Riskiness of Movement Lifestyle Varies Inversely with Adult Survival Probability among Species

AbstractWhy do species differ in their movement lifestyles? Animals that spend more time sitting motionless and acquire food using less conspicuous movements can be more vigilant and less obvious to predators. More active animals that use food types and sites that require more conspicuous behaviors increase vulnerability to predators. Life history theory predicts that aversiveness to mortality risk evolves inversely to adult survival probability. Consequently, we postulated that long-lived species evolved inconspicuous movement lifestyles, whereas shorter-lived species use more conspicuous movement lifestyles. We tested this hypothesis by quantifying the movement lifestyles of nine tropical songbird species. Use of conspicuous movement and foraging behaviors, such as flying and hovering, was greatest in shorter-lived species and decreased with increasing adult survival probability across species. Similarly, foraging speed decreased with increasing adult survival based on a meta-analysis of 64 songbird species. Faster and conspicuous movement lifestyles of shorter-lived species likely increase food acquisition rates, which fits with faster life history strategies that include more feeding trips for young and faster growth. Similarly, slow movement lifestyles of long-lived species fit with the reduced food needs of slower life history strategies. Movement lifestyles may have evolved as an integrated component of the slow-fast life history continuum.

Fine-scale plasticity in nest placement can compensate for poor quality junipers as nesting trees for gray vireos

Understanding the interactions between behavior and habitat characteristics can have important implications for species of conservation concern. Gray vireos (Vireo vicinior) are one example of a species of conservation concern that is understudied in terms of nest survival probabilities and the habitat characteristics that influence them. Our objective was to determine if habitat features such as juniper density, juniper foliage density, or tree height influence nest survival probabilities, and if gray vireo nest placement can mitigate habitat risks. Based on previous work, we expected daily nest survival probabilities to be associated with nest height and surrounding vegetation. We monitored 89 nests in central New Mexico from 2016–2018 to estimate daily nest survival probabilities. We compared variation in nest placement, nest tree characteristics, and surrounding vegetation between failed and successful nests using logistic exposure models and Akaike Information Criteria. Daily and cumulative nest survival probability were 0.983 (95% CI [0.973–0.989]) and 0.575 (95% CI [0.444–0.702]), respectively. Top models predicting nest survival included a negative interaction between nest-tree foliage density and the distance of the nest from the edge of the nesting tree. This suggests that gray vireos can mitigate risks associated with low nest concealment by nesting closer to the interior of the nesting tree.

Could diffuse coevolution explain the generic eggshell color of the brown-headed cowbird?

The brown-headed cowbird (hereafter cowbird) is an avian brood parasite that produces an egg dissimilar to those produced by the majority of its diverse host community. The cowbird’s generic egg may result from a Jack-of-all-trades strategy; however, the evolutionary mechanisms that select for their generic eggs are unclear. Here we propose that the cowbird’s eggshell phenotypes have evolved via diffuse coevolution, which results from community-level selective pressures, rather than via pairwise coevolution that occurs between a particular host species and its brood parasite. Under diffuse coevolution the cowbird’s host community, with varying eggshell phenotypes and recognition abilities, would select for a cowbird eggshell phenotype intermediate to those of its host community. This selection is exerted by hosts that reject cowbird eggs, rather than those that accept them; therefore, we expect cowbird eggshell colors can be approximated by both the phenotypes and rejection abilities of their host community. Here we use eggshell reflectance data from 43 host species to demonstrate that the cowbird eggshell phenotypes are reasonably predicted (within 2 just noticeable differences) by the eggshell phenotypes and rejection rates of their hosts. These findings suggest that cowbird eggshell phenotypes, and potentially those of other some generalist parasites, may evolve via diffuse coevolution. Importantly, this research provides insight into the underlying evolutionary processes that explain observed phenotypic variation and provides a framework for studying selection on both specialist and generalist parasites’ traits.

