Experiments and observations of prothonotary warblers indicate a lack of adaptive responses to brood parasitism

It has been suggested that prothonotary warblers, Protonotaria citrea, respond adaptively to brood parasitism by brown-headed cowbirds, Molothrus ater, even though they lack historical habitat and range overlap with cowbirds. I studied behaviours functioning as potential defences against brood parasitism in the prothonotary warbler, a cavity-nesting host species. Opening sizes preferred by prothonotary warblers were not small enough to exclude cowbirds, and warblers were parasitized heavily in nests with larger openings. Male and female prothonotary warblers were always away from their nests before sunrise when cowbirds laid eggs in their nests. Prothonotary warblers infrequently (∼6% of 560 nests) deserted nests that were parasitized during the egg-laying period, but frequently (56% of 151 nests) deserted nests that were parasitized before a female warbler laid her first egg. Prothonotary warblers also deserted 60-70% of nests where a cowbird egg, warbler egg or die were experimentally added before egg laying. However, the experimental addition of one of these three objects during the egg-laying period did not elicit desertion. The desertion of parasitized nests was not affected by nest site availability as has been reported elsewhere in the literature. This lack of a response to brood parasitism by prothonotary warblers may be an example of evolutionary lag, because it is likely that they have only recently been exposed to widespread parasitism, and they accept parasitism at a high cost to their own reproductive success. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.      

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.     

A generalist brood parasite modifies use of a host in response to reproductive success

Avian obligate brood parasites, which rely solely on hosts to raise their young, should choose the highest quality hosts to maximize reproductive output. Brown-headed cowbirds (Molothrus ater) are extreme host generalists, yet female cowbirds could use information based on past reproductive outcomes to make egg-laying decisions thus minimizing fitness costs associated with parasitizing low-quality hosts. We use a long-term (21 years) nest-box study of a single host, the prothonotary warbler (Protonotaria citrea), to show that local cowbird reproductive success, but not host reproductive success, was positively correlated with the probability of parasitism the following year. Experimental manipulations of cowbird success corroborated that female cowbirds make future decisions about which hosts to use based on information pertaining to past cowbird success, both within and between years. The within-year pattern, in particular, points to local cowbird females selecting hosts based on past reproductive outcomes. This, coupled with high site fidelity of female cowbirds between years, points to information use, rather than cowbird natal returns alone, increasing parasitism rates on highly productive sites between years.     

BEHAVIORAL DEFENSES AGAINST AVIAN BROOD PARASITISM IN SYMPATRIC AND ALLOPATRIC HOST POPULATIONS

The brown-headed cowbird (Molothrus ater) is a widespread, obligate brood parasite of North American passerine birds. In southern Manitoba, where hosts are sympatric with cowbirds, American robins (Turdus migratorius) ejected parasitic eggs from all experimentally parasitized clutches (N = 25) and no eggs were accepted for more than four days. In contrast, robins in northern Manitoba, an area where cowbirds do not breed, accepted parasitic eggs in 33% of nests (N = 18) for at least five days. Acceptance of experimental cowbird eggs by a second host, the yellow warbler (Dendroica petechia), was similar in allopatric (100% of 20 nests) and sympatric (88.6% of 35 nests) populations, but models of a female cowbird elicited greater nest defense by warblers in the area of sympatry. Neither host rejected eggs of conspecifics, thus, rejection of cowbird eggs was not an epiphenomenon of conspecific brood parasitism. These results support the hypothesis that recognition of cowbirds and their eggs evolved as adaptations to counter cowbird parasitism and not some other selection pressure. The expression of anti-parasite defenses by some individuals within allopatric populations further suggests these traits may be controlled genetically but persist in such areas either through the continued introgression of rejecter genes from sympatric populations or because of the low cost of rejection behavior when parasitism is absent or rare.     

Do Hosts Discriminate between Sexually Dichromatic Male and Female Brown‐headed Cowbirds?

