Cowbird Chicks Research Articles

Preferential Begging Responses of Shiny Cowbirds to the Conspecific Chatter Call

Avian brood parasites lay their eggs into the nests of other species, which incubate them and raise the chicks until their independence. Despite living their early weeks of life surrounded by heterospecifics, young brood parasites have the ability to recognize and associate to conspecifics after independence. It has been proposed that the initial conspecific recognition develops when a young parasite encounters a unique species-specific signal that triggers the learning of other aspects of the producer of the signal. For cowbirds (Molothrus spp.), this species-specific signal is hypothesized to be the chatter call. Young birds also could express auditory biases, which in some cases lead to discrimination in favor of conspecific songs. Therefore, the perceptual selectivity for chatters might be also present in nestlings. Our aim was to assess if nestlings of the shiny cowbird (M. bonariensis) present a preferential begging response to conspecific chatter calls. We evaluated if they respond more to the parasitic vocalization than host chicks and if they respond more to the chatter than to heterospecific nonhost calls. We tested shiny cowbird chicks reared by chalk-browed mockingbirds (Mimus saturninus) or house wrens (Troglodytes aedon) and host chicks, as control species. We randomly presented to 6-day-old chicks the following playback treatments: (1) conspecific chatter calls, (2) host calls, used as positive controls, and (3) nonhost calls, used as negative control. We measured if chicks begged during the playback treatments and the begging intensity. When responding to the playback of chatter calls, shiny cowbird chicks begged at a higher frequency and more intensively than host chicks. Shiny cowbird chicks reared by mockingbirds begged more intensively to playbacks of conspecific chatter calls than to host calls, while those reared by wrens begged with a similar intensity to playbacks of conspecific chatter and host calls. On the contrary, wren nestlings begged more intensively to playbacks of the wren call than to chatter calls. Mockingbird nestlings did not beg during any treatment. None of the three species begged during the playback of nonhost calls. Our results show that the chatter call produced a preferential begging response in cowbird nestlings, which may be the beginning of a process of conspecific recognition.

Size matters: shiny cowbirds secure more food than host nestmates thanks to their larger size, not signal exaggeration

Many hosts of obligate brood parasitic birds invest more in parasitic nestlings than they do in their own young. The shiny cowbird, Molothrus bonariensis, a generalist parasite, is fed at a higher proportion than its host nestmates when it is reared in nests of a smaller-bodied host, the house wren, Troglodytes aedon. We test two hypotheses that could account for this differential allocation of food by host parents. The signal exaggeration hypothesis states that cowbird chicks have visual and/or acoustic begging signals that elicit preferential feeding. The size-advantage hypothesis states that hosts preferentially feed large chicks and/or that larger chicks outcompete host chicks in a scramble competition for food. To gain insight into the relative importance of size versus species-specific signals on food allocation by house wrens, we performed audio and video recordings in nests with experimental broods of (1) a 2-day-old cowbird chick and a 2-day-old wren chick (different species, different size), (2) a 2-day-old cowbird chick and an 8-day-old wren chick (different species, same size) and (3) a 2-day-old house wren and an 8-day-old house wren (same species, different size). When cowbirds shared the nest with a same-size wren chick, both chicks received food in equal proportion. In contrast, larger chicks (both cowbirds and wrens) paired with small wren nestmates always received a higher food share. Cowbird begging behaviour and call traits differed from house wrens, but these differences did not always coincide with increased food. We conclude that, at least when cowbird nestlings are young (2 days old), their relatively large size accounts for the larger share of food they receive from house wren hosts, rather than some quality of their begging signal.

Rates of parasitism, but not allocation of egg resources, vary among and within hosts of a generalist avian brood parasite.

