Unparasitized Nests Research Articles

Does cowbird parasitism increase predation risk to American redstart nests?

Brood parasitism and nest predation are major causes of reproductive failure for many bird species nesting in fragmented landscapes. While brood parasites and predators may act independently, they could also interact if brood parasites increase the likelihood that predators detect nests. In this study, we examined the interaction between cowbird parasitism and nest predation in a 10 year study on 466 American redstart Setophaga ruticilla nests in central Alberta, Canada. We used advanced nest survival models to examine the support for three mechanisms that might lead to a positive correlation between brood parasitism and nest predation: 1) the presence of a cowbird nestling might increase the detection of the nest by predators, 2) nests with lower cover are more likely to be detected by both cowbirds and predators, and 3) cowbirds and predators may co‐occur in landscapes of similar structure. Twelve percent of nests were parasitized and those nests had a 16–19% higher rate of failure due to predators compared to unparasitized nests. Daily nest predation rates increased during the nestling stage for both groups, but more strongly for parasitized nests. Loud begging by the cowbird nestling and/or higher parental feeding rates for the cowbird may have increased nest detectability to predators. Brood parasitism and nest predation were also positively related to forest cover, indicating landscape level effects were influential. Most nest predators were forest species and we suspect cowbirds responded positively to forest cover because of the increased abundance of songbird hosts. Nest‐site features had less of an impact on nest predation or brood parasitism, although nests with higher overhead cover were less susceptible to predators. Our study shows how multiple mechanisms, particularly the behavioral effects of the brood parasite nestling and landscape structure, can lead to a positive relationship between nest predation and brood parasitism.

Effects of Parasitism by Brown-Headed Cowbirds (MOLOTHRUS ATER) on Reproductive Success of Three Frequent Hosts in New Mexico

Abstract We studied effects of parasitism by brown-headed cowbirds (Molothrus ater) on three frequent hosts from a pinyon-juniper (Pinus-Juniperus) community in New Mexico. During 1993–1999 and 2001–2002, we measured frequencies of parasitism and examined how differences in size of host, life-history traits, and behavior, as well as aspects of behavior of brown-headed cowbirds (i.e., removal of eggs of host and multiple parasitism), influenced reproductive success of blue-gray gnatcatchers (Polioptila caerulea), plumbeous vireos (Vireo plumbeus), and western tanagers (Piranga ludoviciana). Brown-headed cowbirds parasitized 79% of 288 nests of blue-gray gnatcatchers, 84% of 170 nests of plumbeous vireos, and 76% of 129 nests of western tanagers. Parasitism reduced productivity of hosts through removal of eggs, reduced hatching success, reduced survival of nestlings, and desertion of nest. Relative to unparasitized nests, size of clutch of singly parasitized nests was reduced, on average, by 0.3 eggs for th...

EFFECT OF COWBIRD PARASITISM ON BROOD REDUCTION IN THE BROWN-AND-YELLOW MARSHBIRD

Abstract Brood parasitism usually imposes several costs on individual hosts, one of which is the increased probability of brood reduction. We analyzed the effect of cowbird (Molothrus spp.) parasitism on the probability of brood reduction in the Brown-and-yellow Marshbird (Pseudoleistes virescens), a common host in the Argentinean Pampas. Using a nine-year database of nesting success of the Brown-and-yellow Marshbird, we compared the number of host fledglings produced in parasitized and unparasitized nests, and assessed the effect of year, parasitism (if the nest was parasitized or not), overall brood size (including cowbird and marshbird nestlings), and (interval between the first- and last-hatched young) on the probability of brood reduction. Parasitized nests produced fewer Brown-and-yellow Marshbird fledglings. About 22% of nests suffered brood reduction and 68% of these nests were parasitized by cowbirds. A logistic regression model showed that hatching interval was the main facto...

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.