Host and brood parasite coevolutionary interactions covary with comparative patterns of the avian visual system

In coevolutionary arms-races, reciprocal ecological interactions and their fitness impacts shape the course of phenotypic evolution. The classic example of avian host–brood parasite interactions selects for host recognition and rejection of increasingly mimetic foreign eggs. An essential component of perceptual mimicry is that parasitic eggs escape detection by host sensory systems, yet there is no direct evidence that the avian visual system covaries with parasitic egg recognition or mimicry. Here, we used eye size measurements collected from preserved museum specimens as a metric of the avian visual system for species involved in host–brood parasite interactions. We discovered that (i) hosts had smaller eyes compared with non-hosts, (ii) parasites had larger eyes compared with hosts before but not after phylogenetic corrections, perhaps owing to the limited number of independent evolutionary origins of obligate brood parasitism, (iii) egg rejection in hosts with non-mimetic parasitic eggs positively correlated with eye size, and (iv) eye size was positively associated with increased avian-perceived host–parasite eggshell similarity. These results imply that both host-use by parasites and anti-parasitic responses by hosts covary with a metric of the visual system across relevant bird species, providing comparative evidence for coevolutionary patterns of host and brood parasite sensory systems.

Host personality predicts cuckoo egg rejection in Daurian redstarts Phoenicurus auroreus

In species that are subject to brood parasitism, individuals often vary in their responses to parasitic eggs, with some rejecting the eggs while others do not. While some factors, such as host age (breeding experience), the degree of egg matching and the level of perceived risk of brood parasitism have been shown to influence host decisions, much of the variation remains unexplained. The host personality hypothesis suggests that personality traits of the host influence its response to parasitic eggs, but few studies have tested this. We investigated the relationship between two personality traits (exploration and neophobia) and a physiological trait (breathing rate) of the host, and egg-rejection behaviour in a population of Daurian redstarts Phoenicurus auroreus in northeast China. We first show that exploratory behaviour and the response to a novel object are repeatable for individual females and strongly covary, indicating distinct personality types. We then show that fast-exploring and less neophobic hosts were more likely to reject parasitic eggs than slow-exploring and more neophobic hosts. Variation in breathing rate-a measure of the stress-response-did not affect rejection behaviour. Our results demonstrate that host personality, along the bold-shy continuum, predicts the responses to parasitic eggs in Daurian redstarts, with bold hosts being more likely to reject parasitic eggs.

Monogamy without parental care? Social and genetic mating systems of avian brood parasites

Classic evolutionary theory predicts that monogamy should be intimately linked with parental care. It has long been assumed, therefore, that avian brood parasites-which lay their eggs in the nests of 'host' species and provide little, if any, parental care-should be overwhelmingly promiscuous. However, recent studies have revealed that the social mating systems of brood parasites are surprisingly diverse, encompassing lek polygyny, monogamy, polygamy and promiscuity. What ecological or phylogenetic factors explain this variation, and why are some brood parasites apparently monogamous? Here we review the social and genetic mating systems of all 75 brood parasitic species for which data are available and evaluate several hypotheses that may help explain these patterns. We find that social monogamy is widespread, often co-occurring with territoriality and cooperative behaviour by the mated pair. Comparative studies, though preliminary, suggest that in some species, monogamy is associated with low host density and polygamy with higher host density. Interestingly, molecular data show that genetic and social mating systems can be entirely decoupled: genetic monogamy can occur in parasitic species that lack behavioural pair-bonds, possibly as a by-product of territoriality; conversely, social monogamy has been reported in parasites that are genetically polygamous. This synthesis suggests that social and genetic monogamy may result from very different selective pressures, and that male-female cooperative behaviours, population density and territoriality may all interact to favour the evolution of monogamous mating in brood parasites. Given that detailed descriptive data of social, and especially genetic, mating systems are still lacking for the majority of brood parasitic species, definitive tests of these hypotheses await future work. This article is part of the theme issue 'The coevolutionary biology of brood parasitism: from mechanism to pattern'.

Great spotted cuckoos show dynamic patterns of host selection during the breeding season. The importance of laying stage and parasitism status of magpie nests

Avian brood parasites depend entirely on their hosts to raise their nestlings until independence. Thus, parasite females should select suitable host nests for egg laying according to traits that enhance offspring survival. The availability of nests of certain characteristics influencing the survival of parasitic offspring is, however, temporally dynamic and, thus, patterns of host selection should be evaluated considering characteristics of available host nests the day of parasitism. This allows detecting possible seasonal changes and, therefore, a more realistic picture of host selection by brood parasites. In this paper, we adopt such a new approach and consider daily availability of magpie (Pica pica) host nests at different breeding stage that were or were not parasitized by the great spotted cuckoo (Clamator glandarius). Theory predicts that cuckoos should select host nests at the laying stage. Accordingly, we detected that cuckoos preferred to parasitize magpie nests at the laying stage but, mainly, those that already harbored one or two cuckoo eggs, which may seem counterintuitive. We also showed that patterns of host selection by cuckoos varied during the breeding season, which implies that brood parasite–host interaction is dynamic depending on phenology. These patterns are hidden when not considering the temporally dynamic nature of the availability of host nests of characteristics of interest. We discuss the importance of such patterns and considering diary hosts nests availability for detecting them.