Female brood parasites are recognized as threats to reproductive success by many host species. Male brood parasites may accompany females while they search for nests to parasitize and males depredate nests throughout the nesting cycle. Hence, selection may also favour recognition of males. We examined whether two common host species perceive male brown-headed cowbirds ( Molothrus ater ) as brood parasites, as nest predators, or neither. We quantified visits of male cowbirds to nests of yellow warblers ( Dendroica petechia ) and red-winged blackbirds ( Ageliaus phoeniceus ) to assess the frequency with which these host species interact with male cowbirds. Males were observed near nests during hosts' laying and incubating stages, although less frequently than female cowbirds. No visits by cowbirds occurred while parents cared for nestlings. We then presented models of male and female cowbirds plus a non-threatening control to yellow warblers and red-winged blackbirds during laying and nestling periods. If hosts perceive males and females similarly, they should respond more intensely to the cowbird models during the laying period, when nests are most likely to be parasitized. Both species responded similarly to male and female cowbird models during laying, which suggests that hosts view cowbirds of both sexes as threats. The responses of yellow warblers with nestlings to male cowbirds were strongly influenced by the order of model presentation. Warblers first presented with the male cowbird gave much reduced anti-parasite responses than those that first interacted with the female then the male cowbird. These results suggest that yellow warblers recognized male vs. female cowbirds, but that discrimination was not expressed during laying. By contrast, red-winged blackbirds did not discriminate between male and female cowbirds at either nesting stage.     

Hatching asynchrony, nestling competition, and the cost of interspecific brood parasitism

All parental hosts of heterospecific brood parasites must paythe cost of rearing non-kin. Previous research on nest parasitismby brown-headed cowbirds (Molothrus ater) concluded that competitivesuperiority of the typically more intensively begging and largercowbird chick leads to preferential feeding by foster parentsand causes a reduction in the hosts' own brood. The larger sizeof cowbird nestlings can be the result of at least two causes:(1) cowbirds preferentially parasitize species with smallernestlings and lower growth rates; and/or (2) cowbirds hatchearlier than hosts. I estimated the cost of cowbird parasitismfor each of 29 species by calculating the difference betweenhosts' published brood sizes in nonparasitized and parasitizednests and using clutch size to standardize values. In this analysis,greater incubation length and lower adult mass, surrogate measuresof the hatching asynchrony and size difference between parasiteand hosts, were both related to greater costs of cowbird parasitismwithout bias owing to phylogeny. To establish causality, I manipulatedclutch contents of eastern phoebes (Sayornis phoebe) and examinedwhether earlier hatching by a single cowbird or phoebe egg reducesthe size of the rest of the original host brood. As predicted,greater hatching asynchrony increased the proportion of theoriginal phoebe brood that was lost. This measure of the costof parasitism was partially owing to increased hatching failureof the original eggs in asynchronous broods but was not at allrelated to the size differences of older and younger conspecificnestmates. However, proportional brood loss owing to an earlierhatching conspecific was consistently smaller than brood lossowing to asynchronous cowbirds in both naturally and experimentallyparasitized phoebe nests. These results imply that althoughhatching asynchrony is an important cause of the reduction ofhost broods in parasitized clutches, competitive features ofcowbird nestlings remain necessary to explain the full extentof hosts' reproductive costs caused by interspecific brood parasitism.     

Community-wide patterns of parasitism of a host “generalist” brood-parasitic cowbird

The brown-headed cowbird (Molothrusater) is a generalist obligate brood parasite. Despite intensive study and growing concern over the negative impact of cowbird parasitism on populations of many hosts, very little is known about the factors influencing community-wide patterns of cowbird parasitism. Using systematic nest searches, nest parasitism was studied over two breeding seasons at a study site in northeastern Illinois encompassing grassland, forest-edge, and forest habitat, supporting a diverse avian community. Parasitism was observed for 18 out of 34 altricial bird species found nesting at the study site. A total of 299 cowbird eggs and nestlings were found in 191 of a total of 593 nests. Analyses revealed several ecological and behavioral factors associated with frequency of parasitism and the resulting distribution of cowbird eggs. Much higher frequencies of parasitism were found in edge and forest habitats than in grassland. Within the edge habitat, open nests were parasitized significantly more often than cavity nests. Among open nests in the edge habitat, the two largest species were never parasitized. Host behavior, particularly egg-ejection behavior, was associated with a reduced observed frequency of parasitism, but at least three species known to eject cowbird eggs were sometimes parasitized. For six common hosts capable of rearing cowbirds, we found no correlation between level of parasitism and host nest-survivorship, suggesting that fine-grained assessments of host quality by female cowbirds do not influence patterns of parasitism among acceptable host species, or that differences in host quality are not great and/or predictable enough for such fine-grained assessments. Our results suggest that when a variety of possible nests are available, the level of parasitism on a particular species is a balance between a␣cowbird's preference for a particular species and the effectiveness of host species' defenses. A conceptual model was developed that incorporates the observed correlation of cowbird eggs or nestlings with habitat, nest-type, host species' body mass, and host behavioral defenses. Additional community-wide studies of cowbird parasitism will test if this model is applicable to other avian communities. Received: 20 December 1996 / Accepted: 17 May 1997     