Effects of the introduced parasite Philornis downsi on nestling growth and mortality in the medium ground finch ( Geospiza fortis)

Invasive species have the potential to detrimentally affect native ecosystems by out competing or directly preying upon native organisms, and have been implicated in the extinction of endemic populations. One potentially devastating introduced species in the Galápagos Islands is the parasitic fly Philornis downsi. As larvae, P. downsi parasitize nestling birds and have been associated with high nestling mortality and reduced growth rates. Here I document nestling growth and mortality in a bimodal population of the medium ground finch, Geospiza fortis. Observations were conducted over three years, and under variable ecological conditions. Annual parasite prevalence in nests ranged from 64% to 98%, and nestling mortality in nests with parasites ranged from 16% to 37%. Parasite load and parasite load per nestling follow a skewed distribution with many nests having relatively few parasites, and few nest having many. Parasite load, however, was not correlated with onset of breeding, clutch size, the number of nestlings, nestling survival or fledgling success. Parasite load per nestling, on the other hand, was correlated with clutch initiation date and the proportion of nestlings that died in parasitized nests. Neither nestling size nor growth rate differed between parasitized and unparasitized nests. In addition, male and female beak morphology was not correlated with parasite load, breeding variables or nestling survival. Thus, while overall mortality due to parasitism is high, ecological conditions and possible host defenses may potentially counter some of the detrimental affects of P. downsi on nestling size and growth. These results taken together suggest that parasitism of P. downsi larvae on nestling G. fortis has the potential to lead to large population declines.

Gjøkparasittisme hos heipiplerke: Noen erfaringer fra Øyerfjellet sesongen 2005

In a mountainous area of Øyerfjellet 17 nests of the Meadow Pipit Anthus pratensis were found during the season of 2005. Five or possibly six of these nests were parasitized by the Common Cuckoo Cuculus canorus, giving a parasitizing rate of 29.4% and 35.3%, respectively. The nests were regularly controlled, and two nests were found deserted by the Meadow Pipit probably during the start of the nesting period, whereas three Cuckoo chicks were hatched. Nearly hatched eggs or newly hatched chicks were found ejected from these three Meadow Pipit nests. When comparing the egg clutches from parasitized nests with unparasitized nests, it was roughly estimated that a mean of 1 ½ Meadow Pipit eggs were missing in the parasitized nests. All the five Cuckoo eggs had a pale brownish colour. When considering this brownish colour, it was suggested that mimicry of the Meadow Pipit eggs and the Cuckoo egg were slightly better in the three nests of completed hatching than in the two nests being deserted. The pattern and markings of the eggs showed good mimicry between eggs of Meadow Pipit and Cuckoo except for the pigment concentration at the broad end of many Meadow Pipit eggs.

Spotted Towhee Population Dynamics in a Riparian Restoration Context

Abstract We investigated factors at multiple scales that might influence nest predation risk for Spotted Towhees (Pipilo maculatus) along the Sacramento River, California, within the context of large-scale riparian habitat restoration. We used the logistic-exposure method and Akaike's information criterion (AIC) for model selection to compare predator search, predator abundance, restoration, and temporal effects hypotheses. Our candidate models represented restoration, temporal, nest activity, nest concealment, agriculture, and flood effects. Restoration sites did function as breeding habitat, with nest survival comparable to mature forest sites and some young being produced. The best-supported models contained the covariates cowbird young in host nest and calendar date. All supported models contained the covariate cowbird young, the only variable with a strong effect. In contrast to our prediction, nest survival increased with cowbird young in the nest, implicating adult cowbirds in nest predation events. Nest survival declined throughout the breeding season and increased marginally with nest concealment. We calculated the finite rate of population increase (λ), using a model that incorporated nest success estimates for parasitized and unparasitized nests, double brooding, the locally observed cowbird parasitism rate for Spotted Towhees, number of young fledged from successful parasitized and unparasitized nests, adult survival and a range of juvenile survival values, estimated as percentages of adult survival. Values of λ did not approach the replacement level of one, indicating that this population is not self-sustaining. We recommend extensive habitat restoration in the floodplain landscape, ideally in conjunction with cowbird control, to reduce nest predation and parasitism pressures.

Effects of Shiny CowbirdMolothrus bonariensisparasitism on different components of House WrenTroglodytes aedonreproductive success

