Host activity and the risk of nest parasitism by brown-headed cowbirds

Proportions of nests parasitized by brown-headed cowbirds (Molothrus ater) vary greatly among host species, but factors underlyingthis variation remain poorly understood. Cowbirds are believedto find nests by watching host behavior. We tested the hypothesisthat the activity of hosts during nest building correlateswith the probability of parasitism among and within four sympatrichosts: dusky flycatchers (Empidonax oberholseri), warblingvireos (Vireo gilvus), yellow warblers (Dendroica petechia),and American redstarts (Setophaga ruticilla). Daily probabilityof parasitism varied substantially among these species, from3% for dusky flycatchers to more than four times that for warblingvireos. The four species did not differ in the proportion ofcowbirds fledged from their nests. Differences in nest placementdid not explain differences in probability of parasitism amongor within species. Parasitism frequencies increased among speciesthat made longer nest-building visits, had a greater propensityto perch during nest approach, spent more time near their nests,and had males that vocalized more often near nests. Within species, females that visited their nests less often, spent more timeon the nest per visit, and males that sang more and were activein a smaller area around their nests were more likely to beparasitized by cowbirds. These data support the hypothesisthat cowbirds use the activity of hosts during nest buildingto locate nests.     

Nest desertion by a cowbird host: an antiparasite behavior or a response to egg loss?

Natural selection can favor songbirds that desert nests containingeggs of the parasitic brown-headed cowbird (Molothrus ater).However, the high variability in desertion of parasitized nestswithin species is perplexing in light of the typically highcosts of parasitism. Because nest desertion can also be a responseto partial clutch predation, we first asked if Bell's vireos(Vireo bellii) deserted nests in response to the presence ofcowbird eggs (antiparasite response hypothesis) or to egg removalby predators and female cowbirds (egg predation hypothesis).Second, we asked whether variation in nest desertion was dueto intrinsic differences among individuals or to variation innest contents. We monitored a large number of nests (n = 494)and performed a clutch manipulation experiment to test thesehypotheses. The number of vireo eggs that remained in a nestwas a strong predictor of desertion both within and among pairs.Neither the presence of a single cowbird egg, which leads tonest failure for this host, nor the number of cowbird eggs receivedin a vireo nest influenced nest desertion. Furthermore, vireosdid not desert experimental nests when we immediately exchangedcowbird eggs for vireo eggs but deserted if we removed vireoeggs and replaced them with cowbird eggs the following morning.Desertion of parasitized nests by Bell's vireos can be almostentirely explained as a response to partial or complete clutchloss and does not appear to have been altered by selection frombrood parasitism.     

Nests as ornaments: revealing construction by male sticklebacks

Nests are built by animals from a variety of taxa, and serveas receptacles for eggs and developing offspring. Where nestsare built solely or mainly by one sex, they also have the potentialto serve as extended ornaments, because aspects of constructionpotentially reveal or amplify characteristics of the builderto prospective mates. Here, we develop novel indices to quantifynest structure and examine variation in temporal and structuralaspects of nest construction in relation to morphological,immunological, and physiological traits in male three-spinedsticklebacks, Gasterosteus aculeatus. Wild-caught male sticklebacksthat began construction within 3 days of being transferredto the laboratory built "neater" nests than fish that tooklonger to start, and we present alternative testable hypothesesthat could explain this pattern. Various characteristics ofnest-building males correlated with nest structure. The relativeweight of the building male's kidney—which secretes aglue-like protein used in nest building and whose developmentis androgen-dependent—correlated positively with nest "neatness." We also found males with enlarged spleens (an indicatorof immune stress) to construct less "compact" nests. The structureof a nest may therefore be important not only in determiningits functional capacity, but may also act as a quality-revealingornament. We suggest that females may gain valuable informationregarding male health status and androgen levels from nestinspection.     