Brown-headed cowbirds (Molothrus ater) deposit their eggs into the nests of other birds, which then raise the cowbird chick. Female cowbirds thus have limited options for impacting their offspring's development via maternal effects compared to most other passerines. Cowbirds can impact their offspring's phenotype by choosing among potential host nests, and by adjusting egg resources based on host characteristics. To examine whether cowbirds exhibit either or both of these strategies, we investigated rates of cowbird parasitism and egg investment (egg size, yolk-to-albumen ratio, and yolk testosterone and androstenedione) among and within host species in a shrubland bird community. We found that the probability of being parasitized by cowbirds, controlling for host status as a cowbird egg accepter or rejecter and ordinal date, varied significantly among host species, indicating an apparent preference for some hosts. Parasitism rates did not differ with host size, however, and despite variation in cowbird egg size among host species, this variation was not related to host size or cowbird preference. Among host species with eggs that are larger than those of the cowbird, cowbirds were significantly more likely to parasitize nests with relatively smaller eggs, whereas parasitism rates did not vary with relative egg size in host species with smaller eggs. There was no evidence for variation in cowbird egg components among or within host species. Our data indicate that cowbirds discriminate among host nests, but do not appear to adjust the composition of their eggs based on inter- or intraspecific host variation.

Nest environment modulates begging behavior of a generalist brood parasite

In young birds, the intensity of the begging signal varies according to the chick’s internal condition (i.e., hunger or nutritional state) but may be additionally modulated by the external conditions experienced in the nest (e.g., nestmate competition or feeding success). Flexibility of begging in response to nest environment should be especially beneficial for the chicks of generalist brood parasites, which encounter diverse brood conditions depending on the host nest in which they are reared. We investigated variability in the begging behavior of an extreme generalist brood parasite, the shiny cowbird ( Molothrus bonariensis ). We compared visual and vocal begging variables of cowbird young of 5 days of age reared in nests of 2 common hosts, one smaller (house wren, Troglodytes aedon ) and one larger (chalk-browed mockingbird, Mimus saturninus ) than the parasite. Cowbird chicks were standardized for short-term hunger prior to recording and were similar in mass/condition in each host, but nevertheless begged more intensely in mockingbird than house wren nests, as indicated by increased stretching of the neck, lower latency to beg, increased time spent begging per bout and greater call rate. These results are consistent with young cowbirds adjusting their begging intensity based on the intrabrood competition of a given host, independent of need, an adaptability that likely contributes to their success as parasites utilizing diverse host species.

Shiny cowbirds share foster mothers but not true mothers in multiply parasitized mockingbird nests

Obligate brood parasitic birds, such as cowbirds, evade parental care duties by laying their eggs in the nests of other species. Cowbirds are assumed to avoid laying repeatedly in the same nest so as to prevent intrabrood competition between their offspring. However, because searching for host nests requires time and energy, laying more than one egg per nest might be favoured where hosts are large and can readily rear multiple parasites per brood. Such ‘repeat parasitism’ by females would have important consequences for parasite evolution because young parasites would then incur indirect fitness costs from behaving selfishly. We investigated shiny cowbird (Molothrus bonariensis) parasitism of a large host, the chalk-browed mockingbird (Mimus saturninus), in a population where over 70 % of the parasitized mockingbird nests receive multiple cowbird eggs. We assessed egg maternity directly, using cameras at nests to film the laying of individually-marked females. We also supplemented video data with evidence from egg morphology, after confirming that each female lays eggs of a consistent appearance. From 133 eggs laid, we found that less than 5 % were followed by the same female visiting the nest to lay again or to puncture eggs. Multiple eggs in mockingbird nests were instead the result of different females, with up to eight individuals parasitizing a single brood. Thus, while cowbird chicks regularly share mockingbird nests with conspecifics, these are unlikely to be their maternal siblings. Our results are consistent with shiny cowbird females following a one-egg-per-nest rule, even where hosts can rear multiple parasitic young.

