How common are Sharp-shinned Hawks?

Introduction to Sharp-shinned Hawks

The Sharp-shinned Hawk (Accipiter striatus) is a small hawk of the genus Accipiter. They are found in most of North America. They are widespread and fairly common, but also secretive and easy to overlook. Sharp-shinned Hawks prey on small birds, and their presence can have an effect on local bird populations. They are an important part of many ecosystems. Understanding how common they are can help us appreciate their role.

Population and Distribution

The Sharp-shinned Hawk has a large range across North America. Their breeding range covers most of Canada and Alaska, down through the western United States into Mexico. On the east coast, they breed from Maine down to Florida. Their wintering range covers most of the United States and some of southern Canada.

Within this broad range, they can be found in a variety of habitats. They are most common in forested areas, especially deciduous or mixed woods. They can also be found along forest edges and in wooded suburban areas. Less commonly, they inhabit more open habitats like parks, grasslands, or marshes.

Across their range, Sharp-shinned Hawks are considered common as a breeding species. But their exact population levels are uncertain due to their secretive habits. Partners in Flight estimates a global breeding population of about 1.2 million. Christmas Bird Counts give some indication of winter populations, with recent counts of around 15,000 birds in the U.S. and 30,000 in Canada. Breeding Bird Surveys suggest populations have increased since the 1960s and are still increasing in many regions.

Seasonal and Regional Variation

Sharp-shinned Hawk populations fluctuate seasonally as birds migrate. In the summer breeding season, populations are centered in Canada and the northern U.S. In winter, most migrate south into the southern U.S. and Mexico.

Regional populations also see fluctuations. For example, in the Great Lakes region numbers are lowest in mid-winter, peak in mid-March through May, decline through summer, then rise again in fall.

Seasonal abundance also varies by habitat. In winter, migrant birds increase numbers in open woodland and urban areas. In summer, populations are denser in boreal and montane forests.

Population Density

Population density of Sharp-shinned Hawks varies across their range. Some examples of reported densities include:

• Boreal forest in Alaska: 1-6 pairs per 100 km2
• Maine hardwood forest: 1 pair per 70-190 km2
• Michigan mixed forest: 1 pair per 60-170 km2
• Ontario mixed forest: 1 pair per 60 km2
• Manitoba spruce forest: 1 pair per 120 km2
• British Columbia interior: 1 pair per 8-20 km2
• Northwest Territories boreal: 1 pair per 45 km2
• Oregon Cascades montane forest: 1 pair per 2 km2

In general, densities are lowest in interior continental boreal forests and highest in montane forests and northern hardwoods. But much regional variation exists.

Population Trends

After likely population declines in the early 1900s due to persecution and pesticide use, Sharp-shinned Hawks have rebounded. Breeding Bird Survey (BBS) data indicates populations increased significantly from 1966 to 2015 across North America.

However, trends vary by region:

• Increased by 2.7% per year in the western U.S.
• Increased by 3.8% per year in the eastern U.S.
• No significant change in Alaska and Canada

Christmas Bird Count data also shows increases during 1975-2017 of over 3% per year.

Regional differences may reflect habitat change. Increases in the east may correlate with maturation of woodlands that were cleared for agriculture in the 1800s. Lack of change further north may indicate stable boreal habitats.

Factors Affecting Populations

Several key factors influence Sharp-shinned Hawk populations:

Habitat

Sharp-shinned Hawks require areas with adequate forest cover for nesting and hunting. Habitat loss can cause declines. Their recovery correlates with reforestation in parts of their range.

Prey Availability

Cycles in small bird populations, especially passerines like sparrows, influence Sharp-shinned numbers. When prey is scarce they may breed less or not at all.

Competition

Other accipiters like Cooper’s Hawks occupy similar niches and likely compete for resources. But effects are complex and uncertain.

Climate Change

Warming temperatures could affect prey species distributions, change northern habitats, and shift migration patterns. This may favor the flexible and widespread Sharp-shinned Hawk.

Pollutants

Past pesticide use caused declines. Ongoing exposure to toxins like mercury could still pose risks, but impacts are unclear.

Monitoring Methods

Several methods help monitor Sharp-shinned Hawk populations:

Breeding Bird Surveys

Experts conduct annual 1-2 mile point count surveys along thousands of routes across North America. Sightings provide index of population changes over time.

Christmas Bird Counts

Volunteers perform annual one-day counts within circumscribed areas over the winter. Gives snapshot of wintering numbers.

Migration Counts

Dedicated raptor migration sites tally passing hawks during spring and fall. Reveals migration timing and densities.

Nest Searches

Finding and monitoring nests gives direct data on reproduction and densities. But nests are hard to locate in dense forests.

Telemetry Studies

Tracking tagged birds provides detailed movement data. Small sample sizes limit larger inferences.

Research Needs

Further research could improve understanding of Sharp-shinned Hawk populations:

– Broader nest searching efforts to clarify densities in less-studied areas and habitats.

– Banding studies to better understand migration patterns, survivorship, and dispersal.

– Investigation of competition dynamics with other accipiter hawks.

– Breeding ecology studies to quantify reproduction rates and success.

– Climate change impacts on habitat distributions and prey.

– Genetic analyses to map connectivity of regional populations.

Conclusion

In conclusion, Sharp-shinned Hawks are a fairly common species across most of North America. Their broad range covers diverse habitats from boreal forest to temperate woodlands. Estimated populations number over one million, though local densities vary widely. After declines last century, they have rebounded in many regions but face ongoing threats like habitat loss and climate change. Ongoing monitoring and research is elucidating their status and ecology to support informed conservation efforts. Though common, the secretive Sharp-shinned Hawk remains a fascinating window into the lives of raptors.