Cowbird Parasitism 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.

Evaluation of a Reproductive Index to Estimate Grasshopper Sparrow and Eastern Meadowlark Reproductive Success

We compared an index of reproductive success based on breeding behavior to actual nest fates of grasshopper sparrows ( Ammodramus savannarum ) and eastern meadowlarks ( Sturnella magna ) on 12 plots (4-ha). Concordance of results between the two methods was 58% for grasshopper sparrows and 42% for eastern meadowlarks on a plot-by-plot basis. The indirect method yielded higher estimates of reproductive activity than nest monitoring for the balance of the plots,. There was little evidence that brown-headed cowbird ( Molothrus ater ) parasitism influenced the estimates of reproductive success using the indirect method. We concluded that nests and about-to-fledge nestlings were missed during searches on some plots. It may be appropriate to use an indirect method to more efficiently survey territories and/or plots for species with hard-to-find nests or when monitoring large areas. Use of a reproductive index may be appropriate and more time-efficient than nest searching and monitoring for comparing management effects such as burning, grazing, haying, military training, and other localized disturbances that are likely to affect reproductive success of grasshopper sparrows and eastern meadowlarks. However, nest monitoring may be necessary for more precise estimates of productivity necessary for long-term monitoring. Nest monitoring results are also likely to allow for direct comparisons to results from other studies because the index method requires intimate knowledge of the species being evaluated - a factor that could lead to reduced precision because the experience level of technicians relying only on behavioral cues from study-to-study is likely to vary considerably.

Grassland Bird Responses to Land Management in the Largest Remaining Tallgrass Prairie

Extensive habitat loss and changing agricultural practices have caused widespread declines in grassland birds throughout North America. The Flint Hills of Kansas and Oklahoma--the largest remaining tallgrass prairie--is important for grassland bird conservation despite supporting a major cattle industry. In 2004 and 2005, we assessed the community, population, and demographic responses of grassland birds to the predominant management practices (grazing, burning, and haying) of the region, including grasslands restored under the Conservation Reserve Program (CRP). We targeted 3 species at the core of this avian community: the Dickcissel (Spiza americana), Grasshopper Sparrow (Ammodramus savannarum), and Eastern Meadowlark (Sturnella magna). Bird diversity was higher in native prairie hayfields and grazed pastures than CRP fields, which were dominated by Dickcissels. Although Dickcissel density was highest in CRP, their nest success was highest and nest parasitism by Brown-headed Cowbirds (Moluthrus ater) lowest in unburned hayfields (in 2004). Conversely, Grasshopper Sparrow density was highest in grazed pastures, but their nest success was lowest in these pastures and highest in burned hayfields, where cowbird parasitism was also lowest (in 2004). Management did not influence density and nest survival of Eastern Meadowlarks, which were uniformly low across the region. Nest success was extremely low (5-12%) for all 3 species in 2005, perhaps because of a record spring drought. Although the CRP has benefited grassland birds in agricultural landscapes, these areas may have lower habitat value in the context of native prairie. Hayfields may provide beneficial habitat for some grassland birds in the Flint Hills because they are mowed later in the breeding season than elsewhere in the Midwest. Widespread grazing and annual burning have homogenized habitat-and thus grassland-bird responses-across the Flint Hills. Diversification of management practices could increase habitat heterogeneity and enhance the conservation potential of the Flint Hills for grassland birds.

Gray Jays Accept Brown-headed Cowbird Eggs

Abstract Results of simulated brood parasitism on five Gray Jay (Perisoreus canadensis) nests suggest acceptance of model Brown-headed Cowbird (Molothrus ater) eggs. This finding is contrary to results of experimental parasitism on four other species of jays, also with little or no recent history of parasitism, which eject cowbird eggs. Given that Gray Jays nest in the boreal forest and earlier in the season than cowbirds initiate breeding, it may be that neither Gray Jays nor their congeners have been parasitized regularly in their evolutionary histories, which may explain acceptance of cowbird parasitism.

