Animal Concentrations

OVERVIEW 

Wildlife species may be highly concentrated within relatively small areas seasonally or at various points throughout their life cycles. Fish and wildlife often gather in concentrations for critical activities, such as feeding, breeding/spawning, and migrating. Animals might also form large groups when an important resource is naturally limited in the landscape, such as fresh water in the desert or mineral springs in mineral-poor areas, or as a strategy to deter predators. For example, frogs and toads tend to gather together for a short burst of spring breeding when they have a limited window of opportunity for egg-laying in seasonal ponds. Animals also tend to congregate when their food is concentrated, and migrating animals often gather at critical feeding sites along migration routes to refuel and rest. 

People have long appreciated the spectacle of thousands or millions of animals gathered in one area. Oregonians enjoy wildlife viewing and there are many opportunities to observe animals firsthand throughout the state. For example, there are several popular bird festivals that celebrate seasonal animal gatherings, including wintering Bald Eagles and migrating songbirds, shorebirds, and waterfowl. Salmon, one of the iconic species of the Pacific Northwest, can be seen spawning all across Oregon during the fall months. There are also a number of salmon spawning festivals throughout the state celebrating the return of the fish to their spawning grounds.  

Oregon is home to some remarkable concentrations of wildlife. The Klamath Basin in Southern Oregon and Northern California hosts the largest assemblage 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 pastures year-round. At many of Oregon’s mountain lakes and ponds, western toad tadpoles swarm in large masses in the summer, and can be seen climbing out onto land in large groups in the early fall.  

Similarly, large numbers of migrating fish can be seen when spawning. Salmon and trout species can enter small tributaries in large numbers to build redds and deposit eggs. Pacific herring seasonally return to Yaquina Bay and other estuaries along the coast to spawn. Native cyprinids can develop vibrant spawning colors and provide great viewing opportunities. Freshwater mussels can also occur in very dense beds containing thousands of individuals.    

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. In south central Oregon, Lake Abert has provided essential habitat and food sources for a myriad of migrating birds, once supporting the largest number of Wilson’s Phalaropes in North America. However, this habitat has been threatened by multiple impacts from Climate Change and the Disruption of Disturbance Regimes. In turn, the numbers of birds visiting each year has dropped, demonstrating how fragile these critical habitats are.  

When animals gather in these large groups, they can become particularly vulnerable to threats including climate change, disease, overexploitation, habitat alteration, and human disturbance. Because of the large number of individuals involved, any factors that impact critical sites can affect a large proportion of a population or an entire suite of species. Habitats that support animal concentrations provide important features such as cover and protection, abundant food sources, or nesting and breeding sites. The availability of these habitats may be limited, and impacts to these concentration areas may have a disproportionate effect on wildlife populations by influencing survival and reproduction. The Fish and Wildlife Habitat Mitigation Policy is one tool to help avoid, minimize, or mitigate the impact of development on many these limited, irreplaceable, and essential habitats for fish and wildlife species, populations, or species assemblages. 

Identifying the most important sites is the first step in conserving animal concentrations. One approach to identifying these areas is The National Audubon Society’s Important Bird Areas program, which recognizes the importance of migration stopovers and other areas where birds concentrate. Another approach are the Conservation Opportunity Areas, which include many, but not all, of the state’s animal concentrations. For animal concentrations, appropriate conservation actions depend on the species and site, but should focus on maintaining or restoring important habitat features.  

The table below summarizes some of Oregon’s animal concentrations and the important habitat types and features that support them. 

ANIMAL CONCENTRATIONS, HABITAT TYPES, AND FEATURES 

Animal Concentration	Important Habitat Types	Important Habitat Features
Bald Eagles: wintering	Large lakes, estuaries, bays, and rivers.	Areas of open water for foraging. Large trees or snags within a forest stand are used for communal roosts and for perching.
Bat roost sites (particularly hibernacula, maternal roosts, or diurnal roosts)	Depends on bat species. Includes caves, mines, cliffs, bridges, and tree cavities.	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 generally include warmer sites, such as low valleys and southern slopes, with lower snow depth, further from roads open to motorized vehicles.	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 fences, roads, and development are important.	Herds need connectivity between winter and summer ranges via corridors with forage and cover to provide safe passage.
Freshwater mussel beds	Permanently inundated habitats, such as perennial rivers, streams, lakes, and ponds.	Native freshwater mussels require clean water with low contamination and sedimentation and natural water flow regimes. They need burrowing substrate, stable habitat features, and the presence of specific host fish species.
Nesting colonies (rookeries): Great Blue Herons	Variable. Usually riparian, estuarine, wetland, marsh, or flood-plain habitats.	Herons require large trees near foraging areas (open grassy and wetland habitats) and low levels of human disturbance during the nesting season.
Lampreys (juveniles concentrate in high densities)	Freshwater habitats.	Lampreys may prefer low-gradient floodplain habitats and lower mainstem river channels.
Pond-breeding amphibians (toads, frogs, salamanders)	Ponds and other shallow wetlands. Temporary ponds may dry up each summer but provide essential breeding habitat.	Breeding habitats must retain sufficient water long enough for tadpoles to metamorphose into adults, 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 with suitable perches where prey are often concentrated, e.g., 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 and breeding areas	Big sagebrush	Open areas are used by males for courtship. Typically nest under sagebrush with large canopies. 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 require deep soil for burrowing (Tufted Puffin and Fork-tailed 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 undisturbed open, moist, muddy or sandy areas with high invertebrate prey density.
Snake winter hibernaculum	Burrows, caves, rock crevices, and lumber or rock piles	Habitats that maintain above freezing temperature, adequate ventilation, and moisture.
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 food 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-moving stretches of rivers or side channels.	Maintaining shallow mountain lake habitats, including native aquatic and lakeside vegetation, and slow-moving water is important for young herps.
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 areas.	Large hollow trees and snags are important for nesting and roosting. Chimneys (which ‘imitate’ hollow trees) may also be used.