Support for the nest mimicry hypothesis in Yellow-rumped Thornbills Acanthiza chrysorrhoa

The nest of the Yellow-rumped Thornbill Acanthiza chrysorrhoa (endemic to Australia) consists of a lower domed nest where the young are cared for and an upper cup-shaped structure, referred to as the false nest. Numerous hypotheses have been proposed to explain the primary function of the false nest, but their predictions remain largely untested. Here, we test predictions of the nest mimicry hypothesis (with artificial nests) whereby the empty false nest acts as a decoy, deflecting visual nest predators and/or brood parasites away from the true nest below, thereby increasing nesting success. Clear detection of the false nest by adversaries is an intrinsic aspect of nest mimicry; thus we also test whether conspicuous nests (natural) are favoured by natural selection. Specifically, (1) we compare predation level at artificial domed nests with and without a false nest; and (2) we analyse associations between nest site concealment and nesting outcome in natural nests in a wild population. Supporting the nest mimicry hypothesis, predation level was significantly lower for artificial domed nests with false nests, and for natural nests with lower levels of concealment.     

Testing hypotheses about the function of repeated nest abandonment as a life history strategy in a passerine bird

Nest structures are essential for successful reproduction in most bird species. Nest construction costs time and energy, and most bird species typically build one nest per breeding attempt. Some species, however, build more than one nest, and the reason for this behaviour is often unclear. In the Grey Fantail Rhipidura albiscapa, nest abandonment before egg‐laying is very common. Fantails will build up to seven nests within a breeding season, and pairs abandon up to 71% of their nests before egg‐laying. We describe multiple nest‐building behaviour in the Grey Fantail and test four hypotheses explaining nest abandonment in this species: cryptic depredation, destruction of nests during storm events, and two anti‐predatory responses (construction of decoy nests to confuse predators, and increasing concealment to ‘hide’ nests more effectively). We found support for only one hypothesis – that abandonment is related to nest concealment. Abandoned nests were significantly less concealed than nests that received eggs. Most abandoned nests were not completely built and none received eggs, thus ruling out cryptic predation. Nests were not more likely to be abandoned following storm events. The decoy nest hypothesis was refuted as abandoned nests were constructed at any point during the breeding season and some nests were dismantled and the material used to build the subsequent nest. Thus, Grey Fantails are flexible about nest‐site locations during the nest‐building phase and readily abandon nest locations if they are found to have deficient security.     

Effects of northern pike on patterns of nest use and resproductive behavior of male fathead minnows in a boreal lake

We conducted a two-part study to assets predator avoidance byreproductive male fathead minnows (Pimephales promelos) subjectedto predation threat from northern pike (Esox lucius). First,we determined if patterns of nest use by egg-guarding male minnowsin a boreal lake were related to pike densities. We samplednorthern pike and identified four areas of "high pike-density"and three areas of "low pike-density." We censused natural nestsand placed nest boards in these areas. We found eggs on naturalnests more frequently in areas with low densities of pike thanin areas with high densities of pike. However, we could notfully explain the distribution of nests by predation risk. Second,we evaluated the behavioral response of egg-guarding males toa control stimulus (a piece of wood) or a live pike in a wirecage. We used time to return to the nest after a stimulus asa measure of risk taking. Males took different amounts of riskbased on predation threat; males in the predator treatment tooklonger to return to their nests than control males. Risk takingwas not related to the number or age of the eggs but to distanceto nearest egg-guarding neighbor; males with close neighborsreturned sooner than more isolated males. Males in the predatortreatment had lower total activity and egg rubbing than controlmales after they returned to their nests. We conclude that malefathead minnows altered their reproductive behavior in waysthat reduced predation risk, but the cost of predator avoidancemight include egg predation, lost mating opportunities, or usurpationof nests     

Experimental enlargement of nest size does not increase risk of predation or brood parasitism in the Great Reed Warbler Acrocephalus arundinaceus

We assessed whether nest size affects the probability of nest loss using dyads of large and small (large being twice the size of small) inactive Great Reed Warbler Acrocephalus arundinaceus nests placed at similar sites in Great Reed Warbler territories. Large nests were not predated significantly more frequently than small nests. Experimentally enlarged active Great Reed Warbler nests suffered non‐significantly higher predation compared with non‐manipulated control nests. Our experiments did not support the nest‐size hypothesis and suggested that nest size does not appear to be a factor affecting the risk of nest predation in this species. The probability of brood parasitism by the Common Cuckoo Cuculus canorus was also unaffected by experimental nest enlargement, supporting the commonly accepted hypothesis that the Common Cuckoo searches for suitable host nests by host activity during nest building rather than nest size.     

