Increased cuckoldry as a cost of breeding late for male house wrens (Troglodytes aedon)

One factor hypothesized to influence the reproductive behaviorof individuals is the degree to which reproductive effortsare synchronized with others in the population. We asked whetherthe timing of a pair's breeding cycle, relative to cycles ofpairs on neighboring territories, affected rates of extrapairmating over 2 years in a Wyoming population of house wrens (Troglodytes aedon). Extrapair young (identified using 5 microsatelliteloci) occurred in 74% of nests of 19 pairs whose cycles began later than cycles of one or more neighbors compared to only26% of nests of 27 pairs whose cycles began earlier than, orsimultaneously with, cycles of all neighbors. Extrapair offspringoccurred in 65% of 17 nests belonging to males who initiallysettled and began nesting early relative to neighbors but who were forced to renest late after we removed their first mates.Rates of cuckoldry were not significantly different for forced-lateand naturally late males. Our experimental approach controlledfor possible effects of male quality, clearly demonstratingan effect of timing of breeding on extrapair mating activity.     

Are brighter eggs better? Egg color and parental investment by House Wrens

ABSTRACT Recent work suggests that avian egg color could be a sexually selected signal to males that provides information about female condition, female genetic quality, or maternal investment in eggs. Theory predicts that egg color should influence male investment if it is an honest signal of the marginal fitness returns on paternal investment; a male should invest more in a colorful clutch if that investment increases offspring success more than an equivalent investment in a less colorful clutch. Some experimental support for this hypothesis has been found for species that lay blue eggs containing the pigment biliverdin, a potentially costly antioxidant. However, the brown eggshell pigment protoporphyrin, a pro‐oxidant associated with poor female condition, has received less attention as a potential predictor of female quality or investment. We performed a cross‐fostering experiment with House Wrens (Troglodytes aedon) in southwest Michigan in 2007 to test whether brown egg color was related to female condition or maternal investment, and whether male provisioning of nestlings was related to egg color. We swapped entire clutches between nests and measured egg characteristics and parental provisioning rates. We found that brighter eggs (i.e., those with less brown pigment) were heavier, and that nestlings that hatched from brighter eggs were fed at higher rates by their foster mothers, but not by their foster fathers. This pattern is consistent with the hypothesis that egg color is a potential signal of egg quality and female investment, but we found no evidence of a male response to this potential signal. This lack of a response could be the result of methodological limitations, a nonadaptive biological constraint, or adaptive indifference because chicks from brighter eggs do not actually yield increasing marginal returns on paternal investment. Clearly, additional study is needed to differentiate among these possibilities.     

Female house wrens (Troglodytes aedon) increase the size, but not immunocompetence, of their offspring through extra-pair mating

House wrens are typically socially monogamous, but frequently engage in extra-pair matings leading to multisired broods. Because females do not appear to acquire direct material benefits from their extra-pair mates, we tested the hypothesis that female house wrens derive indirect genetic benefits, such as enhanced immunocompetence (cutaneous immune activity, humoral immunity, and plasma bactericidal activity) and condition (size and haematoserological traits) for their offspring, by mating polyandrously. We predicted that extra-pair young (EPY) should show greater immune responsiveness and better body condition than their within-pair maternal half-siblings (WPY). Contrary to our prediction, WPY had higher cutaneous immune activity than their EPY brood-mates in two of three years, and EPY and WPY did not differ in measures of innate and humoral immunity. WPY also had higher albumin to γ-globulin ratios than EPY; however, they were not in better condition based on other measures. EPY had consistently longer tarsi (a measure of long-bone size) than their WPY half-siblings, suggesting that females engage in extra-pair copulations with larger males. The benefits of large structural size in the study population is unknown, but based on evidence from other passerines, we suggest that structural size may be an important fitness-related trait in house wrens. We conclude that our results are not consistent with the hypothesis that females gain immune-related benefits for their offspring by engaging in extra-pair matings. Further study of the fitness consequences of differences in tarsus length is needed to determine whether females acquire size-related benefits for their offspring from extra-pair mates.     

