Least Bittern (Ixobrychus exilis) Michael Sanders Status: State Threatened
Holland, Ottawa Co., MI. 7/3/2006 © Carl Manning (Click to view a comparison of Atlas I to II)
late 1950s and early 1980s was largely due to The Least Bittern is extremely difficult to habitat loss. The species was detected at only locate due to its secretive nature, inaccessible 10% (11 of 108) of sites inventoried during a habitats and lack of territorial defense calls four-year statewide wetland bird survey of (Woodliffe 1987). Roughly half the size of the coastal wetlands (Sanders et al. 2007). more familiar American Bittern, this small heron is one of Michigan’s most inconspicuous Breeding Biology marsh inhabitants. Least Bitterns breed from Least Bitterns are more habitat-specific than the the eastern half of the United States south to larger American Bittern, preferring freshwater Central America and the West Indies (Kleen et marshes with equal ratios of emergent al. 2004). Its wintering range extends along the vegetation and open water (hemi-marshes) Atlantic coast from Maryland to Texas, through (Gibbs et al. 1992, Monfils 2003). Yocum the Rio Grande Valley, and into the Baja (2003) indicates a preference for marshes Peninsula (Gibbs et al. 1992); the northern dominated by cattails, while Brown and winter limit of Least Bitterns is considerably Dinsmore (1986) suggest that the species may farther south than that of the hardier American be area-dependent, requiring a minimum of 4.9 Bittern (Gibbs et al. 1992). The eastern UP hectares for suitable breeding habitat. represents the northern limit of the species’ North American distribution (Cornell Lab of A Least Bittern nest is an elevated platform Ornithology, 2010). consisting of aquatic vegetation and sticks suspended above shallow water (Gibbs et al. Distribution 1992, Peterjohn and Rice 1991). They are Least Bitterns were historically common in placed in tall, dense stands of emergent Michigan. The Least Bittern was “an abundant vegetation over water 10-75 cm deep, and are bird in all suitable places in the state” in 1900 typically located only a few meters from a (Barrows 1912). Wood (1951) described it as a nearby opening (MNFI 2007). After hatching, common summer resident in southern Michigan the semi-altricial young can fledge at four to but rare and local throughout the Upper five days, but remain nearby to be fed for three Peninsula. Adams et al. (1991) indicated that a to four weeks (Baicich and Harrison, 1997). decline in Least Bitterns in the state between the Population densities within suitable habitat may
© 2011 Kalamazoo Nature Center Least Bittern (Ixobrychus exilis) Michael Sanders Status: State Threatened be large (Keller and Castrale 1998); Yocum across the Great Lakes region. The Least (2003) observed 16 active nests within a 105- Bittern is currently endangered in Illinois and acre impoundment at Nayanqing Point along Indiana, and threatened in Michigan, New York, Saginaw Bay. Departure dates are uncertain Ohio and Ontario (Woodliffe 1987). Habitat because of the secretive nature of this species, loss, particularly the drainage of wetlands for but probably begin in September and continue farming and urban development, is likely the into October (Adams 1991). During migration, primary factor in the species’ decline. Least Bitterns can be observed in small marshes and narrow strips of cattails bordering ponds Conservation Needs and lakes (Peterjohn and Rice 1991). The protection and proper management of large shallow wetlands with abundant emergent Abundance & Population Trends vegetation is the most urgent conservation need Results from MBBA II indicate that the overall of the Least Bittern (Gibbs et al. 1992). range of the species has changed little from Roughly five million acres of palustrine MBBA I. Least Bitterns have remained most wetlands were eliminated in the United States common in the SLP, with the number of from the mid-1950s through the mid-1970s observations declining northward into the NLP. (Gibbs et al. 1992); within Michigan, Cwikiel Relative strongholds continue to be the St. Clair (1998) reported that nearly half of the state’s Delta and areas along Saginaw Bay. The pre-settlement wetlands have disappeared, species remains rare in the UP, where only including almost 70% of its coastal lowlands. seven observations were reported, although one Additionally, several prime breeding areas along was a confirmed breeding in southern Delta the Great Lakes were eliminated due to high County along Lake Michigan. This latitudinal water levels in the mid-1980s (Adams 1991). pattern might be explained by the lack of large expanses of suitable habitat in the Upper Extreme water level manipulations at Peninsula (Sanders et al. 2008). management areas may impede Least Bittern nesting success and recovery efforts (Yocum Total block-level occurrences of Least Bitterns 2003), while the overall impact of exotic and dropped by 16% (96 from 114) despite an nuisance species such as purple loosestrife and overall increase in coverage during the MBBA common reed is uncertain but likely negative II effort; similarly, township-level observations (Gibbs et al. 1992, Monfils 2004). Managing fell 21% (82 from 104). Breeding wetlands explicitly for the hemi-marsh stage confirmations dropped 50%, with only 10 would improve conditions for Least Bitterns and reported during MBBA II, seven of which other wetland species (Evers 1994, MNFI occurred in the SLP. 2009), especially if such areas were protected from chemical contamination and other forms of Analysis of BBS data from 1966-2007 show a pollution that harm the birds and/or their food substantial annual drop in Least Bittern supplies (Gibbs et al. 1992). numbers, although not at a level of statistical significance (-7.3% annually, P = 0.27) in the Upper Midwest region (Sauer et al. 2008). While these and other large-scale surveys do not adequately census secretive marsh birds, BBS data are useful for suggesting trends and patterns (Monfils 2003, Kleen et al. 2004); it is evident that Least Bittern numbers are in decline
© 2011 Kalamazoo Nature Center Least Bittern (Ixobrychus exilis) Michael Sanders Status: State Threatened Literature Cited Peterjohn, B.G. and D.L. Rice. 1991. The Ohio Breeding Bird Atlas. Ohio Department of Adams, R.J., Jr. 1991. Least bittern (Ixobrychus Natural Resources, Columbus, OH. exilis). In Brewer, R., G.A. McPeek, and Sanders, M.A., K.J. Borland, R.L. Rogers, and R.J. Adams, Jr., eds. The Atlas of Breeding D.L. Cuthrell. 2007. Identification of Birds of Michigan. Michigan State critical nesting habitat for wetland birds in University Press. East Lansing. Michigan: Western Upper Peninsula – Year Baicich, P.J. and C.J.O. Harrison. 1997. A Four Report. Report for Michigan Coastal Guide to the Nests, Eggs, and Nestlings of Management Program, Environmental North American Birds. Second Edition. Sciences Division, Michigan Department of Natural World, Academic Press, San Diego, Environmental Quality. Michigan Natural CA. Features Inventory report number 2007-19. Barrows, W.B. 1912. Michigan Bird Life. Sauer, J. R., J. E. Hines, and J. Fallon. 2008. Michigan Agricultural Experiment Station The North American Breeding Bird Survey, Bulletin No. 94. East Lansing. Results and Analysis 1966-2007. Version Bogner, H.E. and G.A. Baldassarre. 2002. 5.15.2008. USGS Patuxent Wildlife Home range, movement, and nesting of least Research Center. Laurel, MD.
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