Old nests are cues for suitable breeding sites in the Eurasian penduline tit Remiz pendulinus

Nest-sites often have a major influence on avian reproductive success. The use of reliable cues that assist nest-site selection should thus be favoured by natural selection. The old nests have been known to serve as a cue in nest-site selection in several species. To find out whether the old nests act as cue in nest-site selection in the Eurasian penduline tit Remiz pendulinus , we carried out two experiments in southern Hungary, where the penduline tits breed around fishponds and build sophisticated pendulous nests on tree branches that often hang over water. In April 2006, we choose 20 groups of two nearby trees, and hung an old nest on one of the trees in each group. The male penduline tits choose 12 of these groups to build a new nest, and every of the twelve nests were built on trees with an old nest. This suggests that the old nests serve as a cue in the selection of breeding sites for males when they enter a habitat. To find out whether the old nests are cues of plentiful nest building material, or to signal high quality breeding areas, we carried out a second experiment in 2007 by selecting 13 groups of three nearby trees. A "worn-out" old nest was hung on one of the trees, a "re-utilize" type of old nest on another tree, whereas the third tree was left without an old nest in each group. The rationale was that while the worn-out material of the old nests is of no use in building the new nests, material of the "re-utilize" nests is good enough to be used for building new nests. Males built a new nest in 10 of the 13 groups, and eight of the new nests were built on trees with an old nest. Of the eight new nests, five were built on trees with a "worn-out" old nest and three on trees with a "re-utilize" type of old nest. It appears that for the penduline tit males it is the presence of an existing old nest and not the quality of the old nest material that serves as cue during the selection of the suitable breeding sites.     

Evaluation of disruptive camouflage of avian cup‐nests

Parent birds employ various strategies to protect their offspring against nest predators. Two well‐researched anti‐nest‐predation strategies involve visual concealment of the nest by way of parental camouflage and egg camouflage. By contrast, camouflage of nest structures is relatively under‐researched, particularly in the case of cup‐nests in trees and bushes. We explored how birds camouflage cup‐nests in nature. Specifically, we tested Hansell’s hypothesis that birds use externally applied pale and white objects such as spider cocoons and lichens to achieve cup‐nest camouflage. To test Hansell’s hypothesis, three complementary experiments were performed: (1) an in situ nest predation experiment; (2) a photo‐based visual search experiment; and (3) contrast analyses using PAT‐GEOM software in IMAGEJ. White paper and chalk spots were used to mimic white objects used by birds in nature. Whereas predation rates in Experiment 1 were not affected by white spots, location rates in Experiment 2 were lower for natural nests with white spots than without white spots. Experiment 3 demonstrated that white spots significantly increased the contrast between different visual elements of nests. It was concluded that white objects can potentially camouflage nests against some nest predators, and that any improved camouflage was probably achieved via disruptive camouflage.     

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.     

Function of egg punctures by Shiny Cowbirds in parasitized and nonparasitized Creamy‐bellied Thrush nests

ABSTRACT Avian brood parasites usually remove or puncture host eggs. Several hypotheses have been proposed to explain the function of these behaviors. Removing or puncturing host eggs may enhance the efficiency of incubation of cowbird eggs (incubation‐efficiency hypothesis) or reduce competition for food between cowbird and host chicks in parasitized nests (competition‐reduction hypothesis) and, in nonparasitized nests, may force hosts to renest and provide cowbirds with new opportunities for parasitism when nests are too advanced to be parasitized (nest‐predation hypothesis). Puncturing eggs may also allow cowbirds to assess the development of host eggs and use this information to decide whether to parasitize a nest (test‐incubation hypothesis). From 1999 to 2002, we tested these hypotheses using a population of Creamy‐bellied Thrushes (Turdus amaurochalinus) in Argentina that was heavily parasitized by Shiny Cowbirds (Molothrus bonariensis). We found that 56 of 94 Creamy‐bellied Thrush nests (60%) found during nest building or egg laying were parasitized by Shiny Cowbirds, and the mean number of cowbird eggs per parasitized nest was 1.6 ± 0.1 (N= 54 nests). At least one thrush egg was punctured in 71% (40/56) of parasitized nests, and 42% (16/38) of nonparasitized nests. We found that cowbird hatching success did not differ among nests where zero, one, or two thrush eggs were punctured and that the proportion of egg punctures associated with parasitism decreased as incubation progressed. Thus, our results do not support the incubation‐efficiency, nest‐predation, or test‐incubation hypotheses. However, the survival of cowbird chicks in our study was negatively associated with the number of thrush chicks. Thus, our results support the competition‐reduction hypothesis, with Shiny Cowbirds reducing competition between their young and host chicks by puncturing host eggs in parasitized nests.     