Repeated Brown-headed Cowbird Parasitism of an Artificial Nest

On 27 May 2007 I discovered two Brown-headed Cowbird (Molothrus ater) eggs in an artificial nest in a wooded fence line on the Tallgrass Prairie Preserve, Osage County, Oklahoma. The artificial nest was one of 20 placed along a transect 10 days earlier and baited with two infertile Blue Quail (Coturnix adansonii) eggs during a study on nest detection and predation. Brown-headed Cowbird parasitism of artificial open-cup nests in the absence of host nesting activity is rare, but has been reported. This occurrence of superparasitism represents the first record wherein more than one cowbird egg was deposited in an artificial nest, indicating that either a single female repeatedly parasitized the nest, or that a second female contributed an egg to the previously parasitized nest.

Habitat selection for parasite‐free space by hosts of parasitic cowbirds

Choice of breeding habitat can have a major impact on fitness. Sensitivity of habitat choice to environmental cues predicting reproductive success, such as density of harmful enemy species, should be favored by natural selection. Yet, experimental tests of this idea are in short supply. Brown‐headed cowbirdsMolothrus atercommonly reduce reproductive success of a wide diversity of birds by parasitizing their nests. We used song playbacks to simulate high cowbird density and tested whether cowbird hosts avoid such areas in habitat selection. Host species that made settlement decisions during manipulations were significantly less abundant in the cowbird treatment as a group. In contrast, hosts that settled before manipulations started and non‐host species did not respond to treatments. These results suggest that hosts of cowbirds can use vocal cues to assess parasitism risk among potential habitat patches and avoid high risk habitats. This can affect community structure by affecting habitat choices of species with differential vulnerability.

Effects of nest-site characteristics and parental activity on cowbird parasitism and nest predation in Brown-and-yellow Marshbirds

Nest-site selection and nest defense are strategies for reducing the costs of brood parasitism and nest predation, two selective forces that can influence avian nesting success and fitness. During 2001–2002, we analyzed the effect of nest-site characteristics, nesting pattern, and parental activity on nest predation and brood parasitism by cowbirds (Molothrus spp.) in a population of Brown-and-yellow Marshbirds (Pseudoleistes virescens) in the Buenos Aires province, Argentina. We examined the possible effects of nest detectability, nest accessibility, and nest defense on rates of parasitism and nest predation. We also compared rates of parasitism and nest predation and nest survival time of marshbird nests during the egg stage (active nests) with those of the same nests artificially baited with passerine eggs after young fledged or nests failed (experimental nests). Most nests (45 of 48, or 94%) found during the building or laying stages were parasitized, and 79% suffered at least one egg-predation event. Cowbirds were responsible for most egg predation, with 82 of 107 (77%) egg-predation events corresponding to eggs punctured by cowbirds. Nests built in thistles had higher rates of parasitism and egg predation than nests in other plant, probably because cowbirds were most active in the area where thistles were almost the only available nesting substrate. Parasitism rates also tended to increase as the distance to conspecific nests increased, possibly due to cooperative mobbing and parental defense by marshbirds. The proportion of nests discovered by cowbirds was higher for active (95%) than for experimental (29%) nests, suggesting that cowbirds used host parental activity to locate nests. Despite active nest defense, parental activity did not affect either predation rates or nest-survival time. Thus, although nest defense by Brown-and-yellow Marshbirds appears to be based on cooperative group defense, such behavior did not reduce the impact of brood parasites and predators. RESUMEN La seleccion del sitio de nidificacion y defensa del nido son estrategias que reducen los costos del parasitismo de cria y predacion de nidos, dos fuerzas selectivas que pueden afectar el exito reproductivo y la eficacia biologica de las aves. Durante 2001–2002, analizamos el efecto de las caracteristicas del sitio de nidificacion, patron de nidificacion, y actividad parental en la predacion de nidos y parasitismo de cria por tordos (Molothrus spp.) en una poblacion de Pseudoleistes virescens en la provincia de Buenos Aires, Argentina. Examinamos los posibles efectos de la detectabilidad, accesibilidad y defensa del nido en las tasas de parasitismo y depredacion de nidos. Asimismo, se compararon las tasas de parasitismo, depredacion, y tiempo de supervivencia de los nidos entre los estadios de huevo (nidos activos) con los de los mismos nidos cebados artificialmente con huevos de passerinos (nidos experimentales). La mayoria de los nidos (45 de 48, o 94%) hallados durante los estadios de construccion y puesta fueron parasitados, donde el 79% de los nidos sufrieron al menos un evento de predacion de huevos. Los tordos fueron responsables de la mayoria de los eventos de predacion de huevos, donde 82 de 107 (77%) eventos de predacion correspondieron a huevos picados por tordos. Los nidos construidos en cardos tuvieron tasas de predacion y parasitismo mas altas que las correspondientes a nidos construidos en otros sustratos, probablemente debido a que los tordos exhiben una mayor actividad en el area donde los cardos son el unico sustrato de nidificacion disponible. La tasa de parasitismo mostro una tendencia a crecer con la distancia al nido mas proximo, probablemente debido a la defensa parental y los despliegues de defensa comunales de Pseudoleistes virescens. La proporcion de nidos descubiertos por tordos fue mayor para nidos activos (95%) que para nidos experimentales (29%), lo cual sugiere que los tordos utilizaron la actividad parental para detectar los nidos. A pesar de la activa defensa del nido, descubrimos que la actividad parental no afecto ni la tasa de predacion ni el tiempo de supervivencia de los nidos. De esta forma, a pesar de que la defensa del nido en Pseudoleistes virescens aparenta estar basada en la defensa cooperativa grupal, tal comportamiento no redujo el impacto de los parasitos de cria o predadores de nidos.