A recoverable cost of brood parasitism during the nestling stage of the American robin (Turdus migratorius): implications for the evolution of egg rejection behaviors in a host of the brown-headed cowbird (Molothrus ater)

Hosts of brood parasitic birds face reduced reproductive success as a direct consequence of rearing parasitic young. The most commonly evolved host behavior to combat costly parasitism is the rejection of foreign eggs. Despite consistent patterns of reduced nesting success in broods parasitized by brown-headed cowbirds (Molothrus ater), most of its host species do not reject foreign eggs. Paradoxically, where it is present, egg rejection is more common among cowbird hosts with larger body sizes, yet cowbird chicks are less likely to outcompete larger host nest mates. This raises a fundamental question whether egg rejection, even in the minority of cowbird hosts, has specifically evolved in response to costly brood parasitism. Here we tested predictions associated with this hypothesis in an egg-rejecter host, the American robin (Turdus migratorius), by assessing whether experimental cowbird parasitism causes reduced nesting success. We cross-fostered cowbird and control, host chicks into robin nests, testing for experimental effects of chick species, brood size, and hatching asynchrony; unmanipulated nests served as additional controls. Rearing a foreign chick in the brood reduced the reproductive output of host robins, however, we detected no effect of parasitizing host nests with a cowbird versus robin chick. Cowbird chicks were significantly less likely to fledge than cross-fostered robin chick controls. These experiments reveal that parasitism itself can exert a cost on robin hosts during the nestling stage, representing a recoverable cost of cowbird parasitism that can be avoided by the host through rejecting parasite eggs prior to hatching. These results support the assumption that foreign egg rejection is an evolved host response to brood parasitism in this system, despite the poor survival rate of cowbird chicks in robin broods.

Shiny cowbirdMolothrus bonariensisegg size and chick growth vary between two hosts that differ markedly in body size

In birds, egg size affects chick growth and survival and it is an important component of reproductive success. The shiny cowbirdMolothrus bonariensisis an extreme generalist brood parasite that uses hosts with a wide range of body masses. Survival of cowbird chicks decreases with host body mass, as competition for food with nestmates is more intense in large than in small hosts. We studied variation in shiny cowbird egg size and chick growth in two hosts that differ markedly in body size: the chalk‐browed mockingbirdMimus saturninus(70–75 g), and the house wrenTroglodytes aedon(12–13 g). We analyzed: 1) if females parasitizing mockingbirds lay larger eggs than those parasitizing wrens, and 2) the association between egg size and chick growth. We experimentally controlled for time of parasitism and number of host chicks and evaluated growth rate of male and female parasite chicks. Shiny cowbirds parasitizing mockingbird nests laid larger eggs than those parasitizing wren nests. Chick body mass after hatching was positively associated with egg size until chicks were five days of age, but there was no association between egg size and growth rate, or asymptotic mass. There were no sexual differences in egg size or body mass at the time of hatching, but growth rate was higher in males than in females leading to sexual dimorphism in asymptotic mass. Differences in egg size between hosts and the effect of egg size on body mass after hatching support the hypothesis that different females are specialized in the use of hosts that differ in body mass.

Do shiny cowbird females adjust egg pecking behavior according to the level of competition their chicks face in host nests?

Interspecific brood parasites, like the shiny cowbird (Molothrus bonariensis), lay eggs in nests of other species. Shiny cowbird females peck and puncture eggs when they parasitize host nests. This behavior increases the survival of cowbird chicks when they have to compete for food with larger nestmates. However, cowbird chicks may benefit from smaller nestmates as they increase food provisioning by parents and the cowbird chicks secure most extra provisioning. We investigated whether egg-pecking behavior by female shiny cowbirds might be adjusted to the competition that their chicks face in host nests. We found that more host eggs are destroyed per cowbird egg laid in a larger-bodied host (chalk-browed mockingbird, Mimus saturninus, 70-75 g) than a smaller-bodied host (house wrens, Troglodytes aedon, 12-13 g). We also tested egg-pecking preferences in choice experiments with female cowbirds in captivity and found cowbirds presented with eggs in artificial nests pecked first and more frequently, and punctured more frequently the larger egg when this was a host egg, but not when this was a cowbird egg. Our results are partially consistent with the hypothesis that shiny cowbird females adaptively adjust their egg pecking behavior according to the competition that their chicks face in host nests.