Host alarm calls attract the unwanted attention of the brood parasitic common cuckoo

It is well known that avian brood parasites lay their eggs in the nests of other bird species, called hosts. It remains less clear, however, just how parasites are able to recognize their hosts and identify the exact location of the appropriate nests to lay their eggs in. While previous studies attributed high importance to visual signals in finding the hosts’ nests (e.g. nest building activity or the distance and direct sight of the nest from vantage points used by the brood parasites), the role of host acoustic signals during the nest searching stage has been largely neglected. We present experimental evidence that both female and male common cuckoos Cuculus canorus pay attention to their host’s, the great reed warbler’s Acrocephalus arundinaceus alarm calls, relative to the calls of an unparasitized species used as controls. Parallel to this, we found no difference between the visibility of parasitized and unparasitized nests during drone flights, but great reed warblers that alarmed more frequently experienced higher rates of parasitism. We conclude that alarm calls might be advantageous for the hosts when used against enemies or for alerting conspecifics, but can act in a detrimental manner by providing important nest location cues for eavesdropping brood parasites. Our results suggest that host alarm calls may constitute a suitable trait on which cuckoo nestlings can imprint on to recognize their primary host species later in life. Our study contributes to the growing body of knowledge regarding the context-dependency of animal signals, by providing a novel example of a beneficial acoustic trait intercepted by a heterospecific and used against the emitter.

Maternal androgens in avian brood parasites and their hosts: Responses to parasitism and competition?

In the coevolutionary dynamic of avian brood parasites and their hosts, maternal (or transgenerational) effects have rarely been investigated. We examined the potential role of elevated yolk testosterone in eggs of the principal brood parasite in North America, the brown-headed cowbird, and three of its frequent host species. Elevated maternal androgens in eggs are a common maternal effect observed in many avian species when breeding conditions are unfavorable. These steroids accelerate embryo development, shorten incubation period, increase nestling growth rate, and enhance begging vigor, all traits that can increase the survival of offspring. We hypothesized that elevated maternal androgens in host eggs are a defense against brood parasitism. Our second hypothesis was that elevated maternal androgens in cowbird eggs are a defense against intra-specific competition. For host species, we found that elevated yolk testosterone was correlated with parasitized nests of small species, those whose nest success is most reduced by cowbird parasitism. For cowbirds, we found that elevated yolk testosterone was correlated with eggs in multiply-parasitized nests, which indicate intra-specific competition for nests due to high cowbird density. We propose experimental work to further examine the use of maternal effects by cowbirds and their hosts.

Are cuckoos maximizing egg mimicry by selecting host individuals with better matching egg phenotypes?

BACKGROUND

Avian brood parasites and their hosts are involved in complex offence-defense coevolutionary arms races. The most common pair of reciprocal adaptations in these systems is egg discrimination by hosts and egg mimicry by parasites. As mimicry improves, more advanced host adaptations evolve such as decreased intra- and increased interclutch variation in egg appearance to facilitate detection of parasitic eggs. As interclutch variation increases, parasites able to choose hosts matching best their own egg phenotype should be selected, but this requires that parasites know their own egg phenotype and select host nests correspondingly.

METHODOLOGY/PRINCIPAL FINDINGS

We compared egg mimicry of common cuckoo Cuculus canorus eggs in naturally parasitized marsh warbler Acrocephalus palustris nests and their nearest unparasitized conspecific neighbors having similar laying dates and nest-site characteristics. Modeling of avian vision and image analyses revealed no evidence that cuckoos parasitize nests where their eggs better match the host eggs. Cuckoo eggs were as good mimics, in terms of background and spot color, background luminance, spotting pattern and egg size, of host eggs in the nests actually exploited as those in the neighboring unparasitized nests.

CONCLUSIONS/SIGNIFICANCE

We reviewed the evidence for brood parasites selecting better-matching host egg phenotypes from several relevant studies and argue that such selection probably cannot exist in host-parasite systems where host interclutch variation is continuous and overall low or moderate. To date there is also no evidence that parasites prefer certain egg phenotypes in systems where it should be most advantageous, i.e., when both hosts and parasites lay polymorphic eggs. Hence, the existence of an ability to select host nests to maximize mimicry by brood parasites appears unlikely, but this possibility should be further explored in cuckoo-host systems where the host has evolved discrete egg phenotypes.