DEVELOPMENT OF VOCALISATIONS IN NESTLING AND FLEDGLING HOUSE WRENS IN NATURAL POPULATIONS

ABSTRACT

We conducted a field study of the ontogeny of vocal signals in the house wren Troglodytes aedon during the nestling and fledgling phases of life. We did spectrographic analyses and quantification of the developmental changes that occurred in the acoustic features of the vocalisations. Evidence of progress to adult-like vocal patterns was of two types. First, nestling calls changed into a harsh-sounding call that resembles the adult chatter call, functionally a warning call. Second, fledglings also uttered subsong, and these vocalisations were similar to notes typical of adult male song. When the vocalisations produced by developing young were broken down into their constituent vocal features, we found that the time course of development was not strictly linear. Instead of a unidirectional change through the course of nestling and fledgling life, the trajectories of the vocal features fluctuated through time and sometimes exhibited abrupt changes. These sudden shifts occurred during nestling life as well as at the time of fledging. We speculate on the possible causes of these abrupt transitions. Changes in acoustic features upon fledging appear to be linked to new social functions.     

Addition of arthropod cocoons to house wren nests is correlated with delayed pairing

Males in the cavity-nesting house wren (Troglodytes aedon) frequentlyadd arthropod cocoons to their nests during building, possiblyas an ornamental cue for female choice. We tested this hypothesisby comparing the time to pairing for males that did and didnot add cocoons to their nests and for males in whose nestswe manipulated the number of cocoons prior to pairing. We alsotested the hypothesis that females acquire fitness-related benefitsby selecting mates based on their use of cocoons. The use ofcocoons by males was not consistently related to habitat, butthe number of cocoons added per nest increased during the courseof the breeding season. Contrary to prediction, the time topairing for males adding cocoons was significantly longer thanthat for males without cocoons in their nests at both unmanipulatedand experimental nests. There was also no consistent fitness-relatedbenefit for females related to the use of cocoons by their mates.Therefore, we conclude that females did not prefer males thatadded cocoons to their nests, and that the increased time topairing for males that add cocoons likely results in fitness-relatedcosts brought about by delayed breeding. Nonetheless, male housewrens routinely use cocoons, and why they do so remains unknown.     

Clutch size and the costs of incubation in the house wren

Trade-offs in the allocation of finite resources among differentstages of a breeding attempt as well as between different reproductiveevents should shape the evolution of life-history traits. Toinvestigate the effects of incubation effort on within-broodand between-brood trade-offs in house wrens (Troglodytes aedon),we manipulated the clutch size that females incubated. We isolatedeffects of incubation by reversing the manipulation at hatchingto allow all parents to provision their natural brood sizes.Females that incubated enlarged clutches had longer incubationperiods than control females, both early and late in the season,suggesting that the experimental treatment increased incubationeffort. Contrary to predictions, however, increased incubationeffort did not adversely affect the allocation of effort tonestling provisioning. Rather, in the early season, but notin the late season, females that incubated enlarged clutchesappeared to allocate more effort to nestling provisioning, producingheavier and larger fledglings than control females. Althoughfemales with enlarged early-season clutches consequently lostmore mass than control females, this was likely an adaptiveresponse to reduce wing loading in anticipation of high provisioningdemands. There were no treatment-related differences in fledglingmass or size, or in female mass loss, in the late season. Thus,elevated incubation demands negatively affected a fitness-relatedtrait (duration of incubation) that may constrain clutch sizebut not the allocation of resources to subsequent stages ofthe same breeding event or to subsequent breeding events. Wesuggest that environmental conditions may mediate clutch-sizeeffects on trade-offs in allocation of resources between incubationand nestling provisioning.     

