Hummingbirds, with their impressive flight ability and competitive aerial contests, make ideal candidates for applying a mechanistic approach to studying community structure. Because flight costs are influenced by abiotic factors that change systematically with altitude, elevational gradients provide natural experiments for hummingbird flight ecology. Prior attempts relied on wing disc loading (WDL) as a morphological surrogate for flight performance, but recent analyses indicate this variable does not influence either territorial behavior or competitive ability. Aerodynamic power, by contrast, can be derived from direct measurements of performance and, like WDL, declines across elevations. Here, I demonstrate for a diverse community of Andean hummingbirds that burst aerodynamic power is associated with territorial behavior. Along a second elevational gradient in Colorado, I tested for correlated changes in aerodynamic power and competitive ability in two territorial hummingbirds. This behavioral analysis revealed that short-winged Selasphorus rufus males are dominant over long-winged Selasphorus platycercus males at low elevations but that the roles are reversed at higher elevations. Several lines of evidence support the hypothesis that the burst rather than sustained aerodynamic performance mediates competitive ability at high elevation. A minimum value for burst power may be required for successful competition, but other maneuverability features gain importance when all competitors have sufficient muscle power, as occurs at low elevations.

The interobserver reliability of histopathological features in the placenta was examined. Two pathologists independently reviewed slides from 250 placentas. The pathologists were given a morphological description of the placenta, but were blinded to clinical status, gestational age and original diagnoses. A protocol for diagnosis and grading of features was first developed and pilot-tested. Definitions and criteria were refined and elaborated. A range of features was examined including inflammatory lesions, features indicative of reduced uterine blood flow and other miscellaneous histopathological changes. Weighted kappa coefficients were calculated. The effect of multiple features on reliability was examined by stratifying on the presence of a second feature and calculating stratum-specific kappa coefficients. Results indicated good to excellent agreement for diagnoses of chorioamnionitis, cord vasculitis, funisitis and villitis (kappa(w) range 0.70-0.83). Agreement between observers was more variable for the diagnosis of reduced uterine blood flow states. Excellent agreement was observed for the diagnosis of meconium staining of the placenta (kappaw = 0.79). In general, lower levels of agreement were observed for features in the presence of a second feature. Reproducible measures are a prerequisite to using placental histopathology for diagnostic and prognostic information. This study demonstrated reliable placental diagnoses can be achieved through a standardised protocol.

The phenomenon of herding is a very general feature of the collective behavior of many species in panic conditions, including humans. It has been predicted theoretically that panic-induced herding in individuals confined to a room can produce a nonsymmetrical use of two identical exit doors. Here we demonstrate the existence of that phenomenon in experiments, using ants as a model of pedestrians. We show that ants confined to a cell with two symmetrically located exits use both exits in approximately equal proportions to abandon it in normal conditions but prefer one of the exits if panic is created by adding a repellent fluid. In addition, we are able to reproduce the observed escape dynamics in detail using a modification of a previous theoretical model that includes herding associated with a panic parameter as a central ingredient. Our experimental results, combined with theoretical models, suggest that some features of the collective behavior of humans and ants can be quite similar when escaping under panic.

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