Sex differences in auditory filters of brown-headed cowbirds (Molothrus ater)

Receiver sensory abilities can be influenced by a number of factors, including habitat, phylogeny and the selective pressure to acquire information about conspecifics or heterospecifics. It has been hypothesized that brood-parasitic brown-headed cowbird (Molothrus ater) females may locate or determine the quality of potential hosts by eavesdropping on their sexual signals. This is expected to produce different sex-specific pressures on the auditory system to detect conspecific and heterospecific acoustic signals. Here, we examined auditory filter shape and efficiency, which influence the ability to resolve spectral and temporal information, in males and females at center frequencies of 2, 3 and 4 kHz. We found that overall, cowbirds had relatively wide filters (lsmean +/- SE: 619.8 +/- 41.6 Hz). Moreover, females had narrower filters (females: 491.4 +/- 66.8, males: 713.8 +/- 67.3 Hz) and greater filter efficiency (females: 59.0 +/- 2.0, males: 69.8 +/- 1.9 dB) than males. Our results suggest that the filters of female cowbirds may allow them to extract spectral information from heterospecific vocalizations. The broader auditory filters of males may reflect limited spectral energy in conspecific vocalizations in this frequency range, and hence, weaker selection for high resolution of frequency in the range of 2-4 kHz.

Community-level patterns of population recruitment in a generalist avian brood parasite, the brown-headed cowbird

The brown-headed cowbird (Molothrus ater) is a generalist brood parasite that typically parasitizes many host species in a single bird community. Population recruitment in a generalist parasite should be diverse with respect to host species; however, host-specific rates of cowbird recruitment have not been reported in any host community, and the determinants of host quality are poorly known. We studied the combined influence of parasitism level, nest abundance, and host quality on community-level patterns of cowbird recruitment in New Mexico, USA. Our objectives were to: (1) evaluate patterns of host use and quality; (2) compare cowbird egg investment and recruitment among host species; (3) identify host species of most importance to cowbird recruitment. Cowbirds parasitized 11 host species, with five "major" hosts experiencing high parasitism levels (>or=1 cowbird egg/nest) and six minor hosts experiencing low parasitism levels (<0.3 cowbird eggs/nest). Parasitism level was not correlated with host species abundance, host mass, host nestling period length, or host success at fledging cowbirds. However, tree-nesting hosts were parasitized more than ground-nesters, and foliage-gleaners more than sally-foragers and ground-foragers. Average estimated survival to fledging of cowbird eggs laid in active host nests was 0.19. Cowbird recruitment was diverse with respect to hosts but was less evenly distributed across the host community than was cowbird egg investment because western tanagers (Piranga ludovicianus) fledged cowbirds more successfully than other hosts. This success in western tanagers was due to high cowbird survivorship in tanager nests and may be associated with the larger body size of tanagers relative to other hosts.

A game-theoretic model of interspecific brood parasitism with sequential decisions

The interaction between hosts and parasites in bird populations has been studied extensively. This paper uses game-theoretic methods to model this interaction. This has been done in previous papers but has not been studied taking into account the detailed sequential nature of this game. We introduce a model allowing the host and parasite to make a number of decisions which will depend on various natural factors. The sequence of events begins with the host forming a nest and laying a number of eggs, followed by the possibility that a parasite bird will arrive at the nest; if it does it can choose to destroy some of the host eggs and lay one of its own. A sequence of events follows, which is broken down into two key stages; firstly the interaction between the host and the parasite adult, and secondly that between the host and the parasite chick. The final decision involves the host choosing whether to raise or abandon the chicks that are in the nest. There are certain natural parameters and probabilities which are central to these various decisions; in particular the host is generally uncertain whether parasitism has taken place, but can assess the likelihood of parasitism based upon certain cues (e.g. how many eggs remain in its nest). We then use this methodology to model two real-world interactions, that of the Reed Warbler with the Common Cuckoo and also the Yellow Warbler with the Brown-headed Cowbird. These parasites have different methods in the way they parasitize the nests of their hosts, and the hosts can in turn have different reactions to these parasites. Our model predictions generally match the real results well, and the model also makes predictions of the effect of changes in various key parameters on the type of parasitic interactions that should occur.

