Detroit River-Western Lake Erie Basin Indicator Project
On this page:
- Status and Trends
- Management Next Steps
- Research/Monitoring Needs
- Links to more information
- Contact Information
- Detroit River-Western Lake Erie Basin Indicator Project home page
- All indicators in alphabetical order
Birds of prey are indicators of ecosystem health due to their terminal position in the food web. Since a number of contaminants bioaccumulate through food webs, avian predators are usually the first wild species to show ill effects, such as failure to reproduce, egg shell thinning and nesting failure, or death through poisoning. Heavy metals and chlorine-based pesticides such as DDT, aldrin, dieldrin and heptachlor have been implicated in causing such wildlife impacts.
The geography of the eastern Great Lakes, combined with the migratory preferences of North American birds of prey, provide unique opportunities to monitor status and trends of raptor populations at the mouth of the Detroit River. The Detroit River is at the intersection of the Atlantic and Mississippi Flyway making it a unique area to survey migrating birds, especially raptors. As the raptors move south from their eastern Canadian breeding grounds, they are blocked by the north shore of Lakes Erie and Ontario. Thermals (rising columns of warm air) do not form over water so the birds are forced in one of two directions: east around Lake Ontario or west around Lake Erie. Those that move west follow the north shore of Lake Erie, until they reach the mouth of the Detroit River. Turning back is not an option so the birds fly over a 4-mile span of water to southeast Michigan, specifically near Lake Erie Metropark and Pointe Mouillee State Game Area. They lose altitude as they cross, making it easier for them to be observed (Figure 1). Volunteer monitoring programs such as Southeastern Michigan Raptor Research (SMRR) and Holiday Beach Migration Observatory (HBMO) have proven invaluable in monitoring fall raptor migrations. Hawk watches are conduced yearly during the months of September, October, and November at narrow points that avoid the expansive lakes (Figure 2) . A total of 23 total raptor species have been observed (16 regularly occurring species).
The early seasons of hawk migration studies by SMRR were exploratory, hours and count locations were not consistent. However, by 1991 these were more standardized, thus trend analysis begins with the 1992 season. All 16 regularly occurring species have increased since 1992 (though more detailed analysis to test for statistical significance is needed). Figure 3 illustrates a general upward trend of the Red-shouldered Hawk sightings (SMRR). While the trend is encouraging, the percentage of immature birds for 2001, 2002, and 2003 was 20%, 11%, and 21%, respectively (SMRR). These percentages of immature birds are low compared to other monitoring areas. The cause or causes of this lower then expected recruitment is unknown and likely requires research on their nesting grounds.
Figure 4 shows significant increase in Turkey Vulture numbers (SMRR). In recent years this species has continuously broken records with every season that passes. In 2004, the count was 63% above the ten year average with a high of 12,131 vultures seen in one day (Figure 5). Most hawk watches (i.e., all but one) throughout the Central Continental Flyway recorded increases as well (Berardi 2004).
Figure 6 shows the substantial increases of Peregrine Falcons, Osprey, and Bald Eagles during the 13-year period on record (SMRR). Hawk watches throughout the Central Continental Flyway have noted this increase as well (Berardi 2004). Osprey and Bald Eagles will spend considerable time in the study area, however only migrating birds are counted because count protocols prevent the inclusion of transient and nesting birds. In 2004, Osprey and Bald Eagles where 65%, and 69%, respectively, above the 10-year averages. In 2004, only 36 Peregrine Falcons were observed – this was 36% below the 10-year average.
The Red-tailed Hawk has shown a very stable population size since 1992 (Figure 7). Counts have fluctuated up and down, with no clear increasing or decreasing trend.
Reaching our long term goal of sustainable raptor populations will require increasing the amount of foraging and nesting habitats conserved and restored. For example, management of Red-shouldered Hawks requires conservation and restoration of habitats such as damp woods, river bottomlands, and swamps with tall trees where they can nest 20-60 feet above the ground. Perches with a wide field of view should be constructed to hunt from in areas where such prey as rodents, birds, frogs, and snakes live (Fergus 2004). However, a quantitative population size target for these species cannot be determined at this time.
SMRR monitoring consistently during fall migration should continue. Improvements to the already existing program should include continued recruitment of volunteers, more funding for paid staff (counters and banders), and more public outreach. Despite the "limited" size of the database, preliminary research efforts in eastern Canada might explain some trends that have already been noted (in particular, the Red-shouldered Hawk adult/immature ratios). Studies should be done to determine which raptor species are nesting in southeast Michigan, including identification of nest locations. More research needs to be undertaken to determine what environmental factors may be contributing to fluctuations in observed numbers.
Berardi, V. 2004. Fall 2003 Central Continental Flyway Report, August 2004. HMANA Hawk Migration Studies Volume XXIX No. 1
Cypher, P. 2005. "Hawk Migration Studies by Southeastern Michigan Raptor Research at the Detroit River" (Unpublished).
Fergus, C. 2004. Pennsylvania Game Commission, Wildlife Notes http://www.pgc.state.pa.us/pgc/cwp/view.asp?a=458&q=150468
Hawk Migration Association of North America Raptors Online Project, 2004.http://www.hawkcount.org
Southeastern Michigan Raptor Research http://www.smrr.net/
Holiday Beach Migration Observatory http://www.hbmo.org
The Hawk Migration Association of North America (HMANA) www.hmana.org
Paul Cypher, President
Southeastern Michigan Raptor Research