The proposal that heterozygosity may increase mutation rates has profound implications for understanding the evolution of microsatellite loci, but currently has limited empirical support. We examined 20 microsatellite mutations identified in an analysis of 12 260 meiotic events across three loci in two populations of a songbird, the house wren (Troglodytes aedon). We found that for an allele of a given length, mutation was significantly more likely when there was a relatively large difference in size between the allele and its homologue (i.e. a large 'allele span'). Our results support the proposal of heterozygote instability at microsatellite loci.