Begging and short-term need in cowbird nestlings: how different are brood parasites?

Because brood parasitic nestlings are usually unrelated to their nestmates and to the provisioning adult, they are free from indirect costs of begging. Consequently, they are predicted to beg more intensely than host nestlings, and some models predict they will beg at an invariantly high level, regardless of short-term need. Previous work has shown intense begging by parasitic cowbirds, but short-term need was not controlled. In this study, we manipulated short-term need and measured begging intensity in two species pairs of host and parasitic nestlings: shiny cowbirds (Molothrus bonariensis) parasitizing larger hosts than themselves (rufous-bellied thrushes, Turdus rufiventris), and brown-headed cowbirds (M. ater) parasitizing smaller hosts than themselves (yellow warblers, Dendroica petechia). All four species increased their begging intensity with short-term need, though the change was much less pronounced between food-deprived and control treatments than between control and hand-feeding treatments. Shiny cowbirds begged more intensely than rufous-bellied thrushes following each treatment. In contrast, brown-headed cowbirds did not beg significantly more intensely than yellow warblers under any of the treatments. Intense begging by both species of parasites was as effective as host begging in stimulating the adults to make provisioning visits, but shiny cowbirds were less successful at acquiring food from adult thrushes. A wide array of factors may underlie the apparent differences in the begging behavior and success of brown-headed and shiny cowbirds, including relative size, experience, and local risk of nest predation. Our experiments clearly demonstrate, however, that these two species of parasitic cowbirds adjust their begging intensity based on short-term need.Communicated by H. Kokko     

Parent-absent begging in the Brown-headed Cowbird (<Emphasis Type="Italic">Molothrus ater</Emphasis>): the role of short-term need and nestmate size

Although it is well-established that nestlings of many altricial species beg when parents are away from the nest, we have a poor understanding of parent-absent begging in brood parasites, including the proximate factors that may influence begging frequency and intensity. In this study, I examined how parent-absent begging was influenced by competitive asymmetries between host and Brown-headed Cowbird (Molothrus ater) nestlings under disparate levels of short-term need. Food-deprived cowbird nestlings begged more frequently and for a greater proportion of parent-absent period than when food-supplemented, with similar patterns observed in hosts of different sizes. In contrast, three metrics of cowbird begging intensity varied relative to host size but not due to differences in short-term need. Cowbirds consistently begged more frequently and intensively than host nestlings for a given level of short-term need, providing evidence that cowbird begging displays are more frequent and intense than non-parasitic nestlings during both feeding visits and parent-absent periods. In sum, the frequency of begging by cowbirds was only influenced by short-term need, whereas begging intensity during parent-absent events was only influenced by the host against which cowbirds competed. This study demonstrates that host size and short-term need had differing influences on the frequency and intensity of parent-absent begging in cowbirds, although both factors are likely important in limiting the evolution of parent-absent begging in cowbirds. Because it appears to provide no immediate benefits yet may decrease fitness, parent-absent begging should be included in future theoretical models investigating the evolution of begging displays in nestling birds.     

Patterns of egg yolk antioxidant co-variation in an avian brood parasite–host system

Despite their importance to offspring fitness outcomes, there has been little previous work on egg-mediated maternal effects in avian brood parasites which lay their eggs in the nests of other species. Here, we examine patterns of egg yolk antioxidant deposition in an avian host (red-winged blackbird, Agelaius phoeniceus) and their brood parasite (brown-headed cowbird, Molothrus ater). Cowbird nestlings compete directly for food with host blackbird chicks and succeed, despite hatching from a smaller egg, by begging more intensely and growing as or more rapidly than their hosts. Dietary-derived antioxidants, such as carotenoids and vitamins E and A, deposited in egg yolk, may enhance growth rate and reduce oxidative stress, and therefore provide a potential mechanism by which female cowbirds could enhance the competitiveness of their young in host nests. However, provision of antioxidants to eggs is costly so we predicted that female cowbirds should adjust the amount of antioxidants in response to variation in host quality. We found that whilst red-winged blackbird clutches parasitised by cowbirds did not differ in the pattern of antioxidant deposition compared to non-parasitised clutches, levels of vitamin E deposited in cowbird eggs were closely matched to that of the individual host clutch in which cowbirds laid their egg. This supports the prediction that female cowbirds adjust the antioxidant concentration of their eggs to increase the likely competitiveness of their offspring in the host nest into which they are laid and may help explain the success of cowbirds in competing with larger host young.     

