Grassland fragmentation and habitat loss are hypothesized to be contributing to widespread grassland bird declines in North America due to the adverse effects of fragmentation on breeding bird abundance and reproductive success. To assess the effects of fragmentation on the reproductive success of grassland birds, we measured rates of nest predation and brood parasitism for four species of birds ( Grasshopper Sparrow [Ammodramus savannaru], Henslow's Sparrow[Ammodramus henslowii], Eastern Meadowlark [ Sturnella magna], and Dickcissel [ Spiza Americana] ) in 39 prairie fragments ranging from 24 to>40,000 ha in size in five states in the mid‐continental United States. Throughout the region, nest‐predation rates were significantly influenced by habitat fragmentation. Nest predation was highest in small (<100 ha ) and lowest in large ( >1000 ha ) prairie fragments. Rates of brood parasitism by Brown‐headed Cowbirds (   Molothrus ater ), however, were not consistently related to fragment size and instead were more strongly related to regional cowbird abundance, being significantly higher in regions with high cowbird abundance. Differences in nest‐predation rates between large fragments ( 54–68% of all nests lost to predators ) and small fragments ( 78–84% lost to predators ) suggest that fragmentation of prairie habitats may be contributing to regional declines of grassland birds. Maintaining grassland bird populations, therefore, may require protection and restoration of large prairie areas.

2005. Ecological traps in isodars: effects of tallgrass prairie management on bird nest success. Á/ Oikos 111: 159 Á/169.Ecological traps occur when habitat selection and habitat suitability (measured in terms of fitness) are decoupled. We developed a graphical model based on isodar theory to distinguish between an ideal distribution and an ecological trap. We tested the model's predictions using data on breeding bird populations in managed tallgrass prairie in Oklahoma. Between 1992 and 1996 we monitored success for 2600 nests of 26 breeding species in undisturbed, grazed, and burned and grazed plots. We also sampled arthropod biomass and nest predator abundance. Using the isodar model we determined that managed plots are ecological traps: compared with success on plots left undisturbed, nest success on plots that were only grazed was lower, and success on plots that were burned and grazed was substantially lower. Yet birds preferred to nest on managed plots, where arthropod abundance was measurably higher. Reptiles were the most abundant taxon of nest predators, and their abundance was highest in managed plots. Consequently, tree-nesting species had higher nest success than shrub-and ground-nesting birds. Nest success also increased with tree height. We concluded that isodar theory is a useful tool for detecting ecological traps if any component of fitness is measured in addition to animal densities. Our study also suggests that (1) human modification of the environment may alter simultaneously food and predator abundance, (2) the former affects nest site selection and the latter nest success, and (3) such ecosystems are likely to become traps for breeding birds.

Bird populations in North America's grasslands have declined sharply in recent decades. These declines are traceable, in large part, to habitat loss, but management of tallgrass prairie also has an impact. An indirect source of decline potentially associated with management is brood parasitism by the Brown-headed Cowbird (Molothrus ater), which has had substantial negative impacts on many passerine hosts. Using a novel application of regression trees, we analyzed an extensive five-year set of nest data to test how management of tallgrass prairie affected rates of brood parasitism. We examined seven landscape features that may have been associated with parasitism: presence of edge, burning, or grazing, and distance of the nest from woody vegetation, water, roads, or fences. All five grassland passerines that we included in the analyses exhibited evidence of an edge effect: the Grasshopper Sparrow (Ammodramus savannarum), Henslow's Sparrow (A. henslowii), Dickcissel (Spiza americana), Red-winged Blackbird (Agelaius phoeniceus), and Eastern Meadowlark (Sturnella magna). The edge was represented by narrow strips of woody vegetation occurring along roadsides cut through tallgrass prairie. The sparrows avoided nesting along these woody edges, whereas the other three species experienced significantly higher (1.9-5.3x) rates of parasitism along edges than in prairie. The edge effect could be related directly to increase in parasitism rate with decreased distance from woody vegetation. After accounting for edge effect in these three species, we found evidence for significantly higher (2.5-10.5x) rates of parasitism in grazed plots, particularly those burned in spring to increase forage, than in undisturbed prairie. Regression tree analysis proved to be an important tool for hierarchically parsing various landscape features that affect parasitism rates. We conclude that, on the Great Plains, rates of brood parasitism are strongly associated with relatively recent road cuts, in that edge effects manifest themselves through the presence of trees, a novel habitat component in much of the tallgrass prairie. Grazing is also a key associate of increased parasitism. Areas managed with prescribed fire, used frequently to increase forage for grazing cattle, may experience higher rates of brood parasitism. Regardless, removing trees and shrubs along roadsides and refraining from planting them along new roads may benefit grassland birds.

Numerous hypotheses have been developed to explain how a brood parasite selects a host nest into which it lays its eggs. Most hypotheses address various aspects of nest placement. We used an extensive dataset to tease apart the relative strength of various hypotheses associated with nest placement and timing. These data were from North American tallgrass prairie and included nearly 2,000 nests of 17 host species known to accept eggs of the brood parasitic Brown-headed cowbird Molothrus ater. Regression tree analyses, with host species as a categorical covariate built around successive logistic regressions, implied that the ''edge effect'' and ''perch proximity'' hypotheses, the latter assessed as distance to woody vegetation, received the strongest support. Hypotheses concerning nest height, livestock proximity, habitat density, nest exposure, laying date, and host clutch size received weaker or subsidiary support, the latter meaning that the hypotheses received significant support only after edge effects or distance to woody vegetation were accounted for first. Host species was associated significantly with parasitism rate, but host species was itself correlated with various vegetation and landscape variables that we assessed. Brood parasitism rate and nest height were associated non-linearly. In addition to a clear hierarchical pattern among factors associated with rates of parasitism, several key explanatory variables had marked interactions, such as prairie edge and extent of woody vegetation or nest height and nest exposure. Such interactions, including between host species and certain landscape and vegetation variables, such as nest height and distance from woody vegetation, suggest caution is warranted when assessing the various competing hypotheses, which are not mutually exclusive. Keywords Brood parasitism Á Edge effects Á Molothrus ater Á Nest placement Á Regression trees Zusammenfassung Es gibt zahlreiche Hypothesen zur Erklärung, wie ein Brutparasit ein Nest für die eigene Eiablage aussucht, wobei sich die meisten dieser Hypothesen an der spezifischen Lage der Nester ausrichten. Wir benutzten eine umfangreiche Datensammlung, um die relative Stärke derjenigen Hypothesen zu entwirren, die sich auf Nestlage und Eiablage-Zeitpunkt beziehen. Die Daten stammten aus der nordamerikanischen Tallgrass-Prärie, von fast 2000 Nestern 17 unterschiedlicher Wirtsarten, von denen bekannt ist, daß sie Eier des Brutparasiten Braunkopf-Kuhstärling (Molothrus ater) in ihren Nestern akzeptieren. Regressionsbaumanalysen mit den Wirtsspezies als kategorialen Kovariaten, um sukzessive logistische Regressionen herum gerechnet, unterstützten am stärksten die Hypothesen zum,, Rand-Effekt'' und zur ,,Sitzplätze-Nähe'' (Entfernung zu Gebüsch-Vegetation). Andere Hypothesen, die die Nesthöhe, die Nähe zu Vieh, die Habitatdichte, die Nestlage, oder den Zeitpunkt der Eiablage und die Gelegegröße des Wirtes heranziehen, wurden schwächer oder nur ,,subsidiär'' unterstützt -wobei letzteres bedeutet, dass diese Hypothesen nur dann signif...

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