Explaining postnatal growth plasticity in a generalist brood parasite

Selection of a particular host has clear consequences for the performance of avian brood parasites. Experimental studies showed that growth rate and fledging mass of brood parasites varied between host species independently of the original host species. Finding correlates of this phenotypic plasticity in growth is important for assessing adaptiveness and potential fitness consequences of host choice. Here, I analyzed the effects of several host characteristics on growth rate and fledging mass of the young of brown-headed cowbird (Molothrus ater), a generalist, non-evicting brood parasite. Cowbird chicks grew better in fast-developing host species and reached higher fledging mass in large hosts with fast postnatal development. A potential proximate mechanism linking fast growth and high fledging mass of cowbird with fast host development is superior food supply in fast-developing foster species. So far, we know very little about the consequences of the great plasticity in cowbird growth for later performance of the adult parasite. Thus, cowbird species could become interesting model systems for investigating the role of plasticity and optimization in the evolution of growth rate in birds.

Does nesting habitat predict hatch synchrony between brood parasitic brown‐headed cowbirdsMolothrus aterand two host species?

Nestling brown‐headed cowbirdsMolothrus atertypically hatch earlier and grow faster than young of the many host species of this generalist obligate brood parasite. However, a cowbird chick also benefits from the presence of some host nest mates as the parasite is provisioned disproportionately more with increasing brood size. Since asynchronous hatching affects both cowbird and host nestlings’ growth and survival, mechanisms that optimize the timing of egg‐laying by female parasites should be prevalent. Several habitat features might facilitate optimal timing of parasitic egg‐laying and we examined whether aspects of host nesting habitat predicted cowbird hatching synchrony. We tested whether synchronous nests were less concealed, closer to perches, and located in areas of higher host density than asynchronous nests using a broad‐scale information theoretic approach. There was no support for these predictions regarding song sparrow (Melospiza melodia; n=55) or yellow warbler (Dendroica petechia; n=67) nests parasitized by brown‐headed cowbirds at Mono Lake, USA. For example, the best statistical models for predicting hatching synchrony in yellow warbler nests included nesting‐patch width and nest‐substrate shrub species. However, these relationships were relatively weak: both synchronous and asynchronous nests were in patches with statistically indistinguishable widths and the two dominant shrub species at our site contained similar proportions of synchronous and asynchronous nests. We conclude that the variability of host nesting habitats does not contribute to a biologically consistent effect on hatching synchrony by this generalist brood parasite.

Utilization of a new host in the screaming cowbird Molothrus rufoaxillaris, a host specialist brood parasite: host switch or host acquisition?

The screaming cowbird Molothrus rufoaxillaris has been long known as a host specialist brood parasite. However, in the past years, the utilization of two new hosts has been documented. We examined the variation in mitochondrial control region sequences from screaming cowbird chicks found in the nests of two hosts, the bay-winged cowbird (Agelaioides badius), which is its regular host, and the chopi blackbird (Gnorimopsar chopi), which is a new host, in Formosa Province, Argentina. If a group of females switched to this new host, we expected to find an association between host use and haplotype frequency distribution, indicating the presence of host-specific female lineages, whereas we expected no such association if the cowbird population incorporated this new host and females use both hosts simultaneously. Haplotype frequency distributions differed between cowbird chicks from the nests of both hosts. This indicates that nest choice by females of this brood parasite is not random and that they preferentially parasitize the nests of the same host species.

Shiny cowbirds synchronize parasitism with host laying and puncture host eggs according to host characteristics

Obligate avian brood parasites lay their eggs in nests of other species, which thereafter provide all parental care. Brood parasites synchronize parasitism with host laying and remove or puncture host eggs to increase their reproductive success, but the benefits of these behaviours may depend on hosts characteristics such as body size. We experimentally analysed the effects of synchronization between parasitism and host laying and reduction in number of host eggs on hatching success and chick survival of shiny cowbirds, Molothrus bonariensis (50 g), in two common hosts that differ in body mass: chalk-browed mockingbirds, Mimus saturninus (75 g), and house wrens, Troglodytes aedon (13 g). We found no effect of synchronization of parasitism or of the number of host eggs removed on parasite hatching success in either host. However, survival of cowbird chicks in mockingbird nests was lower when cowbird chicks hatched after host chicks and when there was no removal of host eggs. In contrast, in wren nests, there was no effect of hatching asynchrony or egg removal on cowbird survival, but asymptotic weight was higher in nests without egg removal. In natural nests, the proportion of cowbird eggs laid during host laying was higher and the number of host eggs punctured per parasitic event was greater for mockingbirds than for wrens. These differences between hosts in the extent of synchronization between parasitism and host laying and the intensity of egg punctures suggest that shiny cowbirds may adaptively adjust these behaviours to host characteristics.