Red-winged blackbird parental investment following brood parasitism by brown-headed cowbirds: is parentage important?

Parental investment by red-winged blackbirds (Agelaius phoeniceus) in response to natural and experi‐mental parasitism by brown-headed cowbirds (Molothrus ater), and in response to freeze-dried, female cowbird mounts presented near redwing nests during the egg-laying period was measured. Two measures of redwing parental investment were used: nest defense effort toward a model predator, and rate of feeding nestlings. There were no significant differences in levels of parental investment among unparasitized nests, naturally parasitized nests, or experimentally parasitized nests. Similarly, parental investment did not differ between redwings that were exposed to the cowbird mount and those that were not exposed to the mount, or among redwings exposed to the cowbird mount at different distances from the nest. This suggests that red-winged blackbirds do not recognize when they have been parasitized, and hence do not associate parasitism with a decrease in their parentage, or that parentage is not an important predictor of parental investment in this species. Received: 24 January 1997 / Accepted after revision: 7 June 1997     

Brood parasitism disproportionately increases nest provisioning and helper recruitment in a cooperatively breeding bird

Obligate avian brood parasites lay their eggs in nests of other species (hosts), which raise parasitic young. Parasitic nestlings are likely to influence host’s parental behaviours as they typically beg for food more vigorously than young host for a given hunger level. However, few studies have tested this idea, with conflicting results. These prior studies were largely limited to biparental hosts, but little is known about the effect of brood parasitism on parental behaviours in hosts that breed cooperatively. We followed a multimodel approach to examine the effect of brood parasitism on nest provisioning and helper recruitment in the baywing (Agelaioides badius), a cooperative breeder parasitised by screaming (Molothrus rufoaxillaris) and shiny (Molothrus bonariensis) cowbirds. Multimodel inference results indicated that feeding visits increased with nestling age, cooperative group size and number of cowbird nestlings in the brood. Brood size had little influence on feeding visits, which further suggests that baywings adjusted their provisioning effort in response to cowbird parasitism. In addition, nests parasitised artificially with shiny cowbird eggs or hatchlings recruited more helpers than unmanipulated nests having only host or screaming cowbird young. Our results provide novel evidence that brood parasitism and cooperative breeding interact in determining the levels of nest provisioning.     

Provisioning rules in tree swallows

Conflict between parents and offspring may result in offspring exaggerating their needs and parents devaluing their begging signals. To determine whether this occurs, it is first necessary to establish the link between need, begging and parental response. The purpose of our study was to examine these relationships in tree swallows (Tachycineta bicolor). Parents preferentially fed nestlings that begged sooner, reached higher and were closer to the front of the nestbox (Fig. 1). Begging intensity of both individuals and entire broods increased with relatively long periods between feeding visits. Within broods, parents responded to increased begging intensity by increasing their feeding rate, although this effect was relatively weak. Large and small nestlings did not differ in their begging behavior and all nestlings, regardless of size, were fed at similar rates. Despite the overall equity in feeding, male parents preferentially fed larger nestlings while female parents fed smaller nestlings. Nestlings did not increase their begging intensity in response to begging by nestmates. Our results suggest that begging is related to need in this species and that parents respond to variation in begging intensity. Received: 4 May 1995/Accepted after revision: 17 December 1995     

