Factors driving territory size and breeding success in a threatened migratory songbird, the Canada Warbler

Abstract

ABSTRACT. Successful conservation of migratory birds demands we understand how habitat factors on the breeding grounds influences breeding success. Multiple factors are known to directly influence breeding success in territorial songbirds. For example, greater food availability and fewer predators can have direct effects on breeding success. However, many of these same habitat factors can also result in higher conspecific density that may ultimately reduce breeding success through density dependence. In this case, there is a negative indirect effect of habitat on breeding success through its effects on conspecific density and territory size. Therefore, a key uncertainty facing land managers is whether important habitat attributes directly influence breeding success or indirectly influence breeding success through territory size. We used radio-telemetry, point-counts, vegetation sampling, predator observations, and insect sampling over two years to provide data on habitat selection of a steeply declining songbird species, the Canada Warbler (Cardellina canadensis). These data were then applied in a hierarchical path modeling framework and an AIC model selection approach to determine the habitat attributes that best predict breeding success. Canada Warblers had smaller territories in areas with high shrub cover, in the presence of red squirrels (Tamiasciurus hudsonicus), at shoreline sites relative to forest-interior sites and as conspecific density increased. Breeding success was lower for birds with smaller territories, which suggests competition for limited food resources, but there was no direct evidence that food availability influenced territory size or breeding success. The negative relationship between shrub cover and territory size in our study may arise because these specific habitat conditions are spatially heterogeneous, whereby individuals pack into patches of preferred breeding habitat scattered throughout the landscape, resulting in reduced territory size and an associated reduction in resource availability per territory. Our results therefore highlight the importance of considering direct and indirect effects for Canada warblers; efforts to increase the amount of breeding habitat may ultimately result in lower breeding success if habitat availability is limited and negative density dependent effects occur.Facteurs influant sur la taille du territoire et le succès reproducteur chez un passereau migrateur menacé, la Paruline du Canada RÉSUMÉ. La conservation réussie d'oiseaux migrateurs passe par la compréhension de l'influence qu'ont les composantes d'habitat des aires de nidification sur le succès reproducteur. Chez les passereaux territoriaux, de nombreuses composantes sont reconnues pour influer directement sur le succès reproducteur. Par exemple, la présence d'une grande disponibilité de nourriture et d'un faible nombre de prédateurs peut avoir des effets directs sur le succès reproducteur. Toutefois, bon nombre de ces mêmes composantes d'habitat peuvent aussi entrainer une densité d'individus c...

Regional habitat needs of a nationally listed species, Canada Warbler (Cardellina canadensis), in Alberta, Canada

“…Land use practices that support retention or recruitment of dense shrubby understory could further enhance the conservation value of managed forests for Canada Warblers. However, a recent study in Alberta found that the Canada Warbler has smaller territories and lower fecundity in areas with high shrub cover, possibly because of increased resource competition (Flockhart et al 2016). Further research is needed to determine whether early seral habitats function as reproductive sources or sinks in western boreal forests, some of which is ongoing (A.…”

“…Hard and soft vegetated linear features were never the dominant land cover and were dropped from the list of local-scale land cover categories. We modeled their effect based on a dummy variable for the presence of a road in the 150-m buffer (i.e., roadside surveys) and the proportion of vegetated linear features, because it is known that their presence can introduce biases when not accounted for (Marques et al 2010, Bayne et al 2016. We also accounted for biases related to survey methodology.…”

Does land use change influence predation of bird nests?

“…) to avoid potential density‐dependent nest predation (Mitchell & Brown ; Flockhart et al . ). We did not use flagging tape to mark artificial nests (Yahner & Wright ).…”

Avian Occupancy Dynamics and Canada Warbler (Cardellina canadensis) Environmental Correlates on a Central Appalachian Elevation Gradient in West Virginia

“…Canada Warblers nest on the forest floor, where high stem densities conceal nests and improve reproductive success (Goodnow and Reitsma 2011), while supporting higher Canada Warbler abundances (Hobson and Bayne 2000). Additionally, Canada Warblers that arrive earlier on their breeding grounds choose sites with greater understory stem density (Hallworth et al 2008b), and territory size decreases with increasing shrub density (Flockhart et al 2016).…”