Cowbird Abundance Research Articles

Long-term decline in Brown-headed Cowbird (Molothrus ater) abundance has not led to less brood parasitism of four riparian songbird species

Long-term decline in Brown-headed Cowbird (Molothrus ater) abundance has not led to less brood parasitism of four riparian songbird species

Collocating pipelines to minimize fragmentation: evaluating ecological costs of a shale gas mitigation practice

AbstractShale gas development occurs in forests of the Appalachian Basin within breeding habitat for forest songbirds. Development requires linear infrastructure (e.g., pipelines, gas access roads) that fragments habitat and reduces core forest. Collocation is a mitigation practice that sites new pipelines adjacent to existing surface disturbance such as forest roads; it reduces core forest loss but may have associated ecological costs, defined as negative effects on native species and ecosystems. We conducted a paired sampling design between forest roads and collocated pipelines (expanded gas access roads collocated with pipelines) to evaluate ecological costs to forest songbirds in 2013 in Pennsylvania, USA. We surveyed 4 focal songbird species: 3 territorial species that varied in habitat requirements and the non‐territorial brown‐headed cowbird (Molothrus ater), an obligate brood parasite. We used spot mapping to survey focal species within linear corridors and the adjacent mature forest. Territory density of forest interior ovenbirds (Seiurus aurocapilla) was significantly lower on collocated pipelines (5.1 fewer territories per 10 ha) compared to forest road sites. We found no effect of collocation on territory density for the early successional species, eastern towhee (Pipilo erythrophthalmus) and chestnut‐sided warbler (Setophaga pensylvanica). Territories of all 3 territorial focal species crossed collocated pipeline sites less frequently than forest roads (ovenbird: 16%, eastern towhee: 14%, chestnut‐sided warbler: 31%) and the barrier effect increased with increasing corridor width. In contrast, brown‐headed cowbird abundance was 15 times greater at collocated pipelines compared to forest roads, suggesting that wider gas corridors provide enhanced access routes for cowbirds. Our study indicates the expansion of forest roads to collocated pipelines exacerbates the negative ecological effects already present with the existing road including increased edge avoidance by a forest interior species, greater barrier effects for all 3 territorial forest songbirds, and increased access for brown‐headed cowbirds into core forest. We support collocation as a mitigation strategy but emphasize restricting overall corridor width to reduce the additional ecological costs associated with this practice.

Spatially structured brown-headed cowbird control measures and their effects on Kirtland’s warbler long-term population sustainability

Context Brown-headed cowbirds (Molothrus ater), through brood parasitism, can exert extrinsic population growth pressures on North American songbirds. Cowbird removal programs may reduce parasitism rates on host species but can be expensive and difficult to implement throughout a host species’ breeding range. Aim We estimated cowbird abundance and nest parasitism rates within Kirtland’s warbler (Setophaga kirtlandii) primary breeding range in Michigan, USA, and determined the maximum sustainable parasitism rate for Kirtland’s warblers under several spatially structured cowbird removal designs. Methods We conducted point counts to estimate cowbird abundance and monitored nests to quantify nest parasitism rates during 2019–2021. We used the modelling software STELLA to determine the maximum sustainable parasitism rate for Kirtland’s warblers under different spatially structured cowbird removal scenarios (complete, core-only, and no removal). Key results Cowbird abundance and parasitism rates remained low following cowbird trap closures in 2018. In the simulation study, complete removal was the most robust scenario with no replications having <1000 Kirtland’s warbler males. The core-only removal scenario had a substantially higher sustainable parasitism rate in the peripheral breeding area than the no removal scenario. Assumed hatch-year dispersal distance had the greatest impact on the maximum sustainable parasitism rate in the core-only scenario. Conclusions Low cowbird abundance and nest parasitism following suspension of cowbird removal efforts showed resuming the removal program may not be required in the short-term. If cowbird abundance increases, however, adaptive cowbird removal programs can be used to sustain Kirtland’s warbler populations long-term. Implications Our results indicate that incorporating spatial structure of host species’ habitat into designing cowbird removal programs may minimise costs of cowbird management while sustaining populations of Kirtland’s warbler and possibly other host species that are affected by brood parasitism.

