Birds respond to structure
Birds do not count trees the way a forester does. What matters to them is the shape of the forest: how tall the canopy is, how much of the space between the ground and the treetops is filled with branches and leaves, how many dead or hollow trees remain, and how open or closed the overhead cover is. Two forests with the same tree species can support quite different bird communities if one has a single even layer and the other has several.
Because of this, ornithologists and foresters often talk about forest birds in terms of the layer or feature they depend on, rather than only in terms of species lists.
Nesting places
Different birds nest in very different places, and a forest that offers only one kind of nest site will support only some of them.
Cavities. Woodpeckers excavate holes in dead or decaying wood, and many other birds, such as chickadees, nuthatches, some owls and some ducks, use holes that already exist. Cavity nesters are often limited by the number of suitable trees. A forest with few standing dead or decaying trees can have plenty of food and still few of these birds. This is one reason that deadwood habitat is considered so important, and why dead standing trees are often left in place when conditions allow.
Canopy nests. Many species build open cup or platform nests high in the crown. Larger birds such as hawks often favor tall, mature trees with strong branches and stable forks. They tend to need trees large enough to hold a heavy nest.
Shrubs and dense low growth. A number of songbirds nest close to the ground or in dense shrubs, where leaves hide the nest from predators. Where deer or livestock eat the understory heavily, these birds can decline. The condition of understory plants matters to them for both cover and food.
The ground. Some forest birds nest on the ground, among leaf litter, roots or fallen logs. They are exposed to ground predators and to disturbance from trampling, so they often depend on thick cover and on quiet, undisturbed areas.
Feeding needs
A nest site is only part of what a bird needs. Adults must find enough food to raise young during a short breeding season, and for many forest species that food is insects.
- Insect eaters (warblers, flycatchers, many woodpeckers) depend on a good supply of caterpillars, beetles, ants and other invertebrates. The timing of leaf growth, which shapes when caterpillars appear, can matter to chick survival. Changes in spring phenology are one of the ways climate shifts may affect nesting success, though the effects differ among species and regions.
- Seed and nut eaters depend on seed crops that can vary greatly from year to year, as in mast seeding, so their numbers and movements may rise and fall with the supply.
- Fruit and berry eaters rely on shrubs and trees that fruit at the right time of year, often at the forest edge or in gaps.
- Predators and scavengers, such as owls and hawks, depend on a healthy supply of small mammals, birds and other prey, and often need large territories.
Structure at larger scales
Birds also respond to the arrangement of habitat across a landscape.
Interior and edge. Some species, often called forest-interior birds, tend to do better away from forest boundaries. Near edges, nests may face more predation or nest parasitism, and the microclimate can differ. Other species prefer edges or open young forest. Neither group is better; they simply need different things. Studies of birds in small forest fragments commonly report declines in some interior species, though results vary by region and by the surrounding landscape.
Age and stage. Young regrowth, mid-aged closed stands and old forest each host different birds. A landscape that contains a mix of stages, including some mature forest, usually supports more species overall than one dominated by a single age. Birds that rely on young, open habitat may benefit from recently disturbed ground, while others depend on large old trees and canopy gaps in mature forest.
Size and connection. Many birds need territories of a certain size, and some need large forest blocks. Connected habitat lets young birds disperse to new areas.
How management can help
Practices that are commonly suggested for bird-friendly management include:
- keeping some large old trees and standing dead trees where it is safe to do so
- retaining a layered structure, with understory and shrubs, rather than clearing everything beneath the canopy
- avoiding disturbance near known nests during the breeding season, which varies by species and region
- keeping a variety of tree species and ages across a property
- protecting streamside strips and wet areas, which often hold more insects and more birds
These ideas are discussed more fully for small properties under wildlife habitat management. Because needs differ so much between species, a local bird guide or wildlife agency is a better source than any general rule for a particular place.
A caution on generalizing
Bird populations respond to many things at once, including conditions far from the forest. Many forest birds migrate, so a decline in one place may reflect conditions on a wintering ground or along a migration route. Simple cause and effect is often hard to prove, and researchers rely on long-term surveys and tools such as sound recorders to track change. Large-scale bird counts organized through citizen science have helped document trends over many years.
The practical message is straightforward. A forest that offers layers, dead and living old trees, dense cover, a reliable food supply and enough connected area will generally support more kinds of birds than a forest that is uniform, tidy or small.