Brood parasite eggs enhance egg survivorship in a multiply parasitized host

Despite the costs to avian parents of rearing brood parasitic offspring, many species do not reject foreign eggs from their nests. We show that where multiple parasitism occurs, rejection itself can be costly, by increasing the risk of host egg loss during subsequent parasite attacks. Chalk-browed mockingbirds (Mimus saturninus) are heavily parasitized by shiny cowbirds (Molothrus bonariensis), which also puncture eggs in host nests. Mockingbirds struggle to prevent cowbirds puncturing and laying, but seldom remove cowbird eggs once laid. We filmed cowbird visits to nests with manipulated clutch compositions and found that mockingbird eggs were more likely to escape puncture the more cowbird eggs accompanied them in the clutch. A Monte Carlo simulation of this 'dilution effect', comparing virtual hosts that systematically either reject or accept parasite eggs, shows that acceptors enjoy higher egg survivorship than rejecters in host populations where multiple parasitism occurs. For mockingbirds or other hosts in which host nestlings fare well in parasitized broods, this benefit might be sufficient to offset the fitness cost of rearing parasite chicks, making egg acceptance evolutionarily stable. Thus, counterintuitively, high intensities of parasitism might decrease or even reverse selection pressure for host defence via egg rejection.     

Phylogenetic analysis of life-history adaptations in parasitic cowbirds

Parasitic cowbirds lay eggs in the nests of other species anddupe them into caring for their young. Unlike other brood parasites,cowbirds have not developed egg mimicry or bizarre chick morphology.However, most of them parasitize a large number of hosts. Severalfeatures of cowbirds have been proposed as more general adaptationsto brood parasitism. In this study, we used a recent molecularphylogeny as a historical framework to test the possible adaptationsof the parasitic cowbird, including egg size, eggshell thicknessand energy content of the eggs, length of the incubation period,and growth pattern of cowbird nestlings. We used a recentlydeveloped extension of independent contrasts to test whetherthe five cowbird species deviate from general allometric equations.We generated prediction intervals for a nonparasite that evolvedin the place of the cowbirds. By using these prediction intervals,we found that parasitic cowbirds had not reduced weight or energycontent of their eggs, nor their incubation period over evolutionarytime. Cowbird chicks and those of nonparasitic relatives hadsimilar growth pattern. The only characteristic that separatedparasitic cowbirds from their nonparasitic relatives was anincrease in eggshell thickness. All these findings were robustand resisted the use of three models of character evolution.The fact that most traits exhibited by cowbirds were inheritedfrom a nonparasitic ancestor does not rule out that they areadvantageous for parasitism. Future research should focus onsuch traits of cowbird relatives and on how these traits preadapteda particular lineage to become parasites.     

Brood parasitism by cowbirds: risks and effects on reproductive success and survival in indigo buntings

We observed brood parasitism by brown-beaded cowbirds (Molothrusater) on indigo buntings (Passerina cyanea) and estimated dieimpact of parasitism on the success of the individual buntingsin their current nests and in their future survival and reproduction.Rates of parasitism over 8 years were 26.6% in 1040 nests and19.8% in 693 nests in two areas in southern Michigan. Risk ofparasitism was high early in the season; half the bunting nestswere begun after the end of the cowbird season. Risk was independentof female age, plant containing the nest, or habitat The immediatecost of parasitism was 1.19 and 1.26 fewer buntings fledgedper nest. Bunting success was lower in parasitized nests withcowbird eggs (nests were more likely to be deserted or predated),lower when the cowbird nestling failed (nests were more likelyto be predated), and lower when the cowbird fledged (fewer buntingsfledged) compared to nonparasitized nests. Costs were due toremoval of a bunting egg when die cowbird laid its own egg andto competition for parental care of the cowbird and buntingnestlings. Buntings that fledged from nests where a cowbirdalso fledged were only 18% as likely to survive and return totheir natal area in the next year as buntings from nests wherea cowbird did not fledge. Long-term effects of cowbird parasitismon adult breeding later in the season, survival to the nextseason, and reproductive success in the next season were negligiblewhen compared between birds that reared a cowbird and birdsthat reared only a bunting brood, or between birds that wereparasitized and birds that escaped parasitism. The results indicatelittle long-term cost of brood parasitism on individual fitnessof adult buntings beyond the impact on the current nest andthe survival of buntings that fledge from it; nearly all costis to the parasitized brood.     