Brood parasitism leads to zero recruitment in the globally endangered Yellow CardinalGubernatrix cristata

SummaryThe Yellow CardinalGubernatrix cristatais an ‘Endangered’ passerine from southern South America. For the past three years a management plan for Yellow Cardinals has been implemented in Argentina, in which rescued individuals from the illegal cage bird trade were released back into suitable habitats within their population of origin. We studied the reproductive success of a mixed population of released and wild Yellow Cardinals in La Pampa province, Argentina, during the reproductive season of 2019. The population was highly parasitized by the brood parasitic Shiny CowbirdMolothrus bonariensis. The frequency of parasitism was 100% and the intensity of parasitism was 4.5 ± 3.4 (mean ± SD) eggs per parasitized nest (range 1–13). No Yellow Cardinal chicks were recruited in the monitored nests, mainly as a consequence of Shiny Cowbird parasitism. The unusually high rate of parasitism led to a poor outcome of the reintroduction programme and indicates the need to update the conservation actions that have been carried out for the Yellow Cardinal so far. Shiny Cowbird abundance varies within the distribution of Yellow Cardinals, related to habitat modification and farming activities. Thus, further research on habitat suitability and assessment of Shiny Cowbird abundance should be incorporated into future strategies for the conservation of the Yellow Cardinal.

Egg rejection behavior does not explain the lack of cowbird parasitism on an eastern North American population of Red-winged Blackbirds

BackgroundRed-winged Blackbirds (Agelaius phoeniceus), hereafter red-wings, are much less frequently parasitized by Brown-headed Cowbirds (Molothrus ater) in eastern North America than in central North America and had not been recorded as hosts in our study area in southeastern Pennsylvania. Although hosts of Old World cuckoos (Cuculidae) often show geographic variation in egg rejection behavior, cowbird hosts typically exhibit uniform responses of all acceptance or all rejection of cowbird eggs. Thus, geographic variation in cowbird parasitism frequencies might reflect a different behavioral response to parasitism by hosts where only some populations reject parasitism. In this study, we tested whether egg rejection behavior may explain the lack of parasitism observed in our eastern red-wing population, which may provide insight into low parasitism levels across eastern North America.MethodsWe parasitized red-wing nests with model cowbird eggs to determine their response to parasitism. Nests were tested across three nest stages and compared to control nests with no manipulations. Because rejection differed significantly by stage, we compared responses separately for each nest stage. We also monitored other songbird nests to identify parasitism frequencies on all potential hosts.ResultsRed-wings showed significantly more rejections during the building stage, but not for the laying and incubation stages. Rejections during nest building involved mostly egg burials, which likely represent a continuation of the nest building process rather than true rejection of the cowbird egg. Excluding these responses, red-wings rejected 15% of cowbird eggs, which is similar to rejection levels from other studies and populations. The overall parasitism frequency on 11 species surveyed in our study area was only 7.4%.ConclusionsEgg rejection behavior does not explain the lack of parasitism on red-wings in our eastern population. Alternatively, we suggest that cowbird preference for other hosts and the low abundance of cowbirds in the east might explain the lack of parasitism. Future research should also explore cowbird and host density and the makeup of the host community to explain the low levels of parasitism on red-wings across eastern North America because egg rejection alone is unlikely to explain this broad geographic trend.