Parasitic cowbirds may defeat host defense by causing rejecters to misimprint on cowbird eggs

Experiments suggest that some hosts that reject parasitic brown-headed cowbird (Molothrus ater) eggs develop egg recognition by imprinting on their own eggs, probably at the first nest naive birds attend. This raises the possibility that parasitism of naive individuals might cause them to mistakenly misimprint on, and accept, cowbird eggs. In an egg-learning experiment designed to cause misimprinting, hosts accepted cowbird eggs when they had laid only 1 egg even though most rejected cowbird eggs when nests had � 2 host eggs. These findings show that cowbirds can cause misimprinting by parasitizing naive hosts. If done oppor- tunistically, misimprinting should become more likely as cowbird abundance increases. In accord with this expectation, simulated brood parasitism showed that 3 host species that usually reject cowbird eggs were more tolerant of cowbird eggs in the Great Plains, where cowbirds reach maximal historic abundance, than in eastern North America where cowbirds are less common. This relation between parasite abundance and host responses is opposite to that found in cuckoo hosts. In addition, these 3 rejecter species had high rates (10.8-30.4%) of natural cowbird parasitism at an Illinois site where cowbirds are extremely, and histor- ically, abundant. These rejecters accepted most cases of researcher-detected parasitism, even though they rarely accept where cowbirds are less common. Despite the potentially high cost resulting from egg rejection, we demonstrate that parasitism of ''rejecters'' can be adaptive for cowbirds particularly when host egg recognition involves learning. Key words: brood parasitism, coevolution, cowbird, egg recognition, imprint, misimprint, Molothrus ater, parasite-host interactions. (Behav Ecol)

Influence of Trees in the Landscape on Parasitism Rates of Grassland Passerine Nests in Southeastern North Dakota

Abstract. Woody vegetation has been linked to increased rates of Brown-headed Cowbird (Molothrus ater) parasitism for some grassland hosts. In northern North Dakota, however, studies reported that parasitism of grassland passerine nests was lower in landscapes with trees than in those without trees. We looked for evidence of this pattern elsewhere, using data from two studies conducted on the Sheyenne National Grassland in southeastern North Dakota. Specifically, we examined the probability of parasitism relative to percent tree cover within 2 km of a nest. We found a negative relationship for grassland passerine nests of all species tested. Our results support the suggestion that cowbirds are less likely to parasitize nests of grassland passerines where tree cover on the landscape is greater. This pattern could be explained by cowbirds switching to alternative hosts in woodlands, but this hypothesis needs further testing.

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...

The cumulative effects of resource development on biodiversity and ecological integrity in the Peace-Moberly region of Northeast British Columbia, Canada

Rapid acceleration of industrial development in north-eastern British Columbia is currently occurring without a comprehensive assessment of the effects it will have on ecological or cultural systems. The cumulative effects of past development are already being felt within the Treaty 8 First Nations of the region but these effects have not been quantified from a landscape point of view. Using habitat modeling and GIS analysis the effects of 35 years of agricultural and industrial development on forest biodiversity and ecological integrity were investigated for a 410,000 ha landscape in north-eastern British Columbia. The study identified a significant increase in area impacted by developments which lead to a shift in landscape structure and significant change in forest biodiversity. Forest fragmentation and habitat loss has resulted in an increase in open, early seral and edge habitats at the expense of contiguous mature forests. The change in landscape structure resulted in a significant increase in species richness as generalist and early seral species responded positively to increases in these habitats. A significant increase in brown-headed cowbird parasitism risk was also detected. Changes in landscape structure, reduction in habitat for 22% of modeled species, increase parasitism and predation risk due to fragmentation, and increased access have resulted in a cumulative effect of recent resource development on ecological integrity that is both additive and synergistic. The propose acceleration of future development will increase the risk to maintaining the biodiversity and ecological integrity in the Peace-Moberly region of British Columbia.