Does nesting habitat predict hatch synchrony between brood parasitic brown‐headed cowbirds Molothrus ater and two host species?

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

Nest Sanitation Plays a Role in Egg Burial by Yellow Warblers

Some hosts of the brown-headed cowbird ( Molothrus ater ) possess defences that eliminate all or most parasitism costs. Yellow warblers ( Dendroica petechia ) bury cowbird eggs, possibly to clean nests rather than serving strictly as an anti-parasite defence, as non-egg-shaped objects have been ejected, buried, or deserted by other hosts. With two experiments, we tested the 'nest sanitation' hypothesis by recording warblers' responses to objects (1) similar in volume, mass, and colour to cowbird eggs, and (2) half the mass and volume (more easily ejected), placed into nests before and during incubation. We compared outcomes at control nests with responses to objects that were dissimilar (stars) and moderately similar (dumbbells) to eggs, and to real cowbird and warbler eggs. We tested whether rejection (1) declines from stars through dumbbells and real eggs, (2) is similar between stages, and (3) non-egg-shaped objects are ejected because this is the least costly rejection method. Large stars were rejected (most buried) significantly more frequently (43.8%) than cowbird eggs (16.3%) in pre-incubation, suggesting that warblers reject objects shaped unlike their own eggs to rid nests of debris. Objects spent less time in nests the more they diverged from eggs. Warblers rarely rejected large stars and dumbbells, and cowbird eggs during incubation, possibly because burial and desertion are too costly by this time. Responses to small stars and dumbbells, and to foreign yellow warbler eggs did not differ between stages; also warblers rejected stars, mostly by ejection and selective burial, more frequently (28.8%) than dumbbells (1.3%) and warbler eggs (0%). Rejection by yellow warblers, especially burial, may keep nests clean, but also functions in rejecting cowbird eggs.     

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

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

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 (≥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.     

Breeding success of common cuckoos Cuculus canorus parasitising four sympatric species of Acrocephalus warblers

We investigated the level of parasitism, egg mimicry and breeding success of cuckoos parasitising four sympatric species of Acrocephalus warblers in southern Moravia, Czech Republic. The parasitism rate was highest in the marsh warbler Acrocephalus palustris (44.8%) followed by great reed warbler A. arundinaceus (33.8%), sedge warbler A. schoenobaenus (26.5%) and reed warbler A. scirpaceus (11.6%). Although the cuckoo eggs showed a high level of mimicry the eggs of the marsh warbler this host species rejected 72% of the cuckoo eggs, resulting in a cuckoo breeding success of only 4.3%. Cuckoo eggs laid in great reed warbler and reed warbler nests showed a similar hatching success, but the cuckoo chicks survived better in great reed warbler nests, resulting in a breeding success of 30.4%, as compared to 16.4% in nests of the reed warbler. The relationship between the level of parasitism, host rejection of cuckoo eggs, cuckoo chick survival and breeding success is discussed for the four host species.     

Referential alarm calling elicits future vigilance in a host of an avian brood parasite

Yellow warblers (Setophaga petechia) use referential ‘seet’ calls to warn mates of brood parasitic brown-headed cowbirds (Molothrus ater). In response to seet calls during the day, female warblers swiftly move to sit tightly on their nests, which may prevent parasitism by physically blocking female cowbirds from inspecting and laying in the nest. However, cowbirds lay their eggs just prior to sunrise, not during daytime. We experimentally tested whether female warblers, warned by seet calls on one day, extend their anti-parasitic responses into the future by engaging in vigilance at sunrise on the next day, when parasitism may occur. As predicted, daytime seet call playbacks caused female warblers to leave their nests less often on the following morning, relative to playbacks of both their generic anti-predator calls and silent controls. Thus, referential calls do not only convey the identity or the type of threat at present but also elicit vigilance in the future to provide protection from threats during periods of heightened vulnerability.     