Need and nestmates affect begging in tree swallows

We conducted an experiment on nestling tree swallows (Tachycineta bicolor) to examine predictions from signalling models for the evolution of conspicuous begging behaviour. Specifically, we examined the relationship between (1) nestling begging intensity and hunger, (2) begging intensity and parental provisioning and (3) begging intensity and nestmate condition. Forty broods of 9-day-old nestlings were removed from their nests for 1 h and assigned to one of the following three treatments: (1) all nestlings in the brood deprived of food (n = 13), (2) all nestlings in the brood fed (n = 11) or (3) half the nestlings in the brood deprived and half fed (n = 16). Videotapes before and after the treatments showed that begging intensity increased in broods in which all of the nestlings had been deprived and decreased in broods in which all of the nestlings had been fed. Deprived nestlings in the half-and-half treatment did not change their begging intensity in response to treatment, while fed nestlings in this treatment group showed a decrease in begging intensity. Parent tree swallows increased their feeding rate to deprived broods and decreased their rate to fed broods. Within broods, parents decreased their feeding rate to fed nestlings, but showed no significant change in feeding to deprived nestlings. Our results suggest that begging intensity is influenced by hunger and that parents appear to respond to variation in begging intensity. The begging of nestmates also appears to influence begging independently of need. These results are consistent with predictions derived from signalling models of begging. Received: 20 June 1997 / Accepted after revision: 19 January 1998     

Brood mate eviction or brood mate acceptance by brood parasitic nestlings? An experimental study with the non-evictor great spotted cuckoo and its magpie host

Some avian brood parasitic nestlings are highly virulent, destroying all host eggs or nestmates, while others accept growing up together with host nestmates. The traditional idea was that all brood parasitic nestlings would benefit from being alone in the host nest. Thus, why do nestlings of some brood parasitic species accept the company of host offspring in the nest? The trade-off hypothesis suggests that brood parasites must balance the costs and benefits of killing host young because of two major potential costs: risk of nest desertion and loss of begging assistance. Here, we test this hypothesis in a non-evictor cuckoo species, the great spotted cuckoo Clamator glandarius and its main host, the magpie Pica pica, by manipulating brood size (1–3 nestlings) and brood composition (only cuckoo, only magpie or mixed) during three consecutive breeding seasons. None of the broods were abandoned by host parents, and cuckoo nestlings alone in the nest tended to grow faster (i.e. wing length). Thus, none of the predictions of the two potential costs on which the trade-off hypothesis is based apply to the great spotted cuckoo–magpie system. Our experimental study could not directly test why chick killing has not evolved in great spotted cuckoos, but the results point in the direction of several possibilities. We suggest that chick killing in great spotted cuckoos may not be adaptive mainly because another, less costly strategy (i.e. outcompeting host nestmates for food), is efficient for successful parasitism of magpie hosts.     

Begging affects parental effort in the pied flycatcher, Ficedula hypoleuca

It has been suggested that nestlings use begging to increase their share of parental resources at the expense of current or future siblings. There is ample evidence that siblings compete over food with nestmates by begging, but only short-term effects of begging on parental provisioning rates have been shown. In this study, we use a new experimental design to demonstrate that pied flycatcher (Ficedula hypoleuca) nestlings that beg more are able to increase parental provisioning rates over the major part of the nestling period, thus potentially competing with future siblings. Parents were marked with microchips so that additional begging sounds could be played back when one of the parents visited the nest. By playing back begging sounds consistently at either male or female visits, a sex difference in provisioning rate that lasted for the major part of the nestling period was induced. If each parent independently adjusts its effort to the begging intensity of nestlings, begging may also be the proximate control mechanism for the sexual division of labour. Received: 24 January 1997 / Accepted after revision: 30 August 1997     

Spectral tuning and perceptual differences do not explain the rejection of brood parasitic eggs by American robins (Turdus migratorius)

By laying their eggs in the nests of other birds, avian brood parasites impose the cost of rearing young upon their hosts. The recognition and rejection of foreign eggs are primary host defenses against costly brood parasitism. Hosts of parasitic brown-headed cowbirds (Molothrus ater) challenge coevolutionary theory because most cowbird hosts accept parasitic eggs despite their drastically different appearance from the hosts’ own eggs. American robins (Turdus migratorius) are one of only 10 % of the over 200 potential cowbird host species to robustly reject parasitic eggs, but the mechanisms driving the sensory bases of foreign egg rejection remain elusive. Our research combined avian visual perceptual modeling and behavioral experimentation to investigate chromatic cues eliciting parasitic egg rejection in American robins. We assessed the effects of perceivable background color differences between real host and model parasite eggs, across all four avian photoreceptors, on rates of rejection of model eggs spanning in color across the entire avian spectral sensitivity range, and including immaculate model eggs matching the natural colors of robin and cowbird eggs. The results suggest that egg rejection in robins is driven by the overall perceivable difference in color between own and artificial eggs, and input from all four single-cone avian photoreceptors affects the rejection decision. The results, however, also reveal that when viewed by the avian eye, natural cowbird eggs appear more similar in background color to robin eggs than predicted by the high rejection rate of these parasitic eggs. This suggests that robins respond specifically to parasitism by cowbirds, despite an apparent lack of sensory tuning toward the detection of the background color of cowbird eggs.     

