flock of birds in the air over a marsh
Photo Credit: Charlie Bruce. Caspian Tern Colony.


Many animals gather together in large groups for migration, breeding, or sheltering, and these concentrations can be vulnerable to disturbance. Identifying the most important sites is the first step in conserving animal concentrations. Approaches include The Audubon Society’s Important Bird Areas program, which recognizes the importance of migration stopovers and other areas where birds concentrate. Conservation Opportunity Areas include many, but not all, of the state’s animal concentrations. For animal concentrations, appropriate conservation actions depend on the species and site, but will focus on maintaining or restoring important habitat features.

Klamath Lake hosts the largest concentration of wintering Bald Eagles in the continental United States, with up to a thousand individuals. At Dean Creek Wildlife Viewing Area, numerous elk congregate in marshy fields during the winter. At many of Oregon’s mountain lakes and ponds, western toad tadpoles swarm in large masses in the summer, and begin to change into frogs and climb out onto land in large groups in the early fall. In Portland, crowds gather nightly every autumn to watch 35,000 migrating Vaux’s Swifts swirl and funnel into an old chimney at Chapman School, the largest known Vaux’s Swift roost in the world.

Estuaries and bays along the Oregon coast and the lakes of southeastern Oregon provide vital stop-over refuges for shorebirds migrating to and from southern wintering areas and nesting locations in Canada and Alaska. Lake Abert may support the largest number of Wilson’s Phalaropes in North America; up to 70,000 birds congregate here in late July.

People have long appreciated the spectacle of thousands or millions of animals gathered in one area. Oregonians enjoy wildlife viewing at several popular festivals that celebrate seasonal animal gatherings, including wintering Bald Eagles and migrating songbirds, shorebirds, and waterfowl.

Fish and wildlife often gather in concentrations for critical activities, such as feeding, breeding, or migrating. Some species breed in colonies, perhaps due to limited, specialized breeding sites or as a strategy to deter predators. Animals congregate when their food is concentrated, and migrating animals flock to a feeding site to refuel and rest.

Animals also might gather when an important resource is naturally limited in the landscape, such as fresh water in the desert or mineral springs in mineral-poor areas. Frogs and toads that breed in seasonal ponds tend to gather together for a short burst of spring breeding because they have a limited window of opportunity for egg-laying. When Pacific tree frogs gather to breed, a springtime chorus erupts as males sing to attract mates.

When animals gather in these large groups, they can become particularly vulnerable to habitat alteration and human disturbance. Because of the large number of individuals involved, any factors that impact highly critical sites can affect a large proportion of a species or an entire suite of species. The table below summarizes important habitat types and features for some of Oregon’s animal concentrations.

Animal Concentrations, Habitat Types, and Features

Animal Concentration Important Habitat Types Important Habitat Features
Bald Eagles: wintering Large lakes and rivers Large trees or snags within a forest stand are used for communal roosts.
Bat roost sites (particularly hibernacula, maternal roosts, or diurnal roosts) Depending on bat species, includes caves, mines, cliffs, bridges, and buildings Roost sites must have suitable temperature and humidity. Lack of human disturbance is critical for Townsend’s big-eared bat and pallid bat.
Deer and elk key winter range areas These vary by ecoregion but usually include warmer sites, such as lower valleys and southern slopes. Wintering areas include diverse forested landscapes with openings and a variety of age classes, perennial grasslands, and sagebrush steppe habitats. Woody vegetation for foraging (e.g., bitterbrush, aspen, alder, willow, oak), and cover for insulation and hiding are needed. Shrubs are important where snow is deep during winter.
Deer and elk herds: migration routes and transition range These vary by ecoregion and combine features of summer and winter ranges. Travel corridors unobstructed by roads and urban areas are important. Herds need forage and cover to provide safe passage between winter and summer ranges.
Freshwater mussel beds Aquatic habitats Freshwater mussels require clean water with low contamination and sedimentation and natural water flow regimes. They are important to tribal culture, filter water, are good indicators of high water quality, and are a key food source for fish, mink, otters, and raccoons.
Nesting colonies (rookeries): Great Blue Herons Riparian habitats Herons require large trees near foraging areas (open grassy and wetland habitats) and low levels of human disturbance during the nesting season. Great Blue Heron nesting colonies are declining and at risk in some areas, particularly in the Willamette Valley.
Lamprey (juveniles concentrate in high densities) Freshwater habitats Lamprey may prefer low-gradient floodplain habitats and lower mainstem river channels.
Pond-breeding amphibians (toads, frogs, salamanders) Ponds and other shallow wetlands. In many areas, these ponds are created by winter and spring rains, then dry up each summer. These temporary ponds provide essential breeding habitat. In order to support breeding amphibians, ponds and shallow wetlands must remain wet long enough for tadpoles to metamorphose, be relatively free of predators or disturbance, and provide sufficient food.
Raptors: migrating and wintering Fields and pastures, grasslands and prairies, sagebrush steppe, and wet meadows; ridges during migration Habitats where prey are often concentrated include open grassy areas for rodents, riparian and deciduous shrub communities for songbirds, lakes for waterfowl, and managed agricultural fields. Raptors use thermals over ridges for soaring.
Salmonid (salmon, steelhead, trout) juvenile rearing areas Estuaries, lakes, rivers, and streams These areas must have suitable habitat complexity, low temperatures, and low fine sediment loads.
Salmonid spawning and holding areas Streams, lakes, and rivers These areas must have suitable habitat complexity and low temperatures.
Greater Sage-Grouse leks Big sagebrush Cover of 15-50% is needed for nesting. Open areas are used by males for courtship. Areas rich in forbs, such as playas, meadows, and higher-elevation sagebrush-steppe habitats, are important for brood-rearing.
Seabird nesting colonies Coastal bluffs, offshore islands and rocks, and sandy islands Depending on the species, colonies may include deep soil for burrowing (Tufted Puffin and storm-petrels), rocky ledges (Common Murres), or unvegetated sandy areas (Caspian Terns). Isolation from mammalian predators and human disturbance is critical.
Seal and sea lion haul-outs and pupping areas Flat offshore rocks and isolated beaches Isolation from human disturbance is important.
Shorebirds: migrating and wintering Wet prairies, flooded fields, mudflats, alkali lakes, shorelines of wetlands and reservoirs, estuaries, and sandy ocean shore Shorebirds need open, moist muddy or sandy areas with high invertebrate prey density.
Songbirds: migrating Deciduous and mixed deciduous-conifer forests, high-elevation deciduous or mixed shrub communities, especially near water, and riparian habitat Migrating songbirds need deciduous trees and shrubs with high invertebrate prey density and cover for insulation and hiding. Forested buttes are important in urban and agricultural landscapes.
Tadpole aggregations (for example, western toad, Oregon spotted frog) Shallow areas in mountain lakes and ponds, slow stretches of rivers or side channels Maintaining shallow mountain lake habitats, including native aquatic and lakeside vegetation, is important.
Waterbird nesting colonies Lakes and marshes with both deep and shallow water Colony characteristics vary by species but include isolated and sparsely vegetated islands (American White Pelican), trees (Snowy Egret), and emergent vegetation (Eared Grebes). Isolation from mammalian predators and human disturbance is important.
Waterfowl and other waterbirds: migrating and wintering Wetlands, lakes, reservoirs, and estuarine bays Waterfowl need diverse water features with high food availability (aquatic plant, invertebrate, or fish) and open water for security.
Vaux’s Swift roosts Late successional conifers, urban and suburban Large hollow trees and snags are important for nesting and roosting. Chimneys (which ‘imitate’ hollow trees) may also be used.