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

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

Density-dependent habitat selection by brown-headed cowbirds (<Emphasis Type="Italic">Molothrus ater</Emphasis>) in tallgrass prairie

Local distributions of avian brood parasites among their host habitats may depend upon conspecific parasite density. We used isodar analysis to test for density-dependent habitat selection in brown-headed cowbirds (Molothrus ater) among tallgrass prairie adjacent to wooded edges, and prairie interior habitat (>100 m from wooded edges) with and without experimental perches. Eight study sites containing these three habitat treatments were established along a geographical gradient in cowbird abundance within the Flint Hills region of Eastern Kansas and Oklahoma, USA. The focal host species of our study, the dickcissel (Spiza americana), is the most abundant and preferred cowbird host in the prairie of this region. Cowbird relative abundance and cowbird:host abundance ratios were used as estimates of female cowbird density, whereas cowbird egg density was measured as parasitism frequency (percent of dickcissel nests parasitized), and parasitism intensity (number of cowbird eggs per parasitized nest). Geographical variation in cowbird abundance was independent of host abundance. Within study sites, host abundance was highest in wooded edge plots, intermediate in the experimental perch plots, and lowest in prairie interior. Cowbirds exhibited a pattern of density-dependent selection of prairie edge versus experimental perch and interior habitats. On sites where measures of cowbird density were lowest, all cowbird density estimates (female cowbirds and their eggs) were highest near (100 m) wooded edges, where host and perch availability are highest. However, as overall cowbird density increased geographically, these density estimates increased more rapidly in experimental perch plots and prairie interiors. Variation in cowbird abundance and cowbird:host ratios suggested density-dependent cowbird selection of experimental perch over prairie interior habitat, but parasitism levels on dickcissel nests were similar among these two habitats at all levels of local cowbird parasitism. The density-dependent pattern of cowbird distribution among prairie edge and interior suggested that density effects on perceived cowbird fitness are greatest at wooded edges. A positive relationship between daily nest mortality rates of parasitized nests during the nestling period with parasitism intensity levels per nest suggested a density-dependent effect on cowbird reproductive success. However, this relationship was similar among habitats, such that all habitats should have been perceived as being equally suitable to cowbirds at all densities. Other unmeasured effects on cowbird habitat suitability (e.g., reduced cowbird success in edge-dwelling host nests, cowbird despotism at edges) might have affected cowbird habitat selection. Managers attempting to minimize cowbird parasitism on sensitive cowbird hosts should consider that hosts in otherwise less-preferred cowbird habitats (e.g., habitat interiors) are at greater risk of being parasitized where cowbirds become particularly abundant.     

Temporal patterns of host availability, brown-headed cowbird brood parasitism, and parasite egg mass

I studied relationships between temporal patterns of host availability, brood parasitism, and egg mass for the parasitic brown-headed cowbird (Molothrus ater). At a study site consisting largely of edge habitat in north-eastern Illinois, I found 834 bird nests from 27 species. A total of 407 cowbird eggs and nestlings were found in these nests over three laying seasons. Nearly all (n= 379; 93%) were found in the nests of seven host species. For these species and all taken together, weekly nest availability generally decreased whereas parasitism frequency generally increased throughout the cowbird laying season, but the proportions of nests parasitized and the mean number of cowbird eggs in them did not. Additionally, no correlation was found between the proportions of nests parasitized and nest availability. Cowbird egg mass generally increased throughout the laying season, indicating that foraging conditions improved and that, early in the laying season, egg mass and quality may be less important than quantity. Consistently high weekly levels of parasitism indicate that cowbird reproduction was less limited by resources needed for egg production and more by the availability of suitable host nests. Fluctuating weekly host availabilities suggest that previously established, constant rates of cowbird egg laying would produce an excess of eggs during periods of low host availability. Further, the low frequency of parasitism (1%) of nests in stages too advanced for successful parasitism, and of abandoned nests, is consistent with the hypothesis that cowbirds' consistently high rate of egg production helps assure an egg is available when an appropriate nest is found. Frequently, nests were parasitized multiple times, raising the possibility that cowbirds were interfering with their own reproduction. A diverse host community increases the possibility that a decline of any one host species is unlikely to affect cowbird reproduction significantly. Received 11 July 1997 / Accepted: 31 March 1998     

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.