Avian brood parasites, including cuckoos and cowbirds, have multiple negative effects on their hosts. We analysed the effects of Shiny CowbirdMolothrus bonariensisparasitism on different components (e.g. egg losses, hatching success, chick survival and nest abandonment) of House WrenTroglodytes aedonreproductive success. We also conducted an experiment to discriminate between two mechanisms that may reduce hatching success in parasitized clutches: lower efficiency of incubation due to the increase in clutch volume and disruption of host incubation by the early hatching of Cowbirds. Egg puncturing by Shiny Cowbirds reduced host clutch size at hatching by 10–20%, and parasitized nests had a decrease in hatching success of 40–80%. Egg losses and hatching failures were positively associated with the intensity of parasitism. Brood reduction was greater in parasitized nests, but the growth rate of the chicks that fledged was similar to that in unparasitized nests. The combined effects of egg losses, hatching failures and brood reduction decreased the number of fledged chicks by 80%. In addition, egg puncturing increased the likelihood of nest abandonment by Wrens. Experimental data showed that hatching failures occurred when there was a combination of: (1) an increase in the volume of the clutch by the addition of the Cowbird egg without removal of host eggs, and (2) the addition of the Cowbird egg before the onset of incubation. This was relatively common in House Wren nests, as Cowbirds generally parasitize before the onset of incubation. Our results indicate that Shiny Cowbird parasitism imposes a major impact on House Wrens, as it affects all components of the Wren's reproductive success.

Host Association in Chrysis fulgida L. and Description of Its Prepupa (Hymenoptera: Chrysididae)

In a study on the host association of the cuckoo wasp, Chrysis fulgida L., at two nesting sites in Northern Italy, it was observed emerging from nests of the mason wasp, Symmorphus crassicornis (Panzer). Although other species of eumenine wasps nested in the same area, none of them was attacked by this parasitoid. Parasitism rate per nest ranged from 0% to 66.3%, 45.8% on average. The number of parasitized cells did not decrease either with the thickness of the nest closing plug or with the length of vestibular cell. No differences in the number of cells per nest or in the length of vestibular cell were found when comparing parasitized and unparasitized nests, suggesting that the wasps probably did not abandon their parasitized nest and that the cuckoo wasp larva was not detected. A review of the host associations of the species of Chrysis of the ignita group suggests that mason wasps are the preferred hosts and that cavity-nesters are more frequently attacked than soil- or mud-nesters. The prepupa of C. fulgida is described, illustrated, and compared with the described mature larvae of Chrysididae. The postdefecating larva of this species exhibits the autapomorphy that defines the genus Chrysis: “normally developed galeae.” The most salient character shown by the mature larva of this species lies in the maxillary palpus, which has 6 sensilla at apex. This character state allows this prepupa to be differentiated from the previously described mature larvae of the genus.

Reassessing the Cowbird Threat

AbstractThe demographic significance of parasitism by Brown-headed Cowbirds (Molothrus ater; hereafter “cowbirds”) has been debated for years, because manipulative studies are few and mathematical models of cowbird-host systems have not led to tests of their predictions. We combine results from a cowbird- removal experiment (Smith et al. 2002, 2003) with a stochastic simulation model that we developed, to reassess the effect of cowbirds on the annual reproductive success (ARS) and nesting success in Song Sparrows (Melospiza melodia). Our model followed the breeding success of individual Song Sparrow nests and extrapolated to output variables including ARS, nest survival, and parasitism rates. We parameterized the model with field data from cowbird control sites (i.e., cowbirds not manipulated) and found that output variables matched those observed. We used the reduced parasitism rates observed on cowbird-removal sites, reran the model, and compared output with observed values. On removal sites, ARS was greater than predicted by the model, which indicates that the model failed to account for some biological phenomenon that occurred when cowbirds were in the system. To assess what this phenomenon might be, we conducted further analyses that indicated that cowbirds may facilitate nest predation by “conventional” predators. Using elasticity analyses, we show that cowbirds have their largest effect on ARS through egg removal and that conventional predators are more important than cowbirds in directly affecting nest survival rates. We report that to assess the effect of cowbirds on nest survival, researchers must avoid using the common technique of comparing survival rates of parasitized and unparasitized nests within populations.Réévaluation de la Menace du Vacher

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.

Genetic elucidation of host use by individual sympatric bronzed cowbirds (Molothrus aeneus) and brown-headed cowbirds (M. ater)

Species of avian brood parasites that use one or several species of hosts are called host specialists and generalists, respectively. To determine host use of individual bronzed cowbirds, Molothrus aeneus (Wagler, 1829), and brown-headed cowbirds, Molothrus ater (Boddaert, 1783), we assigned maternity to eggs using microsatellite DNA markers. We measured patterns of host use by individual sympatric cowbirds. This allowed us to determine whether these species competed for host nests and the number of females laying at nests that were already parasitized by conspecifics. We monitored 1447 nests of 42 potential host species and found that each species of cowbird used primarily four host species, with minimal overlap in the species used, yet at least some individuals acted as generalists. Individual cowbirds tended to avoid laying again at nests each had already parasitized, and multiple parasitism was frequently due to same-day laying by more than one female (19% and 44% of 27 and 39 cases for brown-headed and bronzed cowbirds, respectively). Our results suggest that both cowbirds can differentially parasitize host species; however, host use does not appear to be refined, as many eggs were laid in already-parasitized nests when unparasitized nests of other suitable host species were available.