Eggshell spotting in brood parasitic shiny cowbirds (Molothrus bonariensis) is not linked to the female sex chromosome

For avian brood parasites in which individual females are host-specialists, the arms race between hosts and parasites has favored egg color polymorphism in the parasite, with female lineages laying mimetic eggs that resemble those of the host species they parasitize. Female sex-linked inheritance of egg color fosters evolutionary stability of egg polymorphism if female lineages show both consistent eggshell color and host use. This co-evolutionary relationship is unlikely to occur if individual brood parasites use different hosts or if egg color is not maternally inherited. The shiny cowbird (Molothrus bonariensis) is an extreme generalist brood parasite that shows a very high degree of egg polymorphism. We tested whether egg spotting in this species has female sex-linked inheritance. If genetic factors controlling the expression of egg spotting were present on the female-specific W chromosome, we expected co-segregation between spotting patterns and mtDNA haplotypes, as both W and mtDNA are maternally inherited. In contrast to the known maternal inheritance of spotting patterns in great tits, we found no associations between eggshell spotting and mtDNA haplotypes, which suggests that eggshell spotting is not maternally inherited in this cowbird species.

EGG-LAYING BEHAVIOR IN SCREAMING COWBIRDS: WHY DOES A SPECIALIST BROOD PARASITE WASTE SO MANY EGGS?

Abstract Obligate brood parasites should synchronize parasitism with host laying to maximize egg hatchability and chick survival. While the generalist Shiny (Molothrus bonariensis), Brown-headed (M. ater), and Bronzed (M. aeneus) Cowbirds frequently synchronize parasitism with host laying, specialist Screaming Cowbirds (M. rufoaxillaris) very often fail to do so in nests of their main host, the Bay-winged Cowbird (Agelaioides badius). The poor synchronization of Screaming Cowbird parasitism may be the result of low availability of host nests at the time of laying, higher nest attentiveness by the host during laying, or unpredictable host laying behavior. We used a large set of observational data to test these hypotheses. The rate of Screaming Cowbird parasitism occurring during host prelaying was 31%, while during laying and incubation, it was 50% and 19%, respectively. Synchronization of parasitism was not associated with availability of host nests at laying or with changes in host nest attentiveness thr...

UV reflectance of eggs of brown-headed cowbirds (Molothrus ater) and accepter and rejecter hosts

Many hosts of obligate brood parasites accept parasitic eggs despite the high costs of parasitism. Acceptance is particularly perplexing in brown-headed cowbird (Molothrus ater) (hereafter “cowbird”) hosts because the eggs of cowbirds and most hosts do not appear to match closely in visual characteristics detectable by humans. However, recent evidence suggests that parasite and host eggs may match in their ultraviolet (UV) reflectance, undetectable by humans, and that birds may use UV signals for egg discrimination. We determined whether egg colour matching in UV reflectance separates accepters and rejecters of cowbird parasitism by comparing the total UV (300–400 nm) reflectance of the eggs of 11 host species to cowbird eggs. Eggs of three of five accepter species and five of five rejecter species differed significantly from cowbird eggs in UV reflectance. We found no significant difference in the UV reflectance of the eggs of three closely related pairs of accepter and rejecter species. There also was no significant difference in the UV reflectance of cowbird eggs laid in nests of five host species, and the UV reflectance of cowbird eggs was not significantly correlated with that of host clutches. Thus, we found no support for the UV-matching hypothesis in brown-headed cowbirds and UV reflectance does not appear to separate accepters and rejecters of parasitism. Differences in UV reflectance between cowbird and host eggs, however, provide potential cues for use in egg discrimination. Experimental testing is needed to determine the relative importance of UV reflectance compared to other visual cues.

Sex differences in the effects of captivity on hippocampus size in brown-headed cowbirds (Molothrus ater obscurus).

