Dan L. Reinking

Habitat Edge, Land Management, And Rates Of Brood Parasitism In Tallgrass Prairie

Bird populations in North Americas 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), Henslows 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.

Hierarchical cues in brood parasite nest selection

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.ZusammenfassungEs 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 signifikant unterstützt wurden, wenn die „Rand-Effekte“ oder die Nähe zu buschiger Vegetation als erstes berücksichtigt wurden. Die Wirtsspezies hingen signifikant mit der Parasitismus-Rate zusammen, aber die Wirtsspezies selbst korrelierten mit diversen, ebenfalls untersuchten Vegetations-und Landschafts-Variablen. „Brutparasitismus-Rate“ und „Nesthöhe“ hingen nicht-linear voneinander ab. Zusätzlich zu einem klaren hierarchischen Muster derjenigen Faktoren, die mit der Brutparasitismus-Rate verknüpft sind, stellten sich einige Schlüsselfaktoren als miteinander verknüpfte Variablen heraus, wie z.B. die Lage an Prärie-Rändern und die Ausprägung buschiger Vegetation, oder Nesthöhe und -lage. Solche Zusammenhänge, inklusive der zwischen Wirtsspezies und bestimmten Landschafts- und Vegetations-Variablen wie der Nesthöhe und der Entfernung zu den nächsten Gebüschen, raten bei der Untersuchung der verschiedenen miteinander konkurrierenden, sich nicht gegenseitig ausschließenden Hypothesen zur Vorsicht.

Pallid bands in feathers and associated stable isotope signatures reveal effects of severe weather stressors on fledgling sparrows.

In August 2013, we observed a high incidence (44%) of synchronous bands of reduced melanin (a type of fault bar we have termed “pallid bands”) across the rectrices of juvenile Grasshopper Sparrows (Ammodrammus savannarum) captured near El Reno, Oklahoma. Earlier that year, on May 31, the site was struck by a severe storm which rained hailstones exceeding 5.5 cm diameter and spawned an historic 4.2 km-wide tornado <8 km to the south of the site. We hypothesized that this stressor had induced the pallid bands. An assessment of Grasshopper Sparrow nesting phenology indicated that a large number of nestlings were likely growing tail feathers when the storm hit. The pallid bands were restricted to the distal half of feathers and their widths significantly increased as a function of distance from the tip (i.e., age at formation). We predicted that if stress had caused these pallid bands, then a spike in circulating δ15N originating from tissue catabolism during the stress response would have been incorporated into the developing feather. From 18 juveniles captured at the site in August we measured δ15N and δ13C stable isotope ratios within four to five 0.25–0.40 mg feather sections taken from the distal end of a tail feather; the pallid band, if present, was contained within only one section. After accounting for individual and across-section variation, we found support for our prediction that feather sections containing or located immediately proximal to pallid bands (i.e., the pallid band region) would show significantly higher δ15N than sections outside this region. In contrast, the feathers of juveniles with pallid bands compared to normal appearing juveniles showed significantly lower δ15N. A likely explanation is that the latter individuals hatched after the May 31 storm and had consumed a trophically-shifted diet relative to juveniles with pallid bands. Considering this, the juveniles of normal appearance were significantly less abundant within our sample relative to expectations from past cohorts (z = − 2.03; p = 0.042) and, in as much, suggested widespread nest losses during the storm. Severe weather events may represent major stressors to ground-nesting birds, especially for recent fledglings. We call for others to exploit opportunities to study the effects of severe weather when these rare but devastating stressors impact established field research sites.