Building multiple nests is associated with reduced breeding performance in a south temperate population of Grass Wrens Cistothorus platensis platensis

Grass Wrens Cistothorus platensis build two types of non-breeding nest structures: platforms and dummy nests. Platforms are rudimentary accumulations of grasses concealed between vegetation. Dummy and breeding nests are dome-shaped with a similar structural layer. We used a nest-removal experiment and observational data to evaluate several hypotheses regarding the adaptive significance of building multiple nests in a south temperate population of Grass Wrens. Building non-breeding nests was not a strategy of males to attract additional females, as most of these nests were built after pair formation and both sexes collaborated during building. Building non-breeding nests was not a post-pairing display as the presence of multiple nests did not increase female investment in the breeding attempt: clutch size and female provisioning to nestlings did not differ between experimental and control territories where no non-breeding nests were removed. Similarly, in non-manipulated territories, clutch size and female provisioning were not correlated with the number of non-breeding nests or with males’ nest-building effort. Contrary to this hypothesis, the number of non-breeding nests was associated with delayed clutch initiation and reduced hatching success. The presence of non-breeding nests did not reduce nest predation and brood parasitism, which did not differ between experimental and control territories. We did not detect differences in concealment between non-breeding and breeding nests, suggesting that non-breeding nests were not the result of abandonment before egg-laying to reduce subsequent nest predation. Dummy nests did not provide shelter; they were not used frequently for roosting over the breeding season and were not maintained during the non-breeding season. We suggest that building non-breeding nests may be an attempt by males to manipulate the decision of females to breed with a mate they might otherwise reject or to start reproduction earlier than optimal for the females.     

The heterospecific habitat copying hypothesis: can competitors indicate habitat quality?

According to the "habitat copying" hypothesis, animals use thereproductive performance of conspecifics to assess habitat suitabilityand choose their future breeding site. This is because conspecificsshare ecological needs and thus indicate habitat suitability.Here, we propose the "heterospecific habitat copying" hypothesis,which states that animals should use public information (i.e.,information derived from the performance of others) from con-and heterospecifics sharing ecological needs. In a correlationalapproach we test some assumptions and predictions of this hypothesiswith a data set from two sympatric bird populations, rollers(Coracias garrulus) and kestrels (Falco tinnunculus), usingthe same nest-boxes and exploiting similar food resources. Sincekestrels are residents and breed earlier, we assumed that theyare dominant over rollers for nest-box acquisition. The environmentappears to be patchy for both species and temporally predictablefor kestrels only. Two results suggest that the use of heterospecificpublic information in breeding habitat selection may be at work:(1) an increase in the reoccupancy probability by kestrels ofprevious roller nests with increasing nest success, and (2)an increase in roller breeding population with increasing localkestrel success. Most of the other observed patterns could beexplained by alternative mechanisms such as natal philopatry,breeding fidelity, conspecific attraction, intraspecific habitatcopying, and the effect of interspecific competition.     

Low frequency of observed cowbird parasitism on eastern kingbirds: host rejection, effective nest defense, or parasite avoidance?

