3 years ago

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

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

Wood Thrush

Wood Thrush nestlings — Photo: OMNR they have a full clutch (three to five eggs for most species, but up to eight eggs for some cavity nesters). For many, the eggs will hatch in approximately 12–14 days, and the young will stay in the nest 10–14 days before they are big and strong enough to leave (fledge). Eggs and young birds (nestlings) are popular food for many nest predators. On average, a cup nest has less than a 50 percent chance of seeing any young leave the nest, but primary cavity nesters, like woodpeckers, tend to be more successful. Nest predation accounts for most nest failures (80 percent), though some nests may fail because of bad weather, or young may die of starvation. The suite of nest predators includes: small mammals (mice, shrews, and chipmunks), larger mammals, (squirrels, raccoons, cats, opossums, and weasels), avian predators (American Crow or Blue Jay), and snakes. If a nest does get destroyed, adults will attempt to re-nest until they are successful or they run out of time and need to migrate south. Although nest predation is a natural part of evolutionary history, predation levels can be greatly increased by human activities such that these bird populations are no longer sustainable. In addition to the direct loss of eggs and nestlings to predation, many unsuspecting birds lose out without even knowing it — because of cowbirds. The Brown-headed Cowbird is a brood parasite, common to southern Ontario’s highly fragmented landscape. Brood parasites never build a nest or raise their own young. Instead, they lay their eggs in other birds’ nests, often removing one or two of the host eggs in the process. Female cowbirds are prolific breeders and can lay up to 40 eggs each breeding season. Host parents raise the cowbird young alongside their own; often at the expense of the host’s young (see photo below). Cowbird nestlings usually hatch earlier, are faster growing, and more aggressive than host young, outcompeting them for food, resources, and space. As a result, fewer, often less healthy host young, or sometimes no host young fledge from a parasitized nest. This can substantially lower the number of young produced by each pair, especially when some nests may contain two or three cowbird eggs. Cowbirds are dependent on open fields, crops, and pasture for foraging on seeds and insects. Prior to European settlement, cowbirds were restricted to the buffalo-grazed and firemaintained grasslands of the Great Plains of central North America. However, with the clearing of eastern forests for agriculture and settlement in the 1800s, cowbirds were able to expand their range eastward and in the process exposed many new bird species to nest parasitism. Unlike species that developed with cowbirds and learned to reduce their impact, (like removing cowbird eggs from the nest or abandoning parasitized nests), species with no long term exposure to cowbirds have no defence and can be particularly vulnerable. For example, nearly half of all Wood Thrush nests are parasitized in some parts of southern Ontario. This can significantly reduce Wood Thrush productivity to half that of unparasitized nests. Together nest predation and nest parasitism have the potential to reduce the nesting success for many forest songbirds. This can be especially true in highly fragmented landscapes, where additional food in the form of crops elevates populations of some nest predators (like racoons) and where grass exists for cowbirds Blue-gray Gnatcatcher host parent feeding Brown-headed Cowbird nestling — Photo: G.K. Peck Squirrel eating nestling bird — Photo: Bob McBroom 16 Forest Birds in Ontario

Female American Redstart — Photo: Nick Bartok to forage on. Yet, these are not the only factors affecting productivity. Food availability within a woodlot can have a substantial influence on the nesting success of forest birds. Habitats with poor food resources will result in reductions in the growth of young, their survival, the number of nesting attempts, and the number of young that survive until they are independent. Food can vary annually due to natural variation in insect populations; but it is also highly dependent on vegetation structure and the landscape features. For most species, once the young have fledged from the nest, they are dependent on the parents for food and protection for two to three weeks. This is a time when many young die. The first few days, young cannot fly and stay relatively close to the nest site. Thereafter, they move further away, often into more protected areas with dense, low trees and shrubs, and begin to forage for themselves. By the end of the third week they are totally independent. Finally, at the end of the breeding season, in August or September, adults gradually replace all their old feathers with new ones. This process is called molting. They also build up fat reserves for their fall migration back to the wintering grounds. Prothonotary Warbler at nest cavity — Photo: Mark Peck Habitat Selection Wildlife needs a place to live. For humans, this place is called home, but for wildlife, this place is called habitat. Habitat includes food, water, shelter, and space. When all parts blend together an individual not only survives, but thrives. The quality of the habitat for a bird is based on how well it can provide food, water, cover, and nest sites. Not all habitats are equal, and each bird species has a unique set of requirements. Some, habitat specialists, are very specific in their requirements, like the Prothonotary Warbler which requires cavities in large, living trees or snags (standing dead trees) in flooded woodlands for its nest site. Other species, like the American Robin, can adapt easily to changes in the environment, and are called habitat generalists. This species can nest in the canopy of a large tree in a forest, in the eaves trough of your house, or virtually anywhere in between. Usually, the more varied the habitat conditions, the greater the variety of wildlife or biodiversity that habitat will support. This diversity is intimately linked to the structure, age, and composition of the forests. Vertical structure, an important feature of a habitat, is the extent to which plants are layered within a stand. This arrangement of plant growth forms (trees, vines, shrubs, herbs, etc.) combined with the distribution of trees of different species, age, height, diameter, and crown characteristics determines the vertical structure of the forest. Prothonotary Warbler nesting habitat — Photo: Al Woodliffe Forest Birds in Ontario 17

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