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A land manager's guide to conserving habitat for forest birds in ...

A land manager's guide to conserving habitat for forest birds in ...

Male Brown-headed

Male Brown-headed Cowbird — Photo: Brandon Holden Selection system harvesting causes more subtle changes in vegetation structure than even-aged management and retains a forest bird community similar to uncut mature forests. Gapdependent species, such as the Hooded Warbler and Chestnutsided Warbler increase. However, species that depend on dead and declining trees like cavity nesters, or species like Brown Creeper that depend on large mature trees can suffer after selection or diameter-limit logging, since harvesting removes many of these trees. If the group selection method is used some species like the Wood Thrush, or area sensitive forest interior species (Acadian Flycatchers) may disappear because of the large gaps and perceived changes in habitat structure within their historic territories. Diameter-limit cutting removes most, if not all of the large trees, and although it retains canopy cover, stands are often more open and less dense (30–60 percent basal area removal) than selection harvested stands (20–30 percent of basal area removed). Habitat for shrub layer dependent species (Indigo Bunting, Chestnut-sided Warbler) and generalist species (American Redstart, Brown-headed Cowbird) is often increased. However many species are negatively effected; Brown Creepers and species of conservation concern (Acadian Flycatcher, Cerulean Warbler) tend to be absent from heavy cut stands, while primary cavity nesting species, like woodpeckers, are sparse due to loss of preferred nesting and feeding habitats. (See Bird Response to Forest Management Practices pages 50–53.) Productivity In landscapes fragmented by agriculture, Brown-headed Cowbirds and nest predators can be abundant, and compromise the survival of songbird nests. Logging can compound existing stresses by increasing openness in the canopy and changing forest structure to allow predators and parasites greater access to forest bird nests. Brown-headed Cowbirds are more abundant in clear cuts, shelterwood, and diameter-limit cut stands than in unharvested or in single tree selection harvested forests. For example, in diameter-limit cut stands, not only does the number of nests that are parasitized increase, but also intensity increases. Cowbirds parasitize a greater proportion of nests and lay more eggs in each nest. Though parasitism rates can increase in the fragments of southern Ontario following single tree selection, rates of parasitism are similar to or lower than in uncut forests five or more years after harvest. Predation rates, on the other hand, are more variable between harvest treatments, but tend to be higher on heavily cut sites. Overall, the impact of parasitism and predation on birds nesting in recently harvested forests is species specific, because each species of bird responds slightly differently. Forest interior species like Hooded Warblers that are victims of Brown-headed Cowbird parasitism, suffer reduced nesting success when partially cut forests are invaded by cowbirds. For this species, and those like Rose-breasted Grosbeak, heavier cut sites serve as ecological traps. High availability of open canopy and dense shrub and sapling layers for nesting attracts these species to these sites. Heavily cut sites become traps by attracting high densities of some bird species, who suffer significantly lower nesting success on these sites because of high densities of nest predators or cowbirds. Generalists (species like the American Robin and Northern Cardinal), may not be subject to these same elevated predation and/or parasitism pressures, and therefore experience similar levels of nest success across harvesting intensities. Others Brown-headed Cowbird nestling with Hooded Warbler nestlings — Photo: OMNR Parasitized Veery nest — Photo: Mark Peck 54 Harvesting Effects on Birds

Elderberry — Photo: Robert McCaw like the Wood Thrush will respond positively to the structural changes associated with low intensity selection system harvesting and have higher nesting success in these sites. However, this benefit does not occur until at least five years after harvest, but can persist for a number of years (5–10 years). Predation and parasitism are not the only changes associated with harvesting that can affect avian productivity. Reductions in food availability can strongly influence a parent’s ability to feed its nestlings. This can result in fewer young or less healthy young produced by the pair of birds. Research shows both Northern Cardinals and Rose-breasted Grosbeaks fledge fewer young per nest in diameter-limit sites than uncut sites. This may be due to lower food availability, increased parasitism (as cowbirds typically remove a host egg), or both. Virtually all songbirds eat insects and other invertebrates during the breeding season. When you open up the canopy by harvesting, a forest becomes sunnier, hotter, and drier. This can reduce the biomass of insects that are vulnerable to desiccation. The changes in micro-climate can be extreme as the intensity of harvest increases, and translate into larger changes in forest structure and insect communities. Reductions in the density of mature trees can result in less bark and canopy dwelling insects available for canopy and bark feeders. In selection system harvests where you retain most of the mature canopy and the abiotic environment changes little, insect communities will be more similar to uncut forests. Flying insects, on the other hand, can respond positively to abiotic changes created by canopy gaps, and thus group cuts can create ideal foraging habitat for aerial insectivores, such as flycatchers. As insects are not the only source of food, changes in forest structure and composition may alter availability of fruits and seeds. Growth and regeneration of mid-tolerant tree species like red and white oak, bitternut hickory and black cherry through shelterwood or group selection can increase the availability of tree seeds (also known as mast). Similarly, gaps will often stimulate shrub growth, creating new sources of food, like raspberry or elderberry. Thus harvesting can have positive or negative effects on food availability depending on species-specific foraging preferences and the system employed. Survival of juveniles after they leave or fledge from the nest is another key component influencing avian productivity. During the post-breeding period, adults and juveniles of mature forest bird species start to make use of early successional habitats, where an open canopy creates new growth in the form of a dense shrub layer. These habitats offer greater protection from predators, and for some species, high food sources. For species like Hooded Warbler, heavy diameter-limit cut sites may actually have higher juvenile survival rates than single tree selection or unharvested sites. While for other species like Rose-breasted Grosbeaks, harvest treatment has little impact on fledgling survival parameters. Thus, early successional habitats can provide valuable habitat to juvenile birds of some species but do very little for another. In summary, the intensity of the harvest strongly influences the extent to which harvesting affects bird communities. Evenaged management creates very different breeding conditions and different bird communities, than uneven-aged management. In a continuously forested landscape (like the boreal forest region), even-aged management minimizes many of the dramatic changes associated with harvesting, since the bird species are adapted to infrequent large-scale disturbances. Loss of canopy cover and Rose-breasted Grosbeak fledglings — Photo: Brad Woodworth Harvesting Effects on Birds 55

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