Reducing the conservation reliance of the endangered Kirtland's warbler through adaptive management

ABSTRACTSpecies are considered conservation‐reliant when their continued existence is dependent on human assistance. Conservation reliance challenges the conservation community in terms of their ability to sustain the funding and public‐private partnerships needed for indefinite management. Although increased funding for conservation is critical, reducing conservation reliance through adaptive management represents an attractive alternative. We used a large‐scale ecological experiment as a case study in the use of adaptive management to reduce conservation reliance. For >40 years, the United States Fish and Wildlife Service has trapped and lethally removed an obligate brood parasite, the brown‐headed cowbird (Molothrus ater), to protect the Kirtland's warbler (Setophaga kirtlandii) from the negative effects that brood parasitism has on its reproductive success. To determine if the conservation reliance of the Kirtland's warbler could be reduced through optimization of the cowbird control program, we used an adaptive management approach. In collaboration with stakeholders, we experimentally reduced cowbird trapping effort across nearly all of the Kirtland's warbler breeding range. We monitored the resulting cowbird abundance and rate of parasitism, and then adjusted the scale of trap reductions based on the previous year's results. Despite reducing (2015–2017) and eventually eliminating (2018) cowbird trapping, we detected only 20 cowbirds (2015–2017) and found that just 4 of 514 (<1%) nests were parasitized (2015–2018). Our results indicate that the cowbird control program can at least temporarily be suspended, thereby reducing conservation reliance in the Kirtland's warbler and freeing funds for other management. We urge the conservation community to consider the use of adaptive management to reduce conservation reliance in other threatened and endangered taxa. © 2019 The Wildlife Society.

Oil and natural gas development has mixed effects on the density and reproductive success of grassland songbirds

Oil and natural gas development has increased dramatically in native grasslands over the past 25 years. Some grassland songbirds are less abundant in areas with oil and gas development, but the effects vary among species and geographically within a species' range. The reproductive consequences of nesting in areas with oil and gas development are unknown. We assessed how the density and reproductive success of five species of grassland songbird in Alberta, Canada, varied with distance to oil and gas wells, gravel roads, and trails, and cover of crested wheatgrass (Agropyron cristatum), an aggressive alien plant that often becomes established following anthropogenic disturbance. Crested wheatgrass cover had the greatest impact on the grassland songbird community. Sprague's Pipit (Anthus spragueii) nest survival decreased as the amount of crested wheatgrass increased. As crested wheatgrass cover increased from 0% to 60%, density of Savannah Sparrows (Passerculus sandwichensis) declined by 50%, but they fledged 25% more young from successful nests. Density of Savannah Sparrows was twice as high near wells, and fledging success was 40% higher compared with 700 m away. Distance to gravel roads did not influence the density or reproductive success of any species. Sprague's Pipits and Baird's Sparrows (Ammodramus bairdii) avoided nesting within 100 m of trails, and both species fledged fewer young from successful nests near trails. In contrast, Vesper Sparrows (Pooecetes gramineus) nested close to trails and fledged more young from successful nests near trails. Western Meadowlarks (Sturnella neglecta) were not strongly affected by any variable. Brown-headed Cowbird (Molothrus ater) abundance was three times higher in study plots with wells, although we detected no associated increase in brood parasitism. Our results indicate that the introduction and spread of crested wheatgrass and the creation of access trails to well pads have negative reproductive consequences for primary endemic species such as Sprague's Pipit and Baird's Sparrow, although these results do not extend to other grassland birds. The spread of crested wheatgrass and the disturbance of access trails could be reduced by directional drilling of multiple wells from a single well pad.

Dynamic spatial interactions between the native invader Brown‐headed Cowbird and its hosts

AbstractAimWe investigated the contribution of parasitism by the Brown‐headed Cowbird (Molothrus ater) to its host population dynamics at a subcontinental scale. We predicted that the growth rate of cowbirds would be most strongly related to the abundance of forest insectivorous hosts and that landscape fragmentation would have opposite effects on the parasite and its hosts. Furthermore, due to the species' sedentarity and low rates of adult dispersal, we expected that cowbird population dynamics would be structured at local, rather than large spatial scales.LocationThe Great Plains, United States.MethodsWe modelled jointly the population dynamics of cowbird and its 58 most common hosts, accounting for effects of landscape fragmentation and cowbird dispersal. We used Bayesian inference to evaluate this model on 76 routes of the North American Breeding Bird Survey (BBS), surveyed each year from 2001 to 2011. We then analysed covariations between the demographic dynamics of cowbirds and their hosts in a multivariate ecological trait space.ResultsContrary to most of its hosts, the cowbird had positive growth rates in man‐disturbed, fragmented landscapes, which covaried at small spatial scales. The large‐scale effects of parasitism were limited, as few host species affected, or were affected by cowbird growth rates. However, significant cowbird–host relationships were structured by ecological traits: cowbird growth rates were most increased by the abundance of ecological generalists, while hosts were affected by cowbird abundance irrespective of their position in the functional space.Main conclusionsCowbirds parasitize opportunistically the hosts occurring in disturbed landscapes. The large‐scale effects of nest parasitism therefore depend probably on landscape structure and other environmental factors operating at local scales. We suggest that efforts to counter the local and continental declines of many passerine species should include parasitism among the biotic components of landscape disturbance.