Sheep in wolf's clothing: host nestling vocalizations resemble their cowbird competitor's

Nestlings of many avian brood parasites are virtuosos at mimicking host nestling vocalizations, which, like egg mimicry, presumably ensures acceptance by host parents. Having been accepted, parasitic nestlings then often exaggerate the aspects of the host's display to increase parental care. Host nestlings may, in turn, exaggerate their vocalizations to keep up with the parasite, though this possibility has not been evaluated. We experimentally parasitized song sparrow (Melospiza melodia) nests with a brown-headed cowbird (Molothrus ater) chick to evaluate how host nestlings respond. Vocalizations emitted from experimentally parasitized nests were higher in frequency, and louder, than those from unparasitized nests, consistent with the cowbird exaggerating its signalling. In response, host nestlings exaggerated the frequency and amplitude of their vocalizations, such that they resembled the cowbird's while they 'scaled back' on calls per parental provisioning bout. Sparrows in parasitized nests were fed equally often as sparrows in unparasitized nests, suggesting that exaggerating some aspects of vocalization while scaling back on others can help host nestlings confronted with a cowbird. Our results support the recently proposed hypothesis that signalling in parasitized nests involves a dynamic interaction between parasitic and host nestlings, rather than a one-way process of mimicry by the parasite.

ECOLOGICAL CORRELATES AND SEX DIFFERENCES IN EARLY DEVELOPMENT OF A GENERALIST BROOD PARASITE

Abstract Parents inluence the phenotype of their offspring by determining the environment in which early development occurs. The many factors that affect growth in avian brood parasites provide an excellent context in which to examine how ecological variables and sex differences influence plasticity of early development. We used a model-selection approach to determine the most important variable(s) for explaining patterns in growth rate of the Brown-headed Cowbird (Molothrus ater). Using published growth-rate estimates across various host species, we found that the age-adjusted size of Brown-headed Cowbird chicks increases with increasing hatching synchrony between host and parasite chicks. We also quantified Brown-headed Cowbird growth rates in nests of Song Sparrows (Melospiza melodia) and Yellow Warblers (Dendroica petechia), two common host species at Mono Lake, California, to examine the role of variation in hatching synchrony in broods within host species. Statistical models to explain variation in ...

Partial host fidelity in nest selection by the shiny cowbird (Molothrus bonariensis), a highly generalist avian brood parasite

Obligate avian brood parasites can be host specialists or host generalists. In turn, individual females within generalist brood parasites may themselves be host specialists or generalists. The shiny cowbird Molothrus bonariensis is an extreme generalist, but little is known about individual female host fidelity. We examined variation in mitochondrial control region sequences from cowbird chicks found in nests of four common Argentinean hosts. Haplotype frequency distributions differed among cowbird chicks from nests of these hosts, primarily because eggs laid in nests of house wrens Troglodytes aedon differed genetically from those laid in nests of the other three hosts (chalk-browed mockingbird Mimus saturninus, brown-and-yellow marshbird Pseudoleistes virescens, and rufous-collared sparrow Zonotrichia capensis). These differences in a maternally inherited marker indicate the presence of a nonrandom laying behaviour in the females of this otherwise generalist brood parasite, which may be guided by choice for nest type, as house wrens nest in cavities whereas the other three species are open cup nesters.

