Landscape Modification and Patch Selection: The Demography of Two Secondary Cavity Nesters Colonizing Clearcuts

Abstract

Current forestry practices radically modify habitat, particularly by increasing the frequency of early sera! stages. We examined the demography of two secondary cavity nesters, the Mountain Bluebird (Sialia currucoides) and the Tree Swallow (Tachycineta bicolor), with respect to landscape changes. The study area contained a mosaic of clearcuts ranging from 3 to 27 years in age. The initial distribution of both species was uneven with respect to patch age. Experimental addition of nest boxes: (1) increased the number of nesting pairs of both species, and (2) expanded the age range of occupied patches. Nest-site availability was the primary limiting factor in young patches, but vegetation structure was increasingly important in determining occupation in older patches, Mountain Bluebird patch occupation and Tree Swallow density were best predicted by the proportion of trees 1 to 3m in height. Although some patches contained multiple pairs of Mountain Bluebirds in natural cavities, experimental patches typically contained no more than one pair, even though excess nest boxes were available. The presence of nonbreeding floaters was an indication of nest-site limitation and suggested that Mountain Bluebirds sometimes defer breeding attempts rather than nest close to conspecifics. At low densities, Tree Swallows also were over-dispersed, but pairs saturated available nest boxes as density increased. Clutch size and number of chicks fledged from successful nests did not vary with patch age for either species, although swallows initiated clutches later in younger compared with older patches. Nest predation varied consistently with patch age; mean nest predation for both species was 86% (n = 30 pairs) in the youngest patches, 37% (n = 34) in mid-age patches, and 25% (n = 22) in the oldest patches. The low densities of both species in older patches was not associated with low productivity, suggesting that habitat selection occurred independently of potential productivity in this novel habitat.