Eastern kingbird (Tyrannus tyrannus) nests rarely are parasitizedby brown-headed cowbirds (Molotrus ater). Kingbirds are oneof a dozen or so species known to eject cowbird eggs from theirnests. We hypothesized that either kingbirds eject cowbird eggsso quickly that researchers normally do not detect the eggsduring daily nest inspections, or that cowbirds avoid parasitizingkingbirds. We tested these alternative hypotheses by experimentallyintroducing real cowbird eggs into eastern kingbird nests duringthe pre-egg, early laying, late laying, and incubation stages.We recorded the interval between "parasitism" and ejection ofthe cowbird eggs. Although kingbirds ejected 87 of 88 cowbirdeggs placed in their nests, about 40% of the eggs remained innests for more than 24 h. Thus, during daily nest inspectionswe should have observed cowbird eggs if nests were parasitizedat all. In fact, we detected only one parasitized nest amongthe 402 inspected daily. The time for ejection was longest atnests parasitized early in laying, and shorter at nests parasitizedbefore and after. This variation in ejection times may reflectthe time kingbirds require to learn to recognize their own eggs.Although kingbirds defend their nests aggressively, they donot respond to female cowbirds as unique threats and do notguard their nests before sunrise when cowbirds lay. We concludethat cowbirds avoid parasitizing eastern kingbirds because theireggs most likely will be wasted. The rejection behavior persistspossibly because it is almost cost-free (a maximum of 0.07 kingbirdegg lost or damaged per cowbird egg ejected), or it evolvedin response to conspecific rather than cowbird parasitism. Foreignkingbird eggs introduced into nests at different nest stageswere ejected only during the pre-egg stage. This result supportsthe hypothesis that rejection behavior in eastern kingbirdsevolved in response to cowbird parasitism.     

Public information and conspecific nest parasitism in goldeneyes: targeting safe nests by parasites

Conspecific nest parasitism (CNP) is a widespread alternativereproductive tactic in birds. Several hypotheses have been putforward to explain the evolution and occurrence of CNP, butno generally applicable hypothesis exists. Recent experimentalresults from the common goldeneye (Bucephala clangula), a cavity-nestingduck, have revealed that parasitic females preferentially layeggs in safe nest-sites, implying that nest predation risk isan important ecological determinant of CNP. The present studyfocuses on the mechanisms by which parasites identify safe nest-sites.Predation risk of a given nest-site was predictable betweensuccessive breeding seasons. At the end of the nesting season,females prospected active nest-sites more frequently than nest-sitesthat did not have a nest in the current season. Nest-sites thathad been prospected more frequently by females in year t hada higher probability to be parasitized in year t + 1. The resultssuggest that the use of public information, derived throughnest-site prospecting, enabled parasites to target safe nests.These findings provide a new and potentially generally applicableperspective to understand the evolution and occurrence of CNP.     

Variation in social behavior within a spider mite genus, Stigmaeopsis (Acari: Tetranychidae)

The spider mites belonging to the genus Stigmaeopsis constructextremely dense oval woven roofs (web-nests) over depressionson the lower surface of host leaves and are known to have akind of sociality. The four species that occur on bamboo plantsin Japan show different nest areas. The nest area of Stigmaeopsislongus is the largest, followed by that of S. celarius, S. takahashii,and S. saharai in decreasing order. Smaller nests effectivelyprevent adults of several predator mite species from intruding.We hypothesized that variation in nest size reflects differentanti-intruder adaptations of this mite group in relation totheir sociality. The larger nest makers may adopt an alternativeanti-intruder strategy, namely, counterattack by a large group,so as to compensate for the disadvantage of large nests. S.longus and S. celarius adults effectively defended their largenests against potential predators, and the effects of nest defenseincreased with the number of individuals in a nest. S. takahashiiand S. saharai revealed no counterattack effect. Counterattackabilities that increase with the adult density, and thus, socialitymay compensate for the vulnerability of larger nests.     

Effect of floating nest platforms on the breeding performance of Black Terns

ABSTRACT.   In 2003 and 2004, we placed 41 floating nest platforms on Grassy Lake in southeastern Wisconsin (USA) to test the hypothesis that reproductive success of Black Terns ( Chlidonias niger ) is limited by the quality of suitable nesting habitat. Extreme differences in water levels between these 2 yr provided a natural experiment to evaluate the effectiveness of the nest platforms during a drought year (2003) when natural nesting substrate was abundant, and a flood year (2004) when natural substrate was limited during the peak nesting period. Terns nested on 27 of 41 (66%) of the platforms in 2003 and 26 of 41 (63%) in 2004. No difference in the occupancy rate of platforms and natural nests was evident in 2003, but the pattern of clutch initiations early in the season in 2004 indicated that platforms were preferred over natural substrates. In 2003, nest survival rates did not differ between nests placed on platforms and those placed on natural substrates, but platform nests had significantly higher hatching success and nest survival rates in 2004. Both the Kaplan-Meier and Apparent Nest Success methods of calculating nest survival provided similar estimates. In both years, eggs laid on platforms were significantly larger than those laid on natural substrates, suggesting that platforms were occupied by high-quality birds. Our study indicates that floating nest platforms can be an effective management tool to enhance nesting habitat for Black Terns and other aquatic birds that construct floating nests, primarily because platforms provide nest sites when natural sites are not available due to flooding. Nest platforms also may be useful for addressing questions concerning habitat selection and parental quality.     