An overview and comparative analysis of singing on the nest in North American birds

Bird song is used to defend territories and attract mates and tends to make the singer conspicuous. Singing on the nest (SOTN) has been mentioned since 1904 but has remained largely unexplored. This behavior occurs in 10.1% of breeding birds of North America. Factors that predict the likelihood of SOTN for any species include colonial nesting, incubation sharing and feeding, and taxonomic family. If SOTN is as conspicuous as other forms of singing, this behavior may incur significant costs, such as increased nest predation and parasitism. Data collected from the literature regarding nest vocalizations of adult birds (calls and songs) showed those costs were not significantly different between vocalizing and silent birds. There was greater variance in the rates of nest predation and parasitism for vocalizing birds, which may suggest greater risk. The function(s) of SOTN are not well understood and are mostly untested. Several hypotheses are presented to explain this behavior. This is the first attempt to examine this behavior in any depth and demonstrates the need for further research to explore its function(s) and consequences.

Do cuckoos choose nests of great reed warblers on the basis of host egg appearance?

Prevailing theory assumes cuckoos lay at random among host nests within a population, although it has been suggested that cuckoos could choose large nests and relatively active pairs within host populations. We tested the hypothesis that egg matching could be improved by cuckoos choosing nests in which host eggs more closely match their own, by assessing matching and monitoring nest fate in great reed warblers naturally or experimentally parasitized by eggs of European cuckoos. A positive correlation between cuckoo and host egg visual features suggests that cuckoos do not lay at random within a population, but choose nests and this improves egg matching: naturally parasitized cuckoo eggs were more similar to host eggs as perceived by humans and as measured by spectrophotometry. Our results suggest a hitherto overlooked step in cuckoo-host evolutionary arms races, and have nontrivial implications for the common experimental practice of artificially parasitizing clutches.

Do avian brood parasites eavesdrop on heterospecific sexual signals revealing host quality? A review of the evidence

Eavesdropping can be defined as the extraction of information from the interactions between other individuals. It provides a relatively cheap way of gathering relevant information for fitness enhancement. Here, we propose that obligate avian brood parasites, which always lay their eggs in foreign nests of individuals of other species, may eavesdrop on their host sexual signals to locate nests of high quality individuals in which to lay their parasitic eggs. Sexual signal variation can honestly signal parental quality. Thus, by eavesdropping on sexual signals, parasites may select high quality foster parents for their own offspring. Such a use of sexual signals within host populations by brood parasites differs from signal exploitation theory that proposes that parasite only use signals to locate potential host independently from signaller quality. Here, we review the avian literature concerning host choice within a host species by obligate avian brood parasites and find evidence for host selection within individuals of a host species on the basis of cues potentially functioning as sexually selected traits, or at least revealing parental abilities. We have also found support for the existence of benefits linked to host selection by avian brood parasites. Finally, one study reported on the attenuation of a sexual ornament in host populations under strong pressure by brood parasites. Most of these findings have been interpreted as evidence for host selection by avian brood parasites based on the conspicuousness of sexual signals. We suggest, however, that these findings may in fact reveal eavesdropping on host signalling performance by brood parasites which would use the information extracted to choose the better individuals among conspecifics of a given host. This provides a new perspective for the study of host selection in obligate brood parasites, and raises interesting questions for the study of animal cognition that would deserve experimental studies.

Rapid increase in cuckoo egg matching in a recently parasitized reed warbler population

Parasitic cuckoos lay eggs that mimic those of their hosts, and such close phenotypic matching may arise from coevolutionary interactions between parasite and host. However, cuckoos may also explicitly choose hosts in a way that increases degree of matching between eggs of cuckoos and parasites, with female preference for specific host phenotypes increasing the degree of matching. We tested for temporal change in degree of matching between eggs of the parasitic European cuckoo (Cuculus canorus) and its reed warbler (Acrocephalus scirpaceus) host during 24 consecutive years in a recently parasitized reed warbler population. Cuckoo-host egg matching in an ultraviolet-brownness component yielding most of the chromatic variance of eggs improved during the study period. Improved matching was not due to changes in cuckoo egg phenotype. Cuckoo eggs matched host eggs for ultraviolet-brownness within nests irrespective of duration of sympatry. Ultraviolet-brownness of cuckoo eggs was similar to that of reed warbler eggs at parasitized nests, but differed from that of reed warbler eggs at unparasitized nests. These findings provide tentative support for the cuckoo preference hypothesis suggesting that cuckoo-host egg matching could partially be due to cuckoo females selecting host nests based on the appearance of their eggs.

