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COSEWIC Assessment and Update Status Report on the Least Bittern in Canada 2001


Least Bitterns nest in freshwater marshes, with dense tall aquatic vegetation, interspersed with clumps of woody vegetation and open water.  They are most regular in marshes that exceed 5 ha in area.  Smaller marshes may be used on occasion, but do not sustain populations.  In the northern part of their range they are most strongly associated with cattails (Typha), which is the most common tall emergent (Gibbs et al. 1992), but they may also nest in bulrush (Scirpus), reed grass (Phragmites), horse tail (Equisetum), sedges (Carex), grasses (Graminaceae), Willows (Salix), and dogwood (Cornus) (Peck and James 1983).

Destruction of wetland habitat is the greatest single threat to Least Bitterns (Gibbs et al. 1992).  Drainage for agriculture has been the principal reason for the conversion of more than 70% of southern Ontario's pre-settlement marshes (Bardecki 1981). Losses to urbanization have taken more than 40% of Lake Ontario shoreline marshes, and more than 80% in the most heavily populated sections (McCullough 1981).  More than 90% of the original marshes in southwestern Ontario are now gone (Snell 1978).

In Quebec filling and draining of marshes for agriculture and urban development have been identified as major causes of wetland losses in the St Lawrence Lowlands (Lands Directorate 1986).  In the United States, more than 1.9 million hectares of wetland habitat was lost in only two decades from the mid-1950s to the mid-1970s (Tiner 1984).

Remaining wetlands are still being degraded by continuing development.  In Ontario for instance, the Spring 1995 issue of Seasons magazine, for example, cites an aggregate company threatening a Class 2 wetland, an urban development proposal threatening a Class 1 wetland, and a highway extension proposal impacting various smaller wetlands.  Surviving marshes are often surrounded by development, regularly disturbed by people and their pets, subject to raccoon populations enhanced by urban situations, and generally poorly suited any longer for this bittern.

The new Planning Act in Ontario attempts to streamline planning, but reduces the possibility of input from the Ministry of Natural Resources or the general public, and removes considerable protection from wetlands on the Canadian Shield. Weakened protective legislation in the interests of streamlining processes may save costs, but raises concern about whether we can effectively protect species such as the Least Bittern as development continues to eat away at wetlands.

Runoff from agricultural fields may also pose threats to wetland habitats (Gibbs et al. 1992). However, quantitative data is missing to accurately assess this threat.

Because Least Bitterns tend to fly very low, collisions with cars, fences, and transmission wires are another important source of mortality (Gibbs et al. 1992).  If development is allowed through or too close to wetlands, the habitat is obviously degraded for the bitterns.  But, if wetlands can be left undisturbed and unpolluted, Least Bitterns are relatively tolerant of human presence within reasonably close proximity. Preservation and protection from pollution and runoff are the most urgent long-term needs (Gibbs et al. 1992).