Responses of Great Horned Owls (Bubo virginianus), Barred Owls (Strix varia), and Northern Saw-whet Owls (Aegolius acadicus) to forest cover and configuration in an agricultural landscape in Alberta, Canada

Abstract

Studies on the effects of forest fragmentation on birds have focussed primarily on passerines, with few studies on owls. We assessed the influence of forest amount and configuration on the abundance and distribution of three species of forest owls, Great Horned Owl (Bubo virginianus (Gmelin, 1788)), Barred Owl (Strix varia Barton, 1799), and Northern Saw-whet Owl (Aegolius acadicus (Gmelin, 1788)), in agricultural landscapes with varying amounts of forest cover in central Alberta, Canada. All three species were positively associated with forest cover: Barred Owls were most prevalent in landscapes with >66% forest cover, Great Horned Owls in landscapes with between 36% and 65% forest cover, and Northern Saw-whet Owls in landscapes with between 16% and 100% forest cover. Regression models containing configuration variables were chosen as best models using AIC for all three species. Great Horned Owls were most abundant in landscapes with high heterogeneity: more forest-nonforest edges and higher forest patch area variation. Barred Owls were more likely to occur in landscapes with larger forest patch areas and Northern Saw-whet Owls were more abundant in landscapes that were more connected. These relationships are consistent with predictions based on body size of owls and local habitat relationships described in the literature. Résumé :Les études des effets de la fragmentation des forêts sur les oiseaux se sont intéressées principalement aux passereaux, avec peu de travaux sur les hiboux. Nous évaluons l'influence de l'étendue et de la configuration de la forêt sur l'abondance et la répartition de trois espèces forestières de hiboux, le grand-duc d'Amérique (Bubo virginianus (Gmelin, 1788)), la chouette rayée (Strix varia Barton, 1799) et la petite nyctale (Aegolius acadicus (Gmelin, 1788)), dans des paysages agricoles possédant des étendues diverses de couverture forestière dans le centre de l'Alberta, Canada. Il existe une association positive entre les trois espèces et le couvert forestier : les chouettes rayées prédominent surtout dans les paysages qui présentent plus de 66 % de forêt, les grands-ducs d'Amérique dans les paysages à 36 % -65 % de forêt et les petites nyctales dans les paysages à 16 % -100 % de forêt. Nous avons choisi pour les trois espèces les meilleurs modèles de régression contenant les variables de configuration à l'aide du critère AIC. Les grands-ducs d'Amérique sont plus abondants dans les paysages de forte hétérogénéité avec plus de lisières forêt et non forêt et une variation plus grande des taches forestières. Les chouettes rayées ont plus de chances de se retrouver dans les paysages contenant des taches forestières plus étendues et les petites nyctales sont plus abondantes dans les paysages qui sont plus interconnectés. Ces relations s'accordent avec les prédictions basées sur la taille corporelle des hiboux et des relations d'habitat décrites dans la littéra-ture.[Traduit par la Rédaction]

Factors Associated with Flammulated Owl and Northern Saw-Whet Owl Occupancy in Southern Idaho

“…Thus, saw-whet owls are typically more general in their habitat use than many other forest-dwelling owl species. Grossman et al (2008) found that saw-whet owls occupied a variety of landscapes with the amount of forested area ranging from 16-100% within 0.8 km-radius plots. The positive relationship between saw-whet owls and ponderosa pine and Douglas-fir in our study may also reflect the relatively higher abundance of these two forest types in the BNF.…”

“…Spatially explicit modeling of these relationships is useful for identifying habitat associations (Groce and Morrison 2010), highlighting suitable but unoccupied habitat , and predicting effects of habitat manipulations or restoration (Doherty et al 2008, Grossman et al 2008. Moreover, the scale at which species respond to factors in their environment may differ (Turner et al 2001, Cushman andMcGarigal 2002).…”

Characteristics of Barred Owl (<em>Strix varia</em>) nest sites in Manitoba, Canada

“…128 haney 1997; Mazur et al 1997aMazur et al , 1997bPostupalsky et al 1997;Winton and leslie 2004;Olsen et al 2006;Grossman et al 2008;Singleton et al 2010) and general Barred Owl habitat associations (McGarigal and Fraser 1984;Booth and harrison 1997;Mazur et al 1998;hamer et al 2007;Russell 2008). These studies vary considerably, as described in more detail below, in the way study areas were selected, in the size and habitat fragmentation of the study areas, and in the size and measurement of nest habitat plots.…”

“…Mazur et al (1997b) used a similar small-scale plot (11.3 m radius) with the nest tree at the centre, and reported a somewhat higher mean percentage cover of 57% (Sd 17); this was not significantly different from random plots. Other studies reported yet higher percentage canopy cover within larger Barred Owl home ranges: 96% (Se 1.1) (haney 1997), 62.8% (Winton and leslie 2004), utilized "dense" cover disproportionately (no values given) (Nicholls and Warner 1972), >66% (Grossman et al 2008), and >56% (Singleton et al 2010).…”

“…it is therefore unlikely that Barred Owls choose a nest site based on tree species per se. high percentage forest canopy cover has been cited as a determining factor in Barred Owl selection of breeding habitat, possibly because it provides solar insulation (Nicholls and Warner 1972;haney 1997;Winton and leslie 2004;Grossman et al 2008), but the influence of forest canopy may depend on the size of the area that was measured. in this study, canopy cover was measured within a 30 m circular plot centered on the nest tree, and it did not appear to influence Barred Owl nest tree habitat use: more than half the sample had a canopy cover of ≤50% (Table 2).…”