Assessing the influence of geography, land cover and host species on the local abundance of a generalist brood parasite, the brown‐headed cowbird

AbstractAimThe brown‐headed cowbird is an obligate brood parasite known to exploit a large number of host species and use a variety of habitats. Much attention has been directed towards uncovering the fundamental factors that affect cowbird abundance; however, no study has evaluated these factors in the context of a biogeographic‐scale analysis that takes into account spatial autocorrelation. Our primary objective was to compare the relative influence of geography, land cover and host species on the local abundance of cowbirds.LocationGreat Plains region of the USA.MethodsWe used data from the North American Breeding Bird Survey and the National Land Cover Database to examine the relationships between cowbird abundance and host species, land cover composition and geographic location of a survey route. Multiple regression models were developed for various combinations of these factors. To control for spatial autocorrelation, we used SAM 4.0 (Spatial Analysis in Macroecology) software to implement simultaneous autoregressive modelling of the error term. We then used a model comparison approach to identify the factors that most influence cowbird abundance.ResultsAmong all models examined, host species richness was the single most strong predictor and the sole statistically significant predictor. Cowbird abundance increased with host species richness but did not change in any significant way with non‐host passerine richness or abundance of host species. Models with land cover variables tended to have the poorest fit to the cowbird abundance data.Main conclusionsOur results suggest that cowbirds may be attracted to areas with greater host richness and/or recruit better in such areas, although our data did not allow direct examination of either process. In a greater context, our study demonstrates the utility of a spatially based and geographically extensive analysis in finding range‐wide factors that affect the local abundance of a species.

Broad-scale heterogeneity influences nest selection by Brown-headed Cowbirds

Increasing habitat heterogeneity is widely considered to improve conditions for biodiversity. Yet benefits for native species depend on scale and the effect of heterogeneity on key processes influencing survival and reproduction. We examined the relationship between habitat heterogeneity and brood parasitism at multiple scales in a region characterized by (1) relatively high cowbird abundance, (2) high abundance of our focal species, the grassland obligate Grasshopper Sparrow (Ammodramus savannarum), (3) variation in the structure and composition of grassland habitats, and (4) a gradient of woodland cover in the landscape matrix. Tree cover at broad scales was found to have the greatest impact on parasitism while factors at finer scales were relatively unimportant. We found that for every 1 % increase in tree cover within 1 km of Grasshopper Sparrow nests, the probability of parasitism decreases by 3 %. Parasitism reduced clutch sizes and the number of Grasshopper Sparrows fledged, but survival rates were similar between non-parasitized and parasitized nests. Furthermore, simple population projection models indicated that parasitism has the greatest impact at moderate survival levels and can inhibit the resiliency of this population. Our results support the hypothesis that cowbirds prefer forest hosts, which may reduce parasitism rates on grassland birds in heterogeneous landscapes. Collectively, our findings suggest that the effect of cowbird parasitism may be greater for Grasshopper Sparrows than was previously thought.