Functional reference in an alarm signal given during nest defence: seet calls of yellow warblers denote brood-parasitic brown-headed cowbirds

Field observations and model-presentation experiments have shown that yellow warblers (Dendroica petechia) produce seet calls preferentially in response to brood-parasitic brown-headed cowbirds (Molothrus ater). In this study, we investigated whether seet calls are functionally referential alarm calls denoting cowbirds by determining whether female warblers responded appropriately to seet calls in the absence of a cowbird, whether alarm calling by warblers varied with response urgency, and how warblers in a population allopatric with cowbirds responded to cowbird and avian predator models and seet playbacks. As a control, we presented chip calls, which are elicited by nest predators as well as by non-threatening intruders, but are not strongly associated with cowbirds. Yellow warblers responded differently to playbacks of seet than chip calls. To seet playbacks, almost 60% of females gave seet calls and rushed to sit in their nests, responses typically elicited by cowbirds, whereas these responses were given infrequently in response to chip calls. Yellow warblers seet called equally in situations that simulated low, medium and high risk of parasitism, which suggests that these calls did not vary with response urgency. In a population allopatric with cowbirds, seet calls were rarely produced in response to cowbird or avian nest predator models and never to seet playbacks. These results suggest that seet calls are functionally referential signals denoting cowbirds and that cowbird parasitism was a strong selective pressure in the evolution of functional referentiality in the seet call of yellow warblers.Communicated by W.A. Searcy     

Differential parasitism of least flycatchers and yellow warblers by the brown-headed cowbird

Summary Brown-headed cowbirds (Molothrus ater) parasitized yellow warblers (Dendroica petechia) six times more frequently than least flycatchers (Empidonax minimus) nesting in the same riparian habitat in southern Manitoba. This difference was surprising because least flycatchers were higher quality hosts; flycatchers always accepted cowbird eggs while warblers did so on only about half the occasions. Both hosts were equally good foster parents, at least until fledging; thus, once an egg was accepted there was probably no further advantage in parasitizing one species over the other. The probability that a nest was parasitized decreased with increasing nest height, perhaps as a consequence of the cowbird's habit of searching for nests close to the ground. Since least flycatchers nested higher in the canopy than yellow warblers, a greater proportion of flycatcher nests probably were not detected by cowbirds. Nevertheless, nest height alone could not account fully for the lower incidence of parasitism on flycatchers. Clutch initiation in flycatchers peaked 6 days after warbler clutch initiations and 5 days after that of cowbirds. Despite the later peak in initiations, flycatcher nests were always available as potential hosts over the entire cowbird laying season and it was not until new clutch initiations by warblers declined in late summer that flycatchers were actually used as hosts. Because least flycatchers responded more aggressively than yellow warblers to a model female cowbird at the nest, we concluded that greater nest defense by flycatchers may have also reduced the rate of brood parasitism in this host. Together, our results suggest the large difference in the frequency of parasitism between these two hosts was primarily a product of nest location but that differences in host nest-defense and breeding season asynchrony may have contributed to preferential host selection.Offprint requests to: J.V. Briskie     

