Avian responses to experimental harvest in southern boreal mixedwood shoreline forests: implications for riparian buffer management

Conventional management of shoreline forest in harvested boreal landscapes involves retention of treed buffer strips to provide habitat for wildlife species and protect aquatic habitats from deleterious effects of harvesting. With shoreline forests being considered for harvest in several jurisdictions, it is important to determine the potential impacts of this disturbance on birds. In this study, responses of riparian- and upland-nesting birds to three levels of harvest (0%–50%, 50%–75%, and 75%–100% within 100 m of the water) in shoreline forests around boreal wetlands were assessed 1 year before and each year for 4 years after harvest relative to unharvested reference sites. Upland-nesting species showed variable responses to harvest, with greatest declines in abundance of interior forest nesting species (e.g., Ovenbird, Seiurus aurocapillus L.) with the highest levels of harvest. Shrub-nesting and generalist species increased in abundance in harvest treatments relative to reference sites. Riparian birds showed little response to harvest, suggesting that shoreline forest harvest has little effect on their abundance up to 4 years after harvest. Retention of small buffers may not be an effective management strategy for conservation of birds occupying shoreline forests, particularly interior forest nesting species. We suggest that alternatives to conventional buffer management be explored.

Landbird use of Riparian and Upland Forest Stopover Habitats in an Urban Landscape

Stopover habitat relationships of landbirds are not well known and this constitutes a shortcoming in conservation plans for migratory birds. We compared migrant use of mature upland and riparian forests during spring in an urbanizing landscape within central Ohio. We surveyed stopover migrants along 250-m transects, and quantified both local- and landscape-level habitat characteristics (percent urban and forest land). During spring stopover, migrant abundance was 86% and 118% higher in upland forests for Nearctic-Neotropical transient (long-distance migrants) and temperate transients (short-distance migrants), respectively. Of 27 transient species, 22 species differed in their use of riparian and upland forests, and 20 of those were most abundant in upland forest (e.g., Blue-headed Vireo [Vireo solitarius], Least Flycatcher [Empidonax minimus], Swainson's Thrush [Catharus ustulatus], Nashville Warbler [Vermivora ruficapilla], Magnolia Warbler [Dendroica magnolia], Bay-breasted Warbler [D. castanea], and Ovenbird [Seiurus aurocapillus]). Species richness was 58% and 75% higher in upland forests relative to riparian forests for Neotropical transient and temperate transient groups, respectively. Percent urbanization within 1 km was unrelated to abundance of Neotropical transients and temperate transients. Abundance of Neotropical transients and temperate transients was unrelated to percent forest cover within 1 km. Mature upland and riparian forests differed in that riparian sites had lower percentage of shrub cover, higher percentage of canopy cover, and different species composition of trees. Abundance of Neotropical transients was positively associated with mean canopy height; temperate transients were positively associated with mean number of trees >38 cm DBH and mean percentage of shrub cover. Our data indicate that mature upland forests were more heavily used by migrating landbirds than riparian forests within an urbanizing Midwestern landscape and may represent valuable habitats for the conservation of migratory landbirds within highly fragmented landscapes. Uso de Bosques Riparios y de Llanuras por Aves Terrestres como Hábitats de Paradas Migratorias en un Paisaje Urbano Resumen. Las relaciones de los hábitats que sirven como paradas migratorias para las aves terrestres no están bien conocidas, y esto constituye una deficiencia en los planes de conservación de las aves migratorias. Comparamos el uso de bosques maduros de llanuras y de bosques riparios por parte de aves migratorias durante la primavera en un paisaje que está siendo urbanizado en Ohio central. Registramos aves migratorias en las paradas de migración a lo largo de transectos de 250 m y cuantificamos las características del hábitat tanto a nivel local como a nivel del paisaje (porcentaje de cobertura urbana y de bosque). Durante la parada migratoria de primavera, la abundancia de las aves migratorias en bosques de llanura fue un 86% mayor para las aves transitorias del Neártico-Neotrópico (aves migratorias de larga distancia) y un 118% mayor para las aves transitorias templadas (aves migratorias de corta distancia). De 27 especies transitorias, 22 especies se diferenciaron en su uso de bosque ripario y de llanura, y 20 de éstas fueron más abundantes en bosques de llanura (e.g., Vireo solitarius, Empidonax minimus, Catharus ustulatus, Vermivora ruficapilla, Dendroica magnolia, D. castanea, Seiurus aurocapillus). La riqueza de especies en bosques de llanura con relación a el bosque ripario fue un 58% mayor para los grupos de aves transitorias Neotropicales y un 75% mayor para las aves transitorias templadas. El porcentaje de cobertura de urbanización en un radio de 1 km no se relacionó con la abundancia de aves transitorias Neotropicales ni con las transitorias templadas. La abundancia de las aves transitorias Neotropicales y templadas no se relacionó con la cobertura de bosque dentro de un radio de 1 km. Los bosques maduros de llanura y los bosques riparios se diferenciaron en que los sitios de bosque ripario tuvieron menor cobertura de arbustos, una mayor cobertura de dosel y una composición de especies de árboles diferente. La abundancia de las aves transitorias Neotropicales se relacionó positivamente con la altura promedio del dosel, mientras que las aves transitorias templadas se relacionaron positivamente con el número promedio de árboles con DAP >38 cm y con la cobertura promedio de arbustos. Nuestros datos indican que los bosques maduros de llanuras fueron utilizados con mayor intensidad por aves migratorias terrestres que los bosques riparios en un paisaje urbanizado del medio-oeste, y que estos bosques pueden representar un tipo de hábitat valioso para la conservación de las aves migratorias terrestres en un paisaje altamente fragmentado.

