Aspects of the Ecology of Urban-Nesting Bald Eagles (Haliaeetus leucocephalus) in South-Coastal British Columbia
Bald Eagle (Haliaeetus leucocephalus) populations throughout North America have increased considerably since the ban of DDT in 1972 and eagles now inhabit suburban areas in large numbers. To better understand the ecology of urban populations living in south-coastal British Columbia, we compared nest-site characteristics, reproductive rates, and diets of more than 150 breeding pairs of rural-, suburban- and urban-nesting eagles in the Greater Vancouver area. Three-quarters of the nests were located within 230 m of buildings and roads, or within 31 m of a potential source of disturbance. Urban eagles nested in live, taller trees that were close to the edges of patches, whereas rural eagles used shorter trees and occasionally human-made structures such as transmission towers. Eagles at nests located close to patch edges and in areas with greater human land use had higher reproductive rates than those at isolated nests or in remote rural habitat. Waterfowl and gulls (family Laridae) were common in the diet across the study area, but urban eagles also used alternative sources such as C-O sole (Pleuronichthys coenosus), Bufflehead (Bucephala albeola), and Band-tailed Pigeon (Patagioenas fasciata). Eagles in the Vancouver area have adapted to human-altered landscapes; management strategies should focus on maintaining edge habitat, monitoring population expansion in urban areas, and protecting nest sites. Aspectos de la Ecología de los Individuos de Haliaeetus Leucocephalus Que Nidifican en Lugares Urbanos en la Costa Sur de Columbia Británica Las poblaciones de Haliaeetus leucocephalus a lo largo de todo Norteamérica han aumentado considerablemente desde la prohibición del DDT en 1972 y actualmente las águilas habitan las áreas suburbanas en gran número. Para entender mejor la ecología de las poblaciones urbanas que viven en la costa sur de Columbia Británica, comparamos las características del lugar de cría, las tasas reproductivas y la dieta de más de 150 parejas reproductoras que nidifican en ambientes rurales, suburbanos y urbanos en el área del Gran Vancouver. Tres cuartos de los nidos estuvieron localizados dentro de los 230 m de edificios y carreteras, o a menos de los 31 m de una fuente potencial de molestias. Las águilas urbanas anidaron en árboles altos y vivos, cercanos a los bordes de los parches, mientras que las águilas rurales usaron árboles más bajos y ocasionalmente estructuras antrópicas como torres de transmisión. Las águilas con nidos localizados cerca de los bordes de los parches y en áreas con mayor uso antrópico del suelo tuvieron tasas reproductoras más altas que aquellas con nidos aislados o en hábitats remotos. Las aves acuáticas y las gaviotas (Laridae spp.) fueron comunes en la dieta a lo largo de toda el área de estudio, pero las águilas urbanas también usaron fuentes alternativas de alimento, como el pez plano Pleuronichthys coenosus y las aves Bucephala albeola y Patagioenas fasciata. Las águilas en el área de Vancouver se han adaptado a los paisajes antrópicos; las estrategias de gestión deberían enfocarse en mantener el hábitat de borde, seguir la expansión de las poblaciones en las áreas urbanas y proteger los lugares de cría. [Traducción del equipo editorial]ABSTRACT
RESUMEN
Map of study area, showing the distribution of 112 Bald Eagle (Haliaeetus leucocephalus) nests sampled for nest-site characteristics.
K-means groups for Bald Eagle (Haliaeetus leucocephalus) nest-site characteristics (n ¼ 87). Nests with similar characteristics have matching symbols and show geographical clumping over urban, suburban, and remote rural areas. Compare to Figure 1 for geographical features.
Redundancy analysis after forward selection for binary productivity of Bald Eagles (Haliaeetus leucocephalus) in 2004–2009 with respect to nest-site characteristics (F ¼ 3.77, df ¼ 7, P ¼ 0.047; see Table 1 for a description of abbreviations). Proportion of total variation explained by both predictors is 62.3%. The sequence of years from left to right indicates change in the relationship between nest features and productivity. We represented year symbols by the year number preceded by an X.
Redundancy analysis (RDA) of land-use category (R ¼ remote rural, S ¼ suburban and U ¼ urban) with respect to presence-absence of prey species (see Table 4 for a description of species names) for Bald Eagles (Haliaeetus leucocephalus). Although both RDA predictors explained only 9.8% of the variation in species, the relationship is highly significant (F ¼ 3.155, df ¼ 2, P ¼ 0.001).
Contributor Notes
1 Present address: 209 rue du Golf, Sainte-Marthe-sur-le-Lac, QC, J0N 1P0, Canada; email address: raphael.goulet@ gmail.com
2 Present address: North Saanich, BC, V8L 5J1, Canada.