Nestling American robins compete with siblings by begging

Summary The evolution of intense begging by dependent nestling birds has recently been the subject of several theoretical papers. The interesting problem here is that nestlings should be able to communicate their nutritional status to parents in ways that are less costly energetically and less likely to attract predators. Thus, conspicuous begging behaviour is thought to have evolved as a result of either competition among nestmates or the manipulation of their parents to provide more food than would otherwise be favoured by selection. We studied sibling competition for parental feedings in the American robin (Turdus migratorius). We demonstrate that the probability that an individual nestling received food was related to several indices of begging. When we experimentally prevented parents from feeding part of their brood, both the intensity of begging and the number of feedings subsequently received by food-deprived nestlings increased. Furthermore, the begging intensity of those nestlings that were not food-deprived also increased in response to the begging of their hungrier siblings.Offprint requests to: R. Montgomerie     

Costs of brood parasitism and the lack of defenses on the yellow-winged blackbird - shiny cowbird system

The shiny cowbird (Molothrus bonariensis) is a generalist brood parasite that lays either white-immaculate or spotted egg morphs in eastern Argentina and Uruguay. Some hosts accept both morphs, others accept spotted eggs and reject the white morph, but no host has been found to accept white eggs and reject spotted ones. It has been suggested that the yellow-winged blackbird (Agelaius thilius) may be that type of host. The finding of a white acceptor-spotted rejector species would help to explain the occurrence and maintenance of the parasite egg polymorphism. We studied the incidence of shiny cowbird parasitism on this host, its costs for their reproductive success and the presence of antiparasitic defenses in the yellow-winged blackbird - shiny cowbird system. The parasite affected the reproductive success of the host in two ways. Cowbirds punctured host eggs causing a reduction in clutch size, and yellow-winged blackbirds deserted their nests whenever they suffered high egg loss. In addition, parasitized nests suffered higher predation during the nestling stage, but not during egg stages, indicating that the difference found was related to the presence of the cowbird chick, and not to higher exposure of parasitized nests to both parasites␣and predators. Despite the costs imposed by the parasite, yellow-winged blackbirds have not evolved antiparasitic defenses. This host did not reject any egg morph of the shiny cowbird nor desert parasitized nests unless it had suffered high egg loss. Current explanations for the host lack of defenses, the “time lag” and the “equilibrium” hypothesis, are discussed. Received: 29 August 1997 / Accepted after revision: 10 January 1998     

Age,experience, and enemy recognition by wild song sparrows

Summary Older female Song Sparrows (Melospiza melodia) breeding on Mandarte Island, B.C., Canada, are more often parasitized by a brood parasite, the Brown-headed Cowbird (Molothrus ater), than yearling females breeding for the first time. This may be explained if older Song Sparrows behave differently than yearlings towards searching female cowbirds, and are thus more readily recognized as potential hosts. We tested this hypothesis by measuring the responses of Song Sparrows to a stuffed mount of a female cowbird in 1982, when no cowbirds frequented the island. This mount, and a control (a female junco) were presented near the nests of wild female sparrows of known age. As predicted, adult female sparrows behaved differently towards the cowbird model from yearlings (Table 1). They did not differ in their responses to the junco. Adult males also responded differently to the cowbird and junco (Table 2), but adult males did not differ significantly from yearlings in their response to either model. When birds that responded weakly as yearlings in the absence of cowbirds were retested as 2-year-olds after the recolonization of the island by cowbirds, they responded strongly to cowbird models. Two-year-old birds not exposed to cowbirds as yearlings, were parasitized at a rate intermediate between the rate for experienced adults and that for yearlings. Yearling females were parasitized less often at the beginning of the period of breeding activity by cowbirds than at the end. All these results are consistent with our hypothesis that age-selective parasitism results from differences among age classes in the mobbing responses of Song Sparrows to cowbirds. The greater response of adult sparrows to cowbirds seems non-adaptive, because it apparently results in a loss of reproductive output through selective parasitism. We suggest that this paradox can be resolved if the response to cowbirds near the nest is an instance of a more general acquired response to all potential enemies that approach a nest.     