Nest site attributes and temporal patterns of northern flicker nest loss: effects of predation and competition

To date, most studies of nest site selection have failed to take into account more than one source of nest loss (or have combined all sources in one analysis) when examining nest site characteristics, leaving us with an incomplete understanding of the potential trade-offs that individuals may face when selecting a nest site. Our objectives were to determine whether northern flickers (Colaptes auratus) may experience a trade-off in nest site selection in response to mammalian nest predation and nest loss to a cavity nest competitor (European starling, Sturnus vulgaris). We also document within-season temporal patterns of these two sources of nest loss with the hypothesis that flickers may also be constrained in the timing of reproduction under both predatory and competitive influence. Mammalian predators frequently depredated flicker nests that were: lower to the ground, less concealed by vegetation around the cavity entrance and at the base of the nest tree, closer to coniferous forest edges and in forest clumps with a high percentage of conifer content. Proximity to coniferous edges or coniferous trees increased the probability of nest predation, but nests near conifers were less likely to be lost to starlings. Flickers may thus face a trade-off in nest site selection with respect to safety from predators or competitors. Models suggested that peaks of nest predation and nest loss to eviction occurred at the same time, although a competing model suggested that the peak of nest loss to starlings occurred 5 days earlier than the peak of mammalian predation. Differences in peaks of mammalian predation and loss to starlings may constrain any adjustment in clutch initiation date by flickers to avoid one source of nest loss.

Host Recognition of Brood Parasites: Implications for Methodology in Studies of Enemy Recognition

Various studies have shown that experiments on nest defense and enemy recognition (e.g. recognition of adult brood parasites) can be confounded by many factors. However, no study has described a confounding effect of control dummy type. Here, I show experimentally that the choice of control dummy may influence the results of an experiment and lead to erroneous conclusions. I tested recognition abilities of the Blackcap (Sylvia atricapilla), currently a host rarely used by the Common Cuckoo (Cuculus canorus). Blackcaps responded very differently to two kinds of control dummies: they ignored the Eurasian Blackbird (Turdus merula) dummy, but attacked the Rock Pigeon (Columba livia) dummy as frequently as they attacked the Common Cuckoo. The differing results may be explained by the fact that the Rock Pigeon is more similar to the Common Cuckoo than the Eurasian Blackbird is, and consequently elicited more aggressive behavior than the latter. Thus, absence of discrimination in enemy-recognition studies may reflect a methodological artifact resulting from varying abilities of particular hosts to discriminate along a continuum of recognition cues. This result has serious methodological implications for further research on enemy recognition and aggression in general: a control dummy should not be too similar to the dummy brood parasite; otherwise, the chance of detecting existing recognition abilities is low. Further, I argue that coevolution only increases pre-existing aggression in the particular host species. Therefore, increment analysis (assessing changes in host antiparasitic responses during the nesting cycle while controlling for background aggression to control dummies) provides a more accurate picture of hosts' recognition abilities than the traditional approach (when the total level of antiparasitic response is analyzed).

Brood parasitism by brown-headed cowbirds and the expression of sexual characters in their hosts

Interspecific brood parasites may use the secondary sexual characters of the hosts to decide which species to parasitize. Hence, species with conspicuous and well-recognisable traits may have higher chances of becoming parasitised. Using North American birds and their frequent brood parasite, the brown-headed cowbird Molothrus ater, we tested the relationship between features of song and plumage coloration of hosts and the frequency of brood parasitism while controlling for several potentially confounding factors. Relying on two sets of analysis, we focused separately on the evolutionary view of the parasite and the host. From the cowbird's perspective, we found that males of heavily parasitized species posit songs with low syllable repertoire size, shorter inter-song interval and have brighter plumage. From the host's perspective, a phylogenetic analysis revealed similar associations for features of song, but not for plumage characteristics that were unrelated to brood parasitism. These comparative findings may imply that brood parasites choose novel hosts based on heterospecific signals; and/or host species working against sexual selection escape from brood parasitism by evolving inconspicuous sexual signals. Although our data do not allow us to distinguish between these two evolutionary scenarios, our results suggest that selection factors mediating cowbird parasitism via host recognition by heterospecific signals may have an important role in the evolutionary relationship between brood parasites and their hosts.