Immune-challenged house wren broods differ in the relative strengths of their responses among different axes of the immune system

Single components of the immune system are widely used to assess immune function in free-living vertebrates. However, as different immunological components are triggered by different types of threats and may be regulated independently, there is little reason to assume that they should respond similarly if challenged. We investigated whether three commonly assayed immune responses, cutaneous immune activity (phytohaemagglutinin assay), antibody response (tetanus toxoid immunization), and plasma bactericidal activity (Escherichia coli killing) are positively related in nestling house wrens (Troglodytes aedon). Multivariate analysis revealed significant differences in overall immune responsiveness among broods (i.e. nests), primarily attributable to differences in plasma bactericidal activity. Among broods, humoral immune response was negatively related to cutaneous immune activity and positively related to plasma bactericidal activity. We found no significant relationships among these measures of immunity among individual nestlings within broods. Our results suggest that different broods (i.e. families) invest differentially in the various branches of the immune system. Further study is needed to characterize the roles of maternal, genetic and environmental effects on the expression of this physiological bias.     

Climatic effects on the breeding phenology and reproductive success of an arctic-nesting goose species

Climate warming is pronounced in the Arctic and migratory birds are expected to be among the most affected species. We examined the effects of local and regional climatic variations on the breeding phenology and reproductive success of greater snow geese ( Chen caerulescens atlantica ), a migratory species nesting in the Canadian Arctic. We used a long-term dataset based on the monitoring of 5447 nests and the measurements of 19 234 goslings over 16 years (1989–2004) on Bylot Island. About 50% of variation in the reproductive phenology of individuals was explained by spring climatic factors. High mean temperatures and, to a lesser extent, low snow cover in spring were associated with an increase in nest density and early egg-laying and hatching dates. High temperature in spring and high early summer rainfall were positively related to nesting success. These effects may result from a reduction in egg predation rate when the density of nesting geese is high and when increased water availability allows females to stay close to their nest during incubation recesses. Summer brood loss and production of young at the end of the summer increased when values of the summer Arctic Oscillation (AO) index were either very positive (low temperatures) or very negative (high temperatures), indicating that these components of the breeding success were most influenced by the regional summer climate. Gosling mass and size near fledging were reduced in years with high spring temperatures. This effect is likely due to a reduced availability of high quality food in years with early spring, either due to food depletion resulting from high brood density or a mismatch between hatching date of goslings and the timing of the peak of plant quality. Our analysis suggests that climate warming should advance the reproductive phenology of geese, but that high spring temperatures and extreme values of the summer AO index may decrease their reproductive success up to fledging.     

暖温带乔木和灌木物候变化及对气候变化的响应

根据2003-2014年气象数据和暖温带3种乔木(辽东栎、五角枫和核桃楸)和3种灌木(土庄绣线菊、毛叶丁香和六道木)的物候观测数据资料, 采用气候倾向率和回归分析等方法, 观察乔木和灌木物候变化特征的差异, 分析温度、降水以及乔木、灌木的物候变化趋势, 同时对气象因子与乔木和灌木物候期的相关关系进行研究。结果表明: ①研究期间, 北京东灵山平均气温呈不显著的上升趋势, 气候倾向率为0.200℃·10a–1, 春季(3–5月)和夏季(6-8月)温度显著上升; 降水量呈下降趋势, 平均减少71.630 mm·10a–1, 总体呈暖、干的趋势。②3种乔木的生长季长度都缩短, 辽东栎、五角枫和核桃楸平均生长季长度分别缩短50.70 d·10 a–1、29.83 d·10a–1和22.36 d·10a–1。3种灌木的生长季长度也都缩短, 土庄绣线菊、毛叶丁香和六道木的平均生长季长度分别缩短42.55 d·10a–1、42.76 d·10a–1和38.15 d·10a–1。乔木和灌木的物候变化趋势相同, 整体表现为春季物候推迟, 秋季物候提前, 生长季长度都缩短且生长季长度相差不大。乔木和灌木都表现出芽期推迟最明显, 每10年推迟达19天以上。③乔木和灌木各物候期与气温总体表现为负相关, 即气温升高, 物候期提前, 其相关性显示出夏季(6-8月)温度对植被物候期影响较大, 夏季温度与各物候期表现为正相关, 即夏季温度升高, 物候期推迟。同时乔木和灌木与总体降水没有明显的相关关系, 但秋季物候与不同时段降水表现不同的相关性, 由此可知夏季温度变化对木本植物春季物候(出芽期、展叶期和首花期)的影响更大, 而秋季物候(叶变色期和落叶期)受温度和降水共同影响。