Origin and Availability of Large Cavities for Barrow’s Goldeneye (Bucephala islandica), a Species at Risk Inhabiting the Eastern Canadian Boreal Forest

Abstract

Large secondary-nesting birds such as ducks rely on appropriate cavities for breeding. The main objective of this study was to assess the availability of large cavities and the potential of a managed boreal coniferous landscape to provide nesting trees within the breeding area of the eastern population of Barrow’s Goldeneye (Bucephala islandica), a cavity-nesting species at risk in Canada. Woodpecker surveys were conducted in both conifer and mixed-wood landscapes, and cavities were sought in line transects distributed in unharvested and linear remnant stands of balsam fir (Abies balsamea) and black spruce (Picea mariana) as well as in cutblocks. No Pileated Woodpeckers (Dryocopus pileatus) were detected in the breeding area of Barrow’s Goldeneye, but the species was present in the nearby lowland area in which trembling aspen (Populus tremuloides) is abundant. Only 10 trees (0.2% of those sampled) supported cavities considered suitable for Barrow’s Goldeneye in terms of dimensions and canopy openness. Most of the suitable cavities found during this study were nonexcavated apical (chimney) cavities in relatively short snags that showed advanced states of decay. A diameter-at-breast-height threshold was determined for each tree species, after which the probability of cavity occurrence was enhanced in terms of potential cavity trees for Barrow’s Goldeneye. Remnant linear forest sites had lower potential tree densities than did their unharvested equivalents. Large cavities were thus a rare component in this boreal landscape, suggesting that they may be a limiting factor for this population at risk. Current even-aged forest management that mainly relies on clear-cut practices is likely to further reduce the potential of this landscape to provide trees with suitable cavities. RESUME. Les oiseaux de grande taille qui sont des utilisateurs secondaires de cavites, comme les canards, dependent de cavites adequates pour nicher. L’objectif principal de cette recherche etait d’evaluer la disponibilite de grandes cavites et le potentiel d’un paysage de foret boreale coniferienne amenagee a fournir des arbres dans lesquels le Garrot d’Islande (Bucephala islandica) de la population de l’Est, une espece cavicole en peril au Canada, peut nicher. Des releves de pics ont ete effectues dans des paysages coniferiens et mixtes, et des recherches de cavites ont ete faites dans des transects situes en forets non recoltees et en forets residuelles lineaires de Sapin baumier (Abies balsamea) et d’Epinette noire (Picea mariana), ainsi que dans des parterres de coupe. Aucun Grand Pic (Dryocopus pileatus) n’a ete observe dans l’aire de nidification du Garrot d’Islande, mais cet excavateur primaire etait present dans les basses terres avoisinantes, ou le Peuplier faux-tremble (Populus tremuloides) est abondant. Seulement 10 arbres, soit 0,2 % des arbres echantillonnes, avaient des cavites jugees adequates (bonnes dimensions et ouverture du couvert forestier suffisante) pour le Garrot d’Islande. La plupart des cavites trouvees au cours de cette recherche etaient des cavites apicales non excavees (de type cheminee) dans des chicots relativement courts Universite du Quebec a Montreal, Canadian Wildlife Service, Natural Resources Canada Avian Conservation and Ecology Ecologie et conservation des oiseaux 4(1): 6 http://www.ace-eco.org/vol4/iss1/art6/ en etat de decomposition avance. Un seuil de diametre a hauteur de poitrine, a partir duquel la probabilite d’occurrence d’une cavite augmente, a ete determine pour chaque espece d’arbres afin d’evaluer la disponibilite d’arbres dans lesquels la nidification du Garrot d’Islande etait possible. Les forets residuelles lineaires avaient des densites d’arbres potentiels plus faibles que les forets non recoltees. Les grandes cavites etaient donc des elements rares dans ce paysage boreal, ce qui laisse croire qu’elles puissent representer un facteur limitant pour cette population en peril. L’amenagement forestier actuel (amenagement equienne), qui repose principalement sur la pratique de coupes totales, risque vraisemblablement de diminuer encore plus le potentiel de ce paysage a fournir des arbres avec des cavites adequates.