In brood parasitic cowbirds, hippocampus (Hp) size is correlated with environmental spatial memory demands. Searching for host nests is the presumed causal factor influencing cowbird Hp size, because Hp volumes vary across species, sexes, and seasons according to nest-searching participation. Brown-headed cowbirds have female-only nest searching and, at least in the eastern subspecies, a larger Hp in females than in males, suggesting that nest searching influences cowbird Hp size. We predicted that female brown-headed cowbirds housed in aviaries lacking host nests would have a smaller Hp than wild-caught females whereas males would be unaffected. We found that the Hp was smaller in captive females, but not males, compared to their wild-caught counterparts. This did not appear to be due to general effects of an impoverished environment on all brain regions. Our results imply that interruption of nest searching in cowbirds prevents seasonal increase in Hp size in females. Future studies should isolate which behavioral differences between wild and captive birds contributed to captivity-induced changes in Hp volume in females while not affecting males.

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.

Molothrus ater (Brown-headed Cowbird) Lays Egg in Artificial Nest in Highlands County, Florida

Abstract On April 28, 2001, we found an unidentified egg in an artificial nest in Highlands County, FL. The artificial nests used in this study contained one Coturnix japonica (quail) egg and one tethered clay sham egg. The unidentified egg was added to the nest between days 12 and 18 of exposure. The quail egg was unmarked, but the sham was covered in small beak markings suggesting that a bird had manipulated it. Molothrus ater (Brown-headed Cowbirds) and M. bonariensis (Shiny Cowbirds) occur in the area, but Brown-headeds are far more common; however, we never observed either species near this particular artificial nest. An mtDNA sequence (300 base pairs of cytochrome b) amplified from this egg was compared to DNA sequences from GenBank, and we found a nearly perfect sequence match with the Brown-headed Cowbird. Parasitism by these birds in unattended and artificial nests is unusual, especially when host activity is not mimicked by researchers. This is the first record of Brown-headed Cowbird parasitism...

Brown-headed Cowbird, <em>Molothrus ater</em>, Parasitism and Abundance in the Northern Great Plains

The Brown-headed Cowbird (Molothrus ater) reaches its highest abundance in the northern Great Plains, but much of our understanding of cowbird ecology and host-parasite interactions comes from areas outside of this region. We examine cowbird brood parasitism and densities during two studies of breeding birds in the northern Great Plains during 1990–2006. We found 2649 active nests of 75 species, including 746 nonpasserine nests and 1902 passerine nests. Overall, <1% of nonpasserine nests and 25% of passerine nests were parasitized by Brown-headed Cowbirds. Although the overall frequency of cowbird parasitism in passerine nests in these two studies is considered moderate, the frequency of multiple parasitism among parasitized nests was heavy (nearly 50%). The mean number of cowbird eggs per parasitized passerine nest was 1.9 ± 1.2 (SD; range = 1–8 cowbird eggs). The parasitism rates were 9.5% for passerines that typically nest in habitats characterized by woody vegetation, 16.4% for grassland-nesting passerines, 4.7% for passerines known to consistently eject cowbird eggs, and 28.2% for passerines that usually accept cowbird eggs. The Red-winged Blackbird (Agelaius phoeniceus) was the most commonly parasitized species (43.1% parasitism, 49.6% multiple parasitism, 71.2% of all cases of parasitism). Passerine nests found within areas of higher female cowbird abundance experienced higher frequencies of cowbird parasitism than those found in areas of lower female cowbird abundance. Densities of female cowbirds were positively related to densities and richness of other birds in the breeding bird community.

First Record of Bronzed Cowbird (Molothrus aeneus) Parasitism of the Common Bush-tanager (Chlorospingus ophthalmicus)

I report the first record of Bronzed Cowbird (Molothrus aeneus) parasitism of the Common Bush-tanager (Chlorospingus ophthalmicus). This represents the 97th known host for this cowbird species and the 10th known host from the Family Thraupidae. This record is based on feeding behavior observations and vocalizations recorded in Xalapa, Veracruz, Mexico.

Behavioural Ecology: Niche Construction via Grooming and Extortion?

A recent study shows that brood parasitic cowbirds employ Mafia-like tactics to discourage rejection of their broods by a common host. This may be a new example of animals adaptively 'constructing' key features of their ecological niches.