The influence of parental behavior on vulnerability to nest predation in tropical thrushes of an Andean cloud forest

The Skutch hypothesis predicts that parental activity around the nest may attract the attention of predators and thus, in the tropics where predation pressure may be high, selection favors reduced parental activity. This hypothesis has been questioned by studies demonstrating that parents can decrease the risk of nest predation through nest defense. The link between parental activity and predation risk may be further confounded by nest site characteristics. We examined the effects of parental behavior and nest site on clutch survival in two sympatric tropical thrushes (Myadestes ralloides and Turdus leucops). We compared survival rates of clutches in three treatments: 1) natural nests at the incubation stage, 2) unattended nests (un‐manipulated nests of the same species, with clutches unattended by parents), and 3) exposed clutches (eggs exposed in unconcealed positions, unprotected by the nest). Parental activity had a positive effect on clutch survival, which was revealed by significantly higher survival rate of clutches in attended nests compared to unattended nests. The effect of nest site was less clear: clutches in unattended natural nests survived better than clutches in exposed sites selected by humans, but results were insignificant. We propose that parent birds can exclude a group of opportunistic predators, that are able to destroy unattended clutches. Nest site characteristics may be less important in determining clutch survival in the tropics, where predator guilds are more diverse, making completely safe sites difficult to find. Our results challenge Skutch's hypothesis and point to the need for more data from tropical latitudes.     

Potential influences of climate and nest structure on spotted owl reproductive success: a biophysical approach

Many bird species do not make their own nests; therefore, selection of existing sites that provide adequate microclimates is critical. This is particularly true for owls in north temperate climates that often nest early in the year when inclement weather is common. Spotted owls use three main types of nest structures, each of which are structurally distinct and may provide varying levels of protection to the eggs or young. We tested the hypothesis that spotted owl nest configuration influences nest microclimate using both experimental and observational data. We used a wind tunnel to estimate the convective heat transfer coefficient (h(c)) of eggs in 25 potential nest configurations that mimicked 2 nest types (top-cavity and platform nests), at 3 different wind speeds. We then used the estimates of h(c) in a biophysical heat transfer model to estimate how long it would take unattended eggs to cool from incubation temperature (～36°C) to physiological zero temperature (PZT; ～26°C) under natural environmental conditions. Our results indicated that the structural configuration of nests influences the cooling time of the eggs inside those nests, and hence, influences the nest microclimate. Estimates of time to PZT ranged from 10.6 minutes to 33.3 minutes. Nest configurations that were most similar to platform nests always had the fastest egg cooling times, suggesting that platform nests were the least protective of those nests we tested. Our field data coupled with our experimental results suggested that nest choice is important for the reproductive success of owls during years of inclement weather or in regions characterized by inclement weather during the nesting season.     

What makes a nest-building male successful? Male behavior and female care in penduline tits

Why do females increase parental effort when caring for theoffspring of attractive males? First, attractive males may bepoor fathers so that their females are compelled to increasetheir own contribution in order to fledge some young (the partner-compensationhypothesis). Second, females mated to attractive males may bewilling to increase their parental effort to reap high indirectbenefits for their offspring, and in turn males can decreasetheir own contribution (the differential allocation hypothesis[DAH]). We investigated these hypotheses in the penduline titRemiz pendulinus, a small passerine bird that has sequentialpolygamy by both sexes and strict uniparental care either bythe male or the female. We focused on two sexually selectedmale traits: nest size and nest-building behavior. We show thatmale care is unrelated to nest-building behavior, whereas femalesare more likely to care for the offspring of those males thatspend more time nest building. Females also more likely carefor the offspring of males that build large nests. Consequently,the reproductive success of males increases with nest size andnest-building behavior. Our results are consistent with theDAH and suggest that nest-building behavior and nest size areunder postmating sexual selection in penduline tits.