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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 Northern

Male Northern Cardinal feeding recently fledged young — Photo: Marie Read Ovenbird nest and surrounding habitat — Photo: Ken Elliott Close-up of Ovenbird nest — Photo: Ken Elliott This in turn governs the range of habitats available to different organisms. Generally, forests that have many layers from the ground to the canopy will support a greater array of plant and animal species compared to forests in which the vegetation is concentrated in a single layer. Lack of vegetation structure will have negative effects on species that rely on specific layers of vegetation for food or cover. Territories of different species can overlap horizontally and vertically within the forest. Suites of species that have similar habitat requirements are often grouped according to a nesting or foraging guild. Some birds prefer open, shrubby habitat found in young forests (i.e., early successional guild), whereas others prefer closed canopy forests with large trees and belong to the mature forest. For an individual bird, selecting suitable habitat can have the greatest influence on its ability to survive and reproduce. Faced with a multitude of possible choices in the landscape, how do birds choose the most appropriate habitat? They do this by making a series of choices, first at very large or coarse scales (e.g., all of southern Ontario), then at increasingly finer, smaller scales. For example, an Ovenbird (a small ground nesting bird with bold breast spots, that returns to breed in the woodlots of southern Ontario in the spring), first chooses a forest patch. This may be based on the size of the patch, the surrounding landscape features, and adjacent land uses. Next it will have to choose a territory within that patch. This will be based on the structure of the forest and the availability of food within it. Finally, the Ovenbird will choose a nest site. Ovenbirds nest on the ground and will look for particular vegetation features and structures that may hide the nest from predators and cowbirds. The Reproductive Challenge Although each bird attempts to raise as many young as possible over its lifetime, to maintain a population, births must equal deaths. Each adult, within its lifespan, must replace itself with an offspring of reproductive age. For most songbirds this means a pair must produce one young that survives to the next breeding season. This may not seem difficult over an average life span of two to three years, given that most birds lay three to four eggs per nest, and many can produce more than one successful nest per year (double brooding). However, once you consider rates of nest failure (50–80 percent) and losses due to parasitism (15–50 percent of nests are parasitized and fledge half as many young), the probability of those eggs successfully hatching and the young surviving the winter to return and breed is greatly reduced. As well, once a nestling has left the nest, its survival is often very poor within the first week of life, never mind the additional challenges of migration and over-winter survival! 20 Forest Birds in Ontario

Male American Redstart — Photo: Garth McElroy Forest Food Resources The food available to breeding birds is primarily determined by the composition and structure of the vegetation present. Availability of seeds, fruits and flowers, a direct source of food for many species, is intimately tied to the habitat. Abundance of insects, which serve as a critical food source during the nestling stage, is also determined by vegetation structure, and indirectly links birds with vegetation. Insects provide protein necessary for growth and development of nestlings and fledglings. Caterpillars, in particular, are a major component of the diet of many breeding birds, as they are relatively large and easily captured. Each bird species has a specific foraging strategy to locate and capture insect prey. Foliage gleaners pick caterpillars and aphids off the surface of leaves, whereas bark gleaners pick grubs, spiders, and other insects from the surface and crevices of bark, twigs, and wood. Aerial foragers capture moths, beetles, flies, and ballooning spiders and caterpillars (i.e., insects dangling in the air from strands of silk attached to trees) from the air, and ground feeders primarily search for beetles and ants in the leaf litter on the forest floor. Vegetation structure is very important to foraging birds and can influence the number of species of birds a habitat supports. The canopy, mid-canopy, shrub, and understorey layers of a forest create different foraging microhabitats (see Habitat preferences of common forest birds pages 22–23). Birds like Cerulean Warblers and Scarlet Tanagers forage high in the canopy, whereas other foliage gleaners, like Hooded Warblers and American Redstarts, will forage low in the shrub layer. Native plant diversity further influences available food resources. Each plant species supports a characteristic composition and abundance of insects. Certain bird species may prefer certain tree species over others because prey is more abundant or more easily obtained due to their structure. For instance, foliage gleaners may prefer hardwoods like yellow birch and oak over sugar maple, because sugar maple has lower densities of caterpillars and longer leaf stems that make caterpillars on leaves harder to reach by birds hopping along on branches. The depth of bark crevices also influences prey abundance, and older trees with deeply fissured bark are preferred foraging habitat for species such as the Brown Creeper, which probes the crevices searching for insect prey. Less common species like the Cerulean Warbler tend to be highly selective in habitats used for foraging, while more common species, such as the Red-eyed Vireo, are not as picky about where they forage. Ultimately, the diversity of microhabitats within a forest will strongly influence the bird species that it supports. Maximizing structural diversity and plant diversity, and retaining specialized habitat features like fallen dead wood, cavity trees, snags, and large trees will cascade up the food chain to increase bird diversity. Male Cerulean Warbler with caterpillar — Photo: Greg Lavaty Forest Birds in Ontario 21

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