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

  1. Table of Contents
  2. COSEWIC Assessment Summary
  3. COSEWIC Executive Summary from the 1999 Status Report
  4. Introduction
  5. Population Sizes and Trends
  6. Habitat
  7. Evaluation and Proposed Status
  8. Technical Summary
  9. Acknowledgements
  10. Experts Contacted
  11. Literature Cited
  12. The Author
  13. Addendum

COSEWIC status reports are working documents used in assigning the status of wildlife species suspected of being at risk. This report may be cited as follows:

Please note: Persons wishing to cite data in the report should refer to the report (and cite the author(s)); persons wishing to cite the COSEWIC status will refer to the assessment (and cite COSEWIC). A production note will be provided if additional information on the status report history is required.

COSEWIC 2001. COSEWIC assessment and update status report on the Least Bittern Ixobrychus exilis in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. v + 10 pp.

(http://www.registrelep-sararegistry.gc.ca/sar/assessment/status_e.cfm).

James, R.D. 1999. Update COSEWIC status report on the Least Bittern Ixobrychus exilis in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. 1-10 pp.

Previous report:

Sandilands, A.P. and C.A. Campbell. 1988. COSEWIC status report on the Least Bittern Ixobrychus exilis in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. 40 pp.

Please note the status recommended in the Section "Evaluation and Recommended Status" of the report may differ from the latest status assigned to the species by COSEWIC.

Également disponible en français sous le titre Évaluation et Rapport de situation du COSEPAC sur le petit blongios (Ixobrychus exilis) au Canada – Mise à jour.

Cover illustration:

Least Bittern -- Judie Shore, Richmond Hill, Ontario

©Her Majesty the Queen in Right of Canada 2004

Catalogue No. CW69-14/409-2004E-PDF

ISBN 0-662-38996-4

HTML: CW69-14/409-2004E-HTML

0-662-38997-2

COSEWIC Assessment Summary

Common name : Least Bittern

Scientific name :Ixobrychus exilis

Status :Threatened

Reason for designation : A very small and declining population that depends on high quality marsh habitats that are being lost and degraded across the species’ range.

Occurrence : Manitoba, Ontario, Quebec and New Brunswick

Status history : Designated Special Concern in April 1988. Status re-examined and confirmed in April 1999. Status re-examined and designated Threatened in November 2001. Last assessment based on an existing status report with an addendum.

COSEWIC Executive Summary from the 1999 Status Report

The Least Bittern is strictly associated with freshwater marsh habitats that have experienced tremendous losses during the past century. In 1988, when first designated by COSEWIC, most observers felt that the population was declining, although there were no specific surveys that clearly demonstrated a decline; and the total population was considered to be only in the order of 1000 pairs.

Since that time, there have been no definite data that give a clear idea of population size and trend. The Marsh Monitoring Program in the Great Lakes basin, initiated in 1994, may begin to provide information on this species in future. However, it is uncertain that the survey method, intended to survey many species simultaneously, will be adequate to provide reliable estimates for this species.

Most of the Canadian population breeds in southern Ontario, with few in southern Quebec and Manitoba, and a very few in the Maritimes. There is still a perception that the population is in decline, as the habitats it requires are still being assaulted by developmental and agricultural interests. Sustained high water levels, natural succession and siltation are impacting Great Lakes shoreline marshes, driving birds from some if not all such marshes. Most adjacent U.S. states also consider that the Least Bittern is in decline there.

Because the Least Bittern is a very secretive and quiet bird, it could all but disappear before we become aware of the fact. It is important to continue to review the status of this species and to review current attempts to assess its status, or to find a reliable census method for it.

The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) determines the national status of wild species, subspecies, varieties, and nationally significant populations that are considered to be at risk in Canada. Designations are made on all native species for the following taxonomic groups: mammals, birds, reptiles, amphibians, fish, lepidopterans, molluscs, vascular plants, lichens, and mosses.

COSEWIC comprises representatives from each provincial and territorial government wildlife agency, four federal agencies (Canadian Wildlife Service, Parks Canada Agency, Department of Fisheries and Oceans, and the Federal Biosystematic Partnership), three nonjurisdictional members and the co-chairs of the species specialist groups. The committee meets to consider status reports on candidate species.

Species : Any indigenous species, subspecies, variety, or geographically defined population of wild fauna and flora.

Extinct (X) : A species that no longer exists.

Extirpated (XT) : A species no longer existing in the wild in Canada, but occurring elsewhere.

Endangered (E) : A species facing imminent extirpation or extinction.

Threatened (T) : A species likely to become endangered if limiting factors are not reversed.

Special Concern (SC)* : A species of special concern because of characteristics that make it particularly sensitive to human activities or natural events.

Not at Risk (NAR)* : A species that has been evaluated and found to be not at risk.

Data Deficient (DD)*** : A species for which there is insufficient scientific information to support status designation.

* : Formerly described as “Vulnerable” from 1990 to 1999, or “Rare” prior to 1990.

** : Formerly described as “Not In Any Category”, or “No Designation Required.”

*** : Formerly described as “Indeterminate” from 1994 to 1999 or “ISIBD” (insufficient scientific information on which to base a designation) prior to 1994.

The