Factors Influencing the Incidence of Cowbird Parasitism of Least Bell's Vireos

Least Bell's vireo (Vireo bellii pusillus) is a federally endangered subspecies of Bell's vireo subject to high levels of brood parasitism by brown-headed cowbirds (Molothrus ater). Brood parasitism greatly reduces the reproductive success of the vireo. We examined the relationship of vegetation structure surrounding nests and of activity near the nest to the incidence of brood parasitism by brown-headed cowbirds of least Bell's vireos. We examined vegetation structure at 3 spatial scales: microhabitat (0–1 m from a nest), mesohabitat (1–11.3 m from a nest), and macrohabitat (greater than 11.3 m from a nest). Nests with high microhabitat cover and mesohabitat cover within 5 m of the nest had a lower incidence of parasitism than those with low cover at these scales. Unparasitized nests had fewer trees greater than 8-cm diameter at breast height (dbh) within 11.3 m, and they had less canopy cover within 5 m than parasitized nests. Cowbirds parasitized nests farther from the edge of the riparian habitat more often than nests near the edge. Activity near the nest did not differ significantly between parasitized and unparasitized nests. We suggest that microhabitat cover is the most important habitat feature influencing the incidence of brood parasitism of least Bell's vireos, and we conclude that cover near the nest reduces the chance that a cowbird will observe nesting activity. We suggest that habitat management for improved breeding success of least Bell's vireos focus on increasing the density of understory vegetation.

Coevolutionary Interactions Between Common Cuckoos and Corn Buntings

AbstractAlthough Old World buntings (Emberizinae) may be considered suitable Common Cuckoo (Cuculus canorus) hosts, there is at present no evidence that any of the European species are regularly parasitized. Most historical parasitism records refer to the Yellowhammer (Emberiza citrinella) and Reed Bunting (E. schoeniclus). Both of these species reject almost 100% of experimentally added nonmimetic eggs, and also a considerable proportion of experimentally added conspecific eggs, showing exquisite egg discrimination abilities. In this paper, we report Common Cuckoo parasitism and egg rejection behavior in a Bulgarian population of another Old World Emberizinae, the Corn Bunting (Miliaria calandra). We found this species was regularly parasitized (9%, 8 of 90 nests) and that the parasitism rate was consistent among the three years of our study. Naturally laid Common Cuckoo eggs were fairly good mimics of host eggs and most were accepted (5 of 7). The Corn Bunting proved to be a suitable Common Cuckoo host as we recorded a successfully fledged cuckoo chick. Unlike Yellowhammers and Reed Buntings, Corn Buntings rejected only 42% (16 of 38) of experimentally introduced nonmimetic model Common Cuckoo eggs and none of the experimentally introduced conspecific eggs (n = 13). Parasitized nests had more and higher trees in the vicinity than unparasitized nests and breeding habitat characteristics may explain the difference in egg discrimination abilities between Corn Buntings and other Old World Emberizinae.

Brood parasitism increases provisioning rate, and reduces offspring recruitment and adult return rates, in a cowbird host

Interspecific brood parasitism in birds presents a special problem for the host because the parasitic offspring exploit their foster parents, causing them to invest more energy in their current reproductive effort. Nestling brown-headed cowbirds (Molothrus ater) are a burden to relatively small hosts and may reduce fledgling quality and adult survival. We documented food-provisioning rates of one small host, the prothonotary warbler (Protonotaria citrea), at broods that were similar in age (containing nestlings 8-9 days old), but that varied in composition (number of warbler and cowbird nestlings) and mass, and measured the effect of brood parasitism on offspring recruitment and adult returns in the host. The rate of food provisioning increased with brood mass, and males and females contributed equally to feeding nestlings. Controlling for brood mass, the provisioning rate was higher for nests with cowbirds than those without. Recruitment of warbler fledglings from unparasitized nests was 1.6 and 3.7 times higher than that of fledglings from nests containing one or two cowbirds, respectively. Returns of double-brooded adult male and female warblers decreased with an increase in the number of cowbirds raised, but the decrease was more pronounced in males. Reduced returns of warbler adults and recruitment of warbler fledglings with increased cowbird parasitism was likely a result of reduced survival. Cowbird parasitism increased the warblers' investment in current reproductive effort, while exerting additional costs to current reproduction and residual reproductive value. Our study provides the strongest evidence to date for negative effects of cowbird parasitism on recruitment of host fledglings and survival of host adults.

