EFFECTS OF HABITAT AND LANDSCAPE CHARACTERISTICS ON AVIAN BREEDING DISTRIBUTIONS IN COLORADO FOOTHILLS SHRUB

Abstract

Foothills shrub in the northern Colorado Front Range is a patchy habitat naturally embedded in a mosaic of grassland and conifer forest. We investigated associations of breeding songbirds in foothills shrub with local habitat characteristics and landscape context. Avian distri- butions were quantified using 84 points counted 6 times each over 2 years, and species were categorized as breeding-season residents or non-residents. We quantified habitat structure and shrub species composition using a point-quarter technique and visual estimation of cover, and sampled landscape context within 500 m of study points using aerial orthophotograph images. We used multiple logistic regression to analyze associations between 15 habitat and landscape variables and breeding residence of each of the eight most common breeding bird species: spotted towhee (Pipilo maculatus), brown-headed cowbird (Molothrus ater), green-tailed towhee (Pipilo chlorurus), Virginia's warbler (Vermivora virginiae), yellow-breasted chat (Icteria virens), broad-tailed humming- bird (Selasphorus platycercus), lazuli bunting (Passerina amoena), and blue-gray gnatcatcher (Polioptila caerulea). Five of these species showed significant relationships between breeding residence and at least one local habitat variable, in ways generally consistent with their known habitat require- ments. In a multiple regression with habitat and landscape variables, species richness was positively associated with two habitat characteristics: shrub cover and horizontal heterogeneity. No species was significantly associated with landscape context of shrub, grassland, or coniferous forest at the scale of our analysis, suggesting that shrub patches with appropriate local characteristics were used regardless of their natural landscape setting in this system. We conclude that breeding species in foothills shrub may have evolved tolerance for habitat fragmentation because it represents the natural state of the landscape in which they occur. RESUMEN-El habitat de arbustos de las faldas de las Montafias Rocallosas en el norte de Colorado es un habitat en parches naturalmente incrustado en un mosaico de pastizales y bosques coniferos. Se estudi6 la asociaci6n de aves cantoras que anidan en este hdbitat con caracteristicas del habitat local y con contexto del paisaje. Las distribuciones de aves fueron cuantificadas en 84 lugares, cada lugar contado seis veces durante dos afios, y las especies fueron categorizadas como residentes o no residentes. Cuantificamos la estructura del habitat y la com- posici6n de las especies de arbustos usando la t~cnica de puntos-cuadrados y la estimaci6n visual de cobertura, y muestreamos el contexto del paisaje dentrode 500 m de los puntos fijos usando imagenes ortofotogrificas. Usamos regresi6n mfiltiple logistica para analizar asociaciones entre 15 variables de habitat y del paisaje con la clase de residencia para cada una de las ocho especies mas comunes: Pipilo maculatus, Molothrus ater, Pipilo chlorurus, Vermivora virginiae, Icteria virens,