The Northern Harrier (Circus hudsonius) is a medium-sized raptor found throughout much of North America. Harriers are distinctive birds of prey, recognizable in flight by their long, slender wings and tails. As their name suggests, Northern Harriers hunt by coursing low over open areas, using their acute hearing to locate small mammals, birds, reptiles, and insects in tall grasses and marshes. This hunting strategy requires specific habitat types that provide adequate cover and abundant prey. Understanding the Northern Harrier’s preferred habitats can help conservationists protect vital areas for these declining raptors.
What types of habitats do Northern Harriers prefer?
Northern Harriers thrive in open habitats with low, dense vegetation that allows them to hunt while flying low to the ground. Their preferred habitats include:
- Marshes – Freshwater and brackish marshes provide ideal Northern Harrier habitat. The dense grasses and sedges offer cover for small mammals and birds while allowing the harriers to cruise low over the vegetation.
- Grasslands – Native prairies, pastures, hayfields, and grasslands with low vegetation are commonly used by hunting Northern Harriers. Areas with thick ground cover are especially attractive.
- Agricultural fields – Harriers often forage over harvested croplands, particularly corn and winter wheat. These agricultural habitats mimic grassland environments.
- Coastal habitats – Salt marshes, dunes, meadows, and shorelines are occupied by Northern Harriers, especially outside the breeding season.
- Early successional habitat – Recently logged areas, forest openings, abandoned farmland, and other early successional habitat with low woody regrowth are frequently utilized by Northern Harriers.
- Wetlands – Sedge meadows, bogs, fens, and wetlands with tall emergent vegetation provide cover where the birds can hunt voles, frogs, and other wetland prey.
In general, any habitat with dense, low vegetation and abundant small mammals, birds, reptiles, or insects may attract hunting Northern Harriers.
Why are open habitats important for Northern Harriers?
The Northern Harrier is morphologically and behaviorally adapted to hunt in open habitats close to the ground:
- Their long, broad wings and long tails allow superb maneuverability and gliding close to vegetation.
- Their owl-like facial discs focus sound waves towards their ears, allowing them to detect prey by sound alone.
- Their eyes are placed high and towards the front of their heads, giving wide visual fields for spotting prey while looking down as they fly low.
- Their long legs and toes are ideal for grasping terrestrial prey on the wing.
Dense ground cover provides concealment for harrier prey like voles, sparrows, and short-eared owls. It also obscures the harriers from potential prey. Hunting close to the ground allows Northern Harriers to use vegetation to mask their approach until prey is within striking distance. Open habitats with low vegetation provide the right conditions for the Northern Harrier’s specialized hunting behavior.
What vegetation density do Northern Harriers prefer?
Northern Harrier habitat includes open areas with vegetation dense enough to conceal prey but not so dense that it impedes the harrier’s flight. Specific vegetation density preferences include:
- Grass height of 30-60 cm
- Shrub cover less than 50%
- Trees absent or very sparse
Areas with tall, dense vegetation are less suitable, as prey animals are more difficult to access. Northern Harriers generally avoid forests and other habitats where vegetation is too thick for their coursing hunting style. A mosaic that combines open areas for foraging with scattered dense thickets for roosting and nesting provides optimal habitat. Maintaining a range of seral stages through disturbances like fire, grazing, or mowing is important for preserving Northern Harrier habitat.
How does habitat availability affect Northern Harriers?
The availability of suitable open habitat limits Northern Harrier populations across their range. Destruction and degradation of wetlands have caused significant habitat loss, especially in eastern North America. Agricultural intensification, reduction in native grazing, suppression of natural fire regimes, and reforestation have also reduced suitable habitat.
Smaller harrier home ranges are strongly correlated with higher habitat suitability. The availability of undisturbed nesting sites is a particularly important factor, as Northern Harriers nest on the ground, making their eggs and chicks vulnerable to predators and farming operations.
Fragmentation of their preferred habitat types into smaller parcels can lower prey populations, increase competition, and make harriers more susceptible to predators. Protecting large expanses of marshland, grassland, wetland, and early successional habitat is key for Northern Harrier conservation.
How do Northern Harriers use different habitats?
While open habitats are critical for foraging, Northern Harriers use a mix of habitat types:
- Foraging – Occurs primarily over grasslands, marshes, and wetlands.
- Nesting – Nests are built on the ground, often at wetland edges, in grassy fields, or on islands. Nests are concealed under dense vegetation.
- Roosting – Harriers roost in trees, on the ground, or on fence posts and other low perches, often at the border between open areas and woodlands.
- Migration and wintering – A wide variety of open habitats are occupied during migration and winter, including agricultural fields and coastal areas.
While foraging areas may be defended as territories during the breeding season, at other times Northern Harriers are often found concentrated in productive habitats like tidal marshes and agricultural areas where food is abundant.
How does habitat use vary geographically?
Across their extensive range, Northern Harriers use diverse open habitat types. Some key geographic differences include:
- In northeastern North America, harriers concentrate in large coastal and freshwater marshes.
- Midwestern harriers use wet and dry grasslands, wetlands, and agricultural areas like pastures and winter wheat fields.
- West coast harriers occupy estuarine habitats, open wooded wetlands, and coastal prairies.
- Southeastern harriers favor pine savannas and canebrakes with open understories.
- In Mexico, tropical grasslands and agricultural fields are important habitats.
Despite these regional variations, Northern Harriers consistently rely on habitats that support an abundance of accessible prey. The details of preferred vegetation structure vary, but openness remains key throughout their range.
How does habitat use change seasonally?
Northern Harrier habitat use varies over their annual cycle:
- Breeding (March – July) – Nesting and foraging occur in proximity, with marshes, wetlands, grasslands, and agricultural habitats used most heavily.
- Post-breeding (August – October) – Harriers disperse and use a wider variety of open habitats including barrens, coastal areas, and disturbed sites.
- Winter (November – February) – Coastal areas become more heavily used. Agricultural fields and grasslands in milder regions provide important winter habitat.
- Migration (September – November and February – April) – All types of open habitats along migration routes are utilized during migration stopovers.
Breeding Northern Harriers favor habitats that support nesting and maximize prey availability, since females must provision young at the nest. During migration and winter, prey concentrations drive habitat selection.
Conclusion
Northern Harriers thrive in open habitats that allow a low coursing flight style and provide abundant voles, passerines, reptiles, and other prey. Although they use a diversity of habitat types across their range, ample cover for concealed nesting and roosting alongside open areas for efficient foraging is key. Habitat loss and alteration have caused Northern Harrier declines, making thoughtful management and restoration of marshes, grasslands, coastal habitats, and wetlands essential for these unique raptors. Continuing research and mapping of areas used heavily by Northern Harriers can guide effective conservation planning.