Territory size and overlap in male Ovenbirds: contrasting a fragmented and contiguous boreal forest

We evaluated if male age and body size, density of conspecifics, and arthropod biomass contributed to variation in territory size and overlap of Ovenbirds, Seiurus aurocapillus (L., 1766), in a fragmented and contiguous boreal forest. Territory size and overlap were determined by radio-tracking territorial male Ovenbirds in fragmented (n = 22) and contiguous forest (n = 13) from late May to the end of June 1999 and 2000. Variation in male territory size was most strongly associated with individual characteristics, specifically body size and age. Furthermore, we found strong support for an effect of density of conspecifics on territory overlap, suggesting that the exclusivity of territories and perhaps levels of territoriality were greater for males in contiguous forest than for those in fragments. Our findings (i.e., mean territory size was similar between landscapes and territory overlap was greater in fragments than in contiguous forest) suggest that fragments either have larger areas of unsuitable habitat or are less saturated with Ovenbirds. Furthermore, because resources were not distributed equally among individuals, our results were consistent with the ideal dominance model of habitat selection. Finally, previous studies based on acoustical surveys have likely underestimated space-use requirements in forest passerines.

Using Isotopic Variance to Detect Long-Distance Dispersal and Philopatry in Birds: An Example with Ovenbirds and American Redstarts