Brown-Headed Cowbird Parasitism on Endangered Species: Relationships with Neighboring Avian Species

Loss of reproductive productivity in hosts of the Brown-headed Cowbird (Molothrus ater), a nest parasite, may limit the success of conservation and recovery efforts that focus on restoring avian habitat. We investigated the relationships between parasitism frequency of 2 vireos—the endangered Black-capped Vireo (Vireo atricapillus) and the White-eyed Vireo (Vireo griseus)—and characteristics of the neighboring avian assemblage. Our objective was to identify assemblage characteristics that are useful for predicting parasitism frequency We hypothesized that parasitism frequency of the vireos would decrease with increasing alternate host abundance and increase with increasing abundance of cowbirds. All Black-capped Vireo nests (n = 20) and 81% of White-eyed Vireo nests (n = 27) were parasitized. Parasitism frequency of White-eyed Vireos was positively correlated with the number of cowbirds detected (r = 0.84, df = 4, P = 0.018). Parasitism frequency of White-eyed Vireos was negatively correlated with abundance of the Black-and-white Warbler (Mniotilta varia; r = 0.891, df = 4, P = 0.009) but was not correlated with abundance of other cowbird hosts. The relationship between Black-and-white Warbler abundance and parasitism frequency may be due to chance, given the small sample size and narrow range of parasitism frequencies, but the relationship may also reflect an interaction (requiring further investigation) between parasitism and the vegetation characteristics associated with Black-and-white Warblers. Our results imply that reducing cowbird abundance will reduce parasitism frequency of Black-capped Vireos. When the number of cowbirds in an area predicts parasitism, land managers would benefit from identifying a target cowbird abundance based on the levels of parasitism the host population can tolerate while still meeting productivity and population objectives for the species.

Factors Influencing the Frequency of Nest Parasitism by Brown-Headed Cowbirds in the Northern Sierra Nevada

Nest parasitism by Brown-headed Cowbirds (Molothrus ater) can negatively affect host populations. Landscape-scale factors, such as proximity to residential areas, equestrian riding stables, and grazing allotments, can affect the risk of nest parasitism as well as the abundance of Brown-headed Cowbirds, Recent increases in residential and recreational development along with a reduction in grazing allotments in the northern Sierra Nevada provide an opportunity to reevaluate factors that influence frequency of nest parasitism by Brown-headed Cowbirds, Frequency of nest parasitism increased as the distance to the nearest residential center and distance to the nearest equestrian riding stable decreased. Similarly, Brown-headed Cowbird abundance was negatively correlated with distance to the nearest residential center and distance to the nearest equestrian stable. To reduce the risk of nest parasitism, efforts should be devoted to (1) reducing further residential development at the wildland—urban interface and (2) educating landowners about bird feeders, which frequently attract Brown-headed Cowbirds.

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)

Robust predictions of species distribution: Spatial habitat models for a brood parasite

Empirical habitat selection models aid managers by predicting distribution patterns, but recent critiques urge a fuller application of available methods. We used a suite of habitat modelling techniques to identify factors influencing the distribution of brown-headed cowbirds ( Molothrus ater), a brood parasite capable of causing population declines in some host species. We used data on cowbird occurrence in the Southern Gulf Islands, Canada, to estimate the influence of landscape features on cowbird distribution beginning with a set of a priori models based on literature. Best models were also constructed using stepwise logistic and autologistic regression, and using only widely available data, and these models and their predictive maps were then evaluated. Cowbirds were predicted by proximity to potential feeding areas and landscape context. The best logistic model included landcover, cattle, and distance to urban area and agriculture. Model performance was improved by autologistic regression, and models restricted to widely available data were only slightly poorer. All models were robust to internal validation using bootstrapping and when compared to an independent empirical dataset of parasitism rates of a host on 12 sites monitored for up to 8 years. Our predictive maps for cowbirds should help land managers make spatial predictions about cowbird impact on particular hosts, as well as evaluate land-use decisions that could influence cowbird abundance and host fitness. We evaluated several approaches to habitat modelling of interest to conservationists and make suggestions for future studies when spatial predictions are likely to affect decisions in conservation and land use planning.

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.