Dynamics of calling by tree swallow (Tachycineta bicolor) nestmates

The begging display of nestling passerine birds has become a model for examining the evolution of animal signals. A particular problem for nestlings when transmitting begging signals to parents may be interference from nestmates. The strategies used by nestlings to reduce signal interference have not been studied, yet potentially contribute to the design of these complex displays. In this study, we recorded the begging calls of nestling tree swallows (Tachycineta bicolor) when alone and with a begging nestmate, to determine whether nestlings changed the output, structure or timing of their calls in ways that would reduce acoustic interference. We found that nestlings increased their call rate in the presence of a begging nestmate, but did not alter the length, amplitude or frequency of their calls. They also appeared not to adjust the timing of their calls to avoid those of nestmates. Contrary to expectation, nestling calls became more similar in some aspects when nestmates called together. An increase in call rate in the presence of a begging nestmate should increase the likelihood that a parent detects an individual's calls. However, if all nestlings increase their calling rate in response to competitors, then the overall level of acoustic interference across the brood is potentially increased, an effect exacerbated by the tendency for call similarity to increase when calling together. We discuss how increasing call rate may improve detectability despite this effect and we also examine how an increase in rate and call similarity may serve to produce a strong brood signal.     

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.     

Mechanisms of avian egg recognition: Which egg parameters elicit responses by rejecter species?

Summary Some species of North American passerines nearly always reject nonmimetic eggs placed in their nests and have apparently evolved this behavior in response to brood parasitism. Experiments presented here examined the specific egg parameters to which rejecter species respond, the relative tolerances rejecters show towards nonmimetic eggs and the degree to which rejection is limited to eggs of the brown-headed cowbird (Molothrus ater), the only parasitic bird widespread in North America.Relative to cowbird eggs, American robin (Turdus migratorius) eggs are larger, blue rather than white and immaculate rather than spotted. Experiments using 10 egg models at 137 nests showed that robins respond to each of these differences (Figs. 2 and 3) but do not usually reject an egg that deviates from their own by only one difference. Eggs that differ in ant two of the three parameters are usually rejected. This built-in tolerance reduces the likelihood that robins will reject their own eggs if these are atypical in size or coloration.Small egg size was the most important parameter eliciting rapid rejections (i.e. within 1 day), probably because differnces in size can be detected by both visual and tactile perception. By contrast, small egg size was the least important parameter determining whether eggs were eventually rejected (i.e. within 5 days, Tables 2 and 3). In terms of their eventual response, robins may be more sensitive to egg coloration than to size because the latter parameter is less reliable in distinguishing between robin and cowbird eggs.Experiments were also carried out at 37 nests of the gray catbird (Dumetella carolinensis), whose immaculate blue-green egg is only slightly larger than a cowbird egg. Catbirds are much more responsive to white ground color than to maculation (Table 4), perhaps because color is more reliable in distinguishing between catbird and cowbird eggs.Rejecter species exhibit degrees of tolerance towards foreign eggs that are proportional to the divergence between their eggs and those of the cowbird. Birds with eggs strongly divergent from cowbird eggs benefit from being relatively tolerant because they avoid rejecting their own eggs but still act against cowbird eggs. Species with cowbird-like eggs must be relatively intolerant to maximize the chances that cowbird eggs are rejected.Experiments show that rejection is not specific to cowbird eggs. Thus, birds have apparently responded evolutionarily to brood parasitism by developing recognition of their own eggs, rather than by developing recognition and rejection specific to parasitic eggs.     

Utilization of a new host in the screaming cowbird <Emphasis Type="Italic">Molothrus rufoaxillaris</Emphasis>, 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.     

Sex-related asymmetry in competitive ability of sexually monomorphic barn swallow nestlings

In diverse taxa, offspring solicit parental care using complex displays, which may evolve as reliable signals of condition or as mechanisms to manipulate parental investment. Differential sex allocation may therefore result from adaptive parental decisions or sex-related variation in competitive ability or because of sex-related asymmetries in kin selection. Under normal food provisioning, female barn swallow (Hirundo rustica) nestlings begged more loudly but did not receive more food than male nestlings. After food deprivation, begging call loudness of males but not females increased. Begging loudness positively predicted the number of feedings received by the nestlings, and males gained more mass than females after food deprivation. Male nestlings are more severely affected by chronic food reduction and may therefore accrue a larger benefit compared to females by increasing their food intake under short-term conditions of food scarcity. These results suggest that either females do not increase begging intensity to favour male broodmates which are more vulnerable to prolonged food stress, or that males prevail in scramble competition despite being similar in size to females.