Early Infestation by Bot Flies (Philornis Seguyi) Decreases Chick Survival and Nesting Success in Chalk-Browed Mockingbirds (Mimus Saturninus)

AbstractBot flies (Diptera: Muscidae: Philornis spp.) are a group of flies comprising mostly species with a Neotropical distribution. Their larvae parasitize several species of birds, living subcutaneously on altricial chicks. We investigated the effect of parasitism by bot flies (P. seguyi) on the reproductive success of Chalk-browed Mockingbirds (Mimus saturninus) in temperate grasslands near the southern limit of bot fly distributions. We analyzed seasonal variation of bot fly prevalence during three consecutive years and how the timing and intensity of bot fly infestation affected growth and survival of Chalk-browed Mockingbird nestlings. Bot fly prevalence was 58.3%, 30.7%, and 45.5% each year, and in all years, it increased with time of breeding. Most of the infested nests fledged no chicks. In these nests, chicks had a lower tarsus growth rate than in noninfested nests and died 3–4 days after parasitism. The average time from hatching of the first chick until infestation was 4.4 days. The age of the chicks at the time of infestation was associated positively with nesting success and negatively with intensity of parasitism. Bot fly parasitism also reduced the survival of Shiny Cowbird (Molothrus bonariensis) chicks present in Chalk-browed Mockingbird nests, but the presence of Shiny Cowbird chicks did not affect timing of infestation or fledging success of Chalk-browed Mockingbird chicks. Our results show that an intermediate prevalence of bot fly parasitism produces an important decrease in the reproductive success of Chalk-browed Mockingbirds and suggest that bot flies may play an important role as selective agents in the evolution of host life-history strategies.La Infestación Temprana con Larvas de Philornis seguyi Disminuye la Supervivencia de los Pichones y el Éxito de Nidificación de Mimus saturninus

COSTS OF EGG PUNCTURES AND PARASITISM BY SHINY COWBIRDS (MOLOTHRUS BONARIENSIS) AT CREAMY-BELLIED THRUSH (TURDUS AMAUROCHALINUS) NESTS

Abstract Most studies on cowbird parasitism have focused on its effects on parasitized nests, whereas few have considered the costs at nests that cowbirds visit but at which they do not lay eggs. Shiny Cowbirds (Molothrus bonariensis) peck and puncture host eggs both in nests where they lay eggs and in unparasitized nests. We analyzed the effect of egg punctures in unparasitized and parasitized nests of a large host, the Creamy-bellied Thrush (Turdus amaurochalinus; hereafter “thrush”), as well as the costs of Shiny Cowbird eggs and chicks in this host’s nests. We determined thrush egg survival, hatching success, and chick survival in successful nests, and nest survival during the egg and nestling stages. Frequency of parasitism was 60%, and its intensity 1.6 ± 0.1 eggs nest−1. Number of host eggs punctured was positively associated with intensity of parasitism. The host’s eggs were frequently punctured in parasitized nests (71%) and in unparasitized nests (42%). Egg punctures reduced the number of eggs a...

Costs of Egg Punctures and Parasitism by Shiny Cowbirds (Molothrus Bonariensis) at Creamy-Bellied Thrush (Turdus Amaurochalinus) Nests

AbstractMost studies on cowbird parasitism have focused on its effects on parasitized nests, whereas few have considered the costs at nests that cowbirds visit but at which they do not lay eggs. Shiny Cowbirds (Molothrus bonariensis) peck and puncture host eggs both in nests where they lay eggs and in unparasitized nests. We analyzed the effect of egg punctures in unparasitized and parasitized nests of a large host, the Creamy-bellied Thrush (Turdus amaurochalinus; hereafter “thrush”), as well as the costs of Shiny Cowbird eggs and chicks in this host’s nests. We determined thrush egg survival, hatching success, and chick survival in successful nests, and nest survival during the egg and nestling stages. Frequency of parasitism was 60%, and its intensity 1.6 ± 0.1 eggs nest−1. Number of host eggs punctured was positively associated with intensity of parasitism. The host’s eggs were frequently punctured in parasitized nests (71%) and in unparasitized nests (42%). Egg punctures reduced the number of eggs at hatching in 23% and 49% of unparasitized and parasitized nests, respectively. Nests with egg punctures had a lower survival rate than nests without them, but nest survival was not associated with parasitism. Presence of a Shiny Cowbird egg was associated with a decrease in the hatching success of host eggs, but presence of a Shiny Cowbird chick did not have any detrimental effect on either the survival and growth rate of host chicks in successful nests or the survival of the whole nest. Our results show that egg punctures were the primary determinant of thrush reproductive success. Consequently, comparison of unparasitized and parasitized nests gives an incomplete estimation of the effects of Shiny Cowbirds on host reproductive success, because the cost of egg punctures is also important in nests where there is no Shiny Cowbird egg laying.Costos de la Perforación de Huevos y el Parasitismo por Molothrus bonariensis en Nidos de Turdus amaurochalinus