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

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...

EGG DESTRUCTION AND EGG REMOVAL BY AVIAN BROOD PARASITES: ADAPTIVENESS AND CONSEQUENCES

AVIAN BROOD PARASITES possess an array of adaptations that help them to succeed at their alternative breeding strategy, and these adaptations reduce host fitness in several ways. Host eggs may not hatch because brood parasites have relatively short incubation periods that allow them to hatch sooner, and this may cause hosts to stop incubating their eggs (Briskie and Sealy 1990, Hauber 2003). If host eggs hatch, their nestlings may be outcompeted by parasitic nestlings that are often larger (reviewed in Lorenzana and Sealy 1999, Peer et al. 2005). Common Cuckoo (Cuculus canorus) nestlings eliminate competition by evicting host nestlings or eggs, and Honeyguide (Indicator spp.) and Striped Cuckoo (Tapera naevia) nestlings have specialized mandibular hooks used to kill host young (Davies 2000). Brood parasites also remove or puncture host eggs, and this reduces the host's clutch size and sometimes causes the host to abandon its nest (Davies and Brooke 1988, Sealy 1992, Peer and Sealy 1999a). Some parasites may occasionally depredate nests, thereby causing total nest failure (Soler et al. 1995, Arcese et al. 1996). Egg removal and egg puncture are among the least understood of these adaptations. In a study published in this issue of The Auk, Astie and Reboreda (2006) investigated the effects of egg puncture by Shiny Cowbirds (Molothrus bonariensis). Shiny Cowbirds punctured host eggs in parasitized as well as unparasitized nests, which reveals that the effects that parasites have on hosts are complex and cannot always be fully understood simply by comparing host reproductive success at parasitized and unparasitized nests.

Impact of ectoparasitic blowfly larvae (Protocalliphora spp.) on the behavior and energetics of nestling Blue Tits

Abstract The nests of altricial bird species are host to a variety of nest ectoparasites that may develop high infestation levels and have a negative impact on chick growth, hematocrit, metabolic capacity, and survival before or after fledging. If ectoparasites affect chick development and fitness, then one would expect chicks to express behavior that might serve to limit the impact of parasites. We compared the behavior of nestling Blue Tits (Parus caeruleus) in nests with and without ectoparasitic blowfly larvae (Protocalliphora spp.). In parasitized nests, only 14.5% of the 15-s sampling periods were devoted to rest, whereas all chicks were resting in 32.5% of sampling periods for unparasitized nests. When nests were classed as active, repositioning occurred nearly twice as often in parasitized nests (83.5%) as in unparasitized ones (48.5%). Comfort behavior (e.g., preening) occurred more frequently in unparasitized nests (35.1%) than in parasitized (18.1%). The proportion of time spent repositioning i...

BROWN-HEADED COWBIRDS SKEW HOST OFFSPRING SEX RATIOS

Predators (or parasites) can have both direct and indirect effects on prey (or host) demography. Recent theory suggests that the impact of either effect may be greater if predators and parasites skew the sex ratio of survivors. Brown-headed Cowbirds (Molothrus ater) are obligate brood parasites. Many studies have shown that adult cowbirds directly cause host offspring mortalities. Cowbird young may indirectly cause additional mortalities by increasing competition in the nest. The sex ratio of surviving host offspring will be skewed if one sex is a poorer competitor and thus more likely to die. We studied whether and how cowbird parasitism affects host offspring sex ratios in Song Sparrow (Melospiza melodia) nests. The proportion of female sparrows in parasitized nests (0.28 ± 0.01) was half that in unparasitized nests (0.57 ± 0.05) at six days post-hatch and at fledging (0.27 ± 0.06 vs. 0.52 ± 0.11). Consistent with their being at a competitive disadvantage, female sparrows were smaller and lighter than male sparrows in parasitized mixed-sex (both sexes present) nests. This indirect effect of cowbirds on the sex ratio of surviving host offspring may dramatically affect host demography. We suggest that predator- or parasite-mediated sex ratio biases could be important to the demography of many prey or host species.