Robert C. Dobbs

SOURCES OF EGG-SIZE VARIATION IN HOUSE WRENS (TROGLODYTES AEDON): ONTOGENETIC AND ENVIRONMENTAL COMPONENTS

Abstract Evolutionary theory predicts that heritability of fitness-related traits should be low. Egg size in birds is a fitness-related trait, but exhibits high heritability. One possible explanation for the apparent failure of natural selection to exhaust variability is that variation in egg size is mediated by female condition. In this study, we estimated repeatability of egg size within and between successive clutches in a wild, double-brooded population of House Wrens (Troglodytes aedon) in central Illinois, and examined the relationship of egg size with female size and condition. Repeatability of egg volume and mass in individual birds was high within years and between successive years (>0.77), suggesting a substantial heritable component to variation in egg size. However, egg mass was also significantly correlated with female condition. We suggest that the high repeatability values largely reflect permanent but nongenetic (ontogenetic) variation in egg size among females, which is reflected in the positive correlation of egg size with female condition. If variation in egg size is attributable to a combination of nonheritable, ontogenetic variation and variation due to current environmental conditions among females, then selection may not produce an evolutionary response in egg size.

Variation in Foraging Behavior Among Nesting Stages of Female Red-Faced Warblers

Foraging rates and maneuvers were examined in breeding female Red-faced Warblers (Cardellina rubrifrons) among egg-laying, incubation, and nestling stages. All measures varied among nesting stages, with prey attack rate and search speed significantly increasing from egg-laying to incubation through the nestling stage. During egg-laying and incubation, birds gleaned stationary prey from a fixed perch, but shifted to hover-sallying for stationary prey during the nestling period. These dynamic behavioral patterns may reflect responses to variable time constraints and energetic costs associated with different stages of the nesting cycle.

THE NEST, NESTING BEHAVIOR, AND FORAGING ECOLOGY OF THE RUSTY-WINGED BARBTAIL (PREMNORNIS GUTTULIGERA)

Abstract We discovered and monitored a nest of the Rusty-winged Barbtail (Premnornis guttuligera) on the eastern slope of the Andes in Napo Province, Ecuador. The nest, in a hollow Cyathea tree-fern snag, was a deep cup composed entirely of Cyathea tree-fern petiole scales (ramenta). A single nestling fledged successfully on 13 March 2002. During the latter half of the nestling period, adults visited the nest with food a mean of 4.7 ± 1.9 (SD) times/h and removed fecal sacs 2.2 ± 1.1 times/h. Nest visitation rates generally decreased throughout the day. Adult Rusty-winged Barbtails foraged by gleaning from or probing into suspended dead leaves or moss, often while hanging onto the substrate, and did not hitch up trunks or creep along branches. Nest structure, composition, and location, and foraging behavior all raise doubts about the taxonomic placement of Premnornis in the Margarornis treerunner-barbtail assemblage.

FEMALE CONDITION: A PREDICTOR OF HATCHING SYNCHRONY IN THE HOUSE WREN?

Abstract The degree of hatching synchrony in clutches of passerine birds frequently varies among species and among individuals of the same species. Many hypotheses have been proposed to explain why some eggs hatch several days after others in a clutch. We tested one of these hypotheses, the energetic-constraints hypothesis, which proposes that females in poor physical condition postpone initiating incubation and hatch their clutches synchronously, whereas females in good condition begin incubation early and hatch their clutches asynchronously. We tested the hypothesis using the House Wren (Troglodytes aedon) because recent studies have found little difference in productivity between synchronously and asynchronously hatching clutches in this species, suggesting that the degree of hatching synchrony varies for reasons unrelated to nestling growth and survival. We used logistic regression to test the dependence of the degree of hatching synchrony on each of two measures of female condition. We found no relationship between female condition and degree of hatching synchrony. These results and two other lines of evidence are inconsistent with the energetic-constraints hypothesis as an explanation for variation in degree of hatching synchrony in this House Wren population. ¿Predice la Condición de las Hembras la Sincronía de Eclosión en Troglodytes aedon? Resumen. Con frecuencia, el grado de sincronía en la eclosión de nidadas de aves paserinas varía entre especies y entre individuos de la misma especie. Se han propuesto muchas hipótesis para explicar por qué algunos huevos eclosionan varios días después que otros en una misma nidada. Pusimos a prueba una de esas hipótesis, la de las limitaciones energéticas, que propone que hembras en mala condición física postponen el inicio de la incubación y sus nidadas eclosionan sincrónicamente, mientras que aquellas en buena condición inician la incubación temprano y sus nidadas eclosionan asincrónicamente. Pusimos a prueba la hipótesis usando a Troglodytes aedon, pues estudios recientes en esta especie han encontrado pocas diferencias en productividad entre nidadas de eclosión sincrónica y asincrónica, lo que sugiere que el grado de sincronía en la eclosión varía por motivos no relacionados con el crecimiento y supervivencia de los pichones. Utilizamos regresión logística para evaluar el grado de dependencia de la sincronía de eclosión en dos medidas de condición física de las hembras. No encontramos ninguna relación entre la condición de las hembras y el grado de sincronía en la eclosión. Estos resultados y otras dos líneas de evidencia son inconsistentes con la hipótesis de las limitaciones energéticas como una explicación para la variación en el grado de sincronía de eclosión en esta población de T. aedon.