Phylogenetic analysis of life-history adaptations in parasitic cowbirds

Parasitic cowbirds lay eggs in the nests of other species and dupe them into caring for their young. Unlike other brood parasites, cowbirds have not developed egg mimicry or bizarre chick morphology. However, most of them parasitize a large number of hosts. Several features of cowbirds have been proposed as more general adaptations to brood parasitism. In this study, we used a recent molecular phylogeny as a historical framework to test the possible adaptations of the parasitic cowbird, including egg size, eggshell thickness and energy content of the eggs, length of the incubation period, and growth pattern of cowbird nestlings. We used a recently developed extension of independent contrasts to test whether the five cowbird species deviate from general allometric equations. We generated prediction intervals for a nonparasite that evolved in the place of the cowbirds. By using these prediction intervals, we found that parasitic cowbirds had not reduced weight or energy content of their eggs, nor their incubation period over evolutionary time. Cowbird chicks and those of nonparasitic relatives had similar growth pattern. The only characteristic that separated parasitic cowbirds from their nonparasitic relatives was an increase in eggshell thickness. All these findings were robust and resisted the use of three models of character evolution. The fact that most traits exhibited by cowbirds were inherited from a nonparasitic ancestor does not rule out that they are advantageous for parasitism. Future research should focus on such traits of cowbird relatives and on how these traits preadapted a particular lineage to become parasites. Key words: adaptation, cowbirds, eggshell, egg size, Icteridae, incubation period, independent contrasts, nestling development, phylogeny. [Behav Ecol 15:109–119 (2004)]

Reproductive Success of Shiny Cowbird (Molothrus Bonariensis) Parasitizing the Larger Brown-and-Yellow Marshbird (Pseudoleistes Virescens) in Argentina

Abstract Shiny Cowbirds (Molothrus bonariensis) often parasitize larger hosts. It has been proposed that larger hosts are preferred by that parasite because they provide higher reproductive success, but available data are quite variable. We studied the reproductive success of Shiny Cowbirds in nests of Brown-and-yellow Marshbirds (Pseudoleistes virescens), a larger and often multiply parasitized host. To estimate the extent of interspecific competition, we compared the hatching success of parasite eggs in nests with and without reduction of the clutch size of the host as a result of egg punctures inflicted by the parasite, and the survival and growth of parasite chicks reared with and without host chicks. To estimate the extent of intraspecific competition, we compared Shiny Cowbird egg losses, hatching success, and chick survival in singly versus multiply parasitized nests. Reproductive success of Shiny Cowbirds was 8% with depredation causing 80% of losses. Clutch reduction due to egg punctures were higher in multiply than in singly parasitized nests, but it did not improve hatching success of parasite eggs. Neither survival nor growth of parasite chicks was affected by the presence of host chicks. Shiny Cowbird hatching success and chick survival did not differ between singly and multiply parasitized nests. Parasite chicks were smaller than same-age Brown-and-yellow Marshbird chicks. However, because parasite chicks hatched one or two days before host chicks and had a higher growth rate, they were the larger chicks in the nest. Overall Shiny Cowbird reproductive success in Brown-and-yellow Marshbird nests was apparently higher than that reported in other smaller or similar-sized hosts. We think that host life-history traits like large clutch size, a longer incubation period, and slower growth rate of chicks are responsible for the high reproductive success of Shiny Cowbirds with Brown-and-yellow Marshbirds.