Understanding movements of individual birds between breeding sites (breeding dispersal) or between natal sites and the site of first breeding (natal dispersal) is crucial to the modeling of population dynamics. Unfortunately, these aspects of demography are poorly understood for avian species in general, and for migratory songbirds in particular. This is because it is often impossible to sample broadly enough to relocate marked birds that have moved. We used stable-hydrogen (δD) and carbon (δ13C) isotope analyses of the feathers of 139 American Redstarts (Setophaga ruticilla) and 193 Ovenbirds (Seiurus aurocapillus) to evaluate evidence for individuals molting feathers at locations other than their breeding sites from the previous year. We sampled outer rectrices from breeding populations at three extensive boreal forest sites (Prince Albert National Park and Duck Mountain, Saskatchewan, and Lac La Biche, Alberta) and at three isolated forest tracts (Cypress Hills, and Moose Mountain, Saskatchewan, and Turtle Mountain, Manitoba) in western Canada. Based on outlier analysis of δD measurements, we found evidence for long-distance dispersal ranging from 0–29% of individuals. For both species, second-year birds had higher variance in δD values suggesting they had a higher probability of originating from elsewhere compared to after-second-year birds.Utilización de la Variación Isotópica para Detectar Dispersión de Larga Distancia y Filopatría en las Aves: Un ejemplo con Seiurus aurocapillus y Setophaga ruticillaResumen. Entender los movimientos de aves individuales entre sitios de cría (dispersión reproductiva) o entre el sitio de nacimiento y el sitio del primer evento reproductivo (dispersión natal) es crucial para modelar la dinámica de poblaciones. Desafortunadamente, estos aspectos demográficos están poco entendidos para las especies de aves en general y para las aves canoras migratorias en particular. Esto se debe a que es usualmente imposible realizar muestreos lo suficientemente amplios como para relocalizar a las aves marcadas que se desplazaron. Usamos análisis de isótopos estables de hidrógeno (δD) y carbono (δ13C) de las plumas de 139 individuos de Setophaga ruticilla y de 193 individuos de Seiurus aurocapillus para evaluar la presencia de individuos que han mudado sus plumas en localidades distintas a las de sus sitios reproductivos del año anterior. Muestreamos las rectrices externas en poblaciones reproductivas de tres sitios extensos de bosque boreal (Parque Nacional Prince Albert y Duck Mountain, Saskatchewan, y Lago La Biche, Alberta) y de tres parches de bosque aislados (Cypress Hills y Moose Mountain, Saskatchewan, y Turtle Mountain, Manitoba) en el oeste de Canadá. Basados en análisis de datos extremos (outliers) de medidas de δD, encontramos evidencia de dispersión de larga distancia que comprendió entre el 0–29% de los individuos. Para ambas especies, las aves del segundo año de vida presentaron mayor varianza en los valores de δD, sugiriendo que poseen una mayor probabilidad de haberse originado en algún otro lugar comparado con aves de más de dos años de vida.

Prior Breeding Success Affects Return Rates of Territorial Male Ovenbirds

I examined the hypothesis that male Ovenbirds (Seiurus aurocapillus) make breeding dispersal decisions based on prior breeding experience at a site. I determined the reproductive success of color-banded male Ovenbirds at sites in fragmented and unfragmented landscapes in Missouri from 1992–1995. I documented which individuals returned and I recorded their reproductive success upon return as well as the success of birds new to each plot. I obtained similar results in both landscapes. Males with different histories of reproductive success returned at different rates. Only 2 of 22 males that were paired but failed to raise young returned in the following year, whereas the return rates of unpaired males (41%, n = 37) and males that successfully raised young (54%, n = 57) did not differ significantly (P = 0.19). The patterns were more consistent with the hypothesis that return rates were due to decisions about dispersal rather than difference in survival of individuals in these groups.El Éxito Reproductivo Previo Afecta Índices de Regreso de Machos Territoriales de Seiurus aurocapillusResumen. Examiné la hipótesis de que individuos de Seiurus aurocapillus hacen decisiones de dispersión reproductiva basadas en las experiencias reproductivas previas en un sitio. Determiné el éxito reproductivo de machos con anillos de color en lugares en terrenos fragmentados y no fragmentados de Missouri entre 1992 y 1995. Documenté cuáles individuos regresaron y su éxito reproductivo al regresar, y también el éxito de pájaros nuevos en cada sitio. Obtuve resultados semejantes en ambos paisajes. Los machos con historias de éxito reproductivo diferentes presentaron índices de regreso diferentes. Solamente 2 de 22 machos apareados que no produjeron cría volvieron al año siguiente, mientras que los índices de regreso de machos no apareados (41%, n = 37) y de los machos que produjeron crías exitosamente (54%, n = 57) no difirieron significativamente (P = 0.19). Los patrones concuerdan más con la hipótesis de que los índices de regreso se deberieron a las decisiones de dispersión y no a la diferencia de supervivencia de individuos en estos grupos.