Temporal fluctuation in abundance of Brown-headed Cowbirds and their hosts in riparian habitat in the Okanagan Valley, British Columbia, Canada

en

Conflicting Temporal Changes in the Frequency and Intensity of Cowbird Parasitism on Four Common Hosts in Ontario over 130 Years

AbstractWe tested the hypothesis proposed by Friedmann (1963) that multiple parasitism (nests with more than one parasitic egg) by Brown-headed Cowbirds (Molothrus ater) is a recent phenomenon associated with increased densities of cowbirds. We used Ontario Nest Records Scheme data to quantify frequencies of parasitism and multiple parasitism on four common host species over the last 130 years. Frequency of multiple parasitism significantly increased over all decades only in Chipping Sparrows (Spizella passerina). We also analyzed data separately before and after 1970 because at about this time cowbird abundance began to stabilize and then decline. Pre-1970, multiple parasitism significantly increased on Chipping Sparrows and Song Sparrows (Melospiza melodia), but neither had significant trends post-1970. There were no significant trends in multiple parasitism on Red-eyed Vireos (Vireo olivaceus) or Northern Cardinals (Cardinalis cardinalis). For all time periods analyzed, there were no significant trends in overall frequency of parasitism on any species. Our results provide limited support for the hypothesis that temporal variation in parasitism patterns has followed changes in cowbird density over the past century.

Conflicting Temporal Changes in the Frequency and Intensity of Cowbird Parasitism on Four Common Hosts in Ontario over 130 Years

Abstract We tested the hypothesis proposed by Friedmann (1963) that multiple parasitism (nests with more than one parasitic egg) by Brown-headed Cowbirds (Molothrus ater) is a recent phenomenon associated with increased densities of cowbirds. We used Ontario Nest Records Scheme data to quantify frequencies of parasitism and multiple parasitism on four common host species over the last 130 years. Frequency of multiple parasitism significantly increased over all decades only in Chipping Sparrows (Spizella passerina). We also analyzed data separately before and after 1970 because at about this time cowbird abundance began to stabilize and then decline. Pre-1970, multiple parasitism significantly increased on Chipping Sparrows and Song Sparrows (Melospiza melodia), but neither had significant trends post-1970. There were no significant trends in multiple parasitism on Red-eyed Vireos (Vireo olivaceus) or Northern Cardinals (Cardinalis cardinalis). For all time periods analyzed, there were no significant trend...

Geographic Variation in Brown-Headed Cowbird (Molothrus Ater) Parasitism on Dickcissels (Spiza Americana) in Great Plains Tallgrass Prairie

AbstractThe incidence of brood parasitism by Brown-headed Cowbirds (Molothrus ater; hereafter “cowbirds“) within host species typically reflects the continental pattern in cowbird abundance across North America, where parasitism is heaviest in the Great Plains. However, we found considerable variation in cowbird parasitism on Dickcissel (Spiza americana) nests within a subregion of the Great Plains (the Flint Hills), where the highest levels of cowbird parasitism on grassland bird nests had been previously reported. Local parasitism frequencies on Dickcissel nests varied latitudinally across the Flint Hills, ranging from 0% to 92% of nests parasitized. Interestingly, we found no obvious patterns in habitat or host attributes that were associated with this steep geographic gradient in brood parasitism. Cowbird parasitism on Dickcissel nests was not correlated with the vertical density of local prairie vegetation, mean nest distance to edge, proportion of forest to grassland habitat surrounding study sites (≤5 to 10 km), geographic variation in host abundance, or Dickcissel density or nest initiation dates. Parasitism frequencies and intensities (number of cowbird eggs per parasitized nest) on Dickissel nests were only significantly related to variation in local female cowbird density. Dickcissel clutch size and apparent fledging success were negatively correlated with local cowbird parasitism levels. Geographic patterns in cowbird abundance within and among regions should be considered when establishing conservation areas for grassland birds or other cowbird hosts of concern.

Cowbird (Molothrus spp.) Ecology: A Review of Factors Influencing Distribution and Abundance of Cowbirds across Spatial Scales

Cowbird (Molothrus spp.) Ecology: A Review of Factors Influencing Distribution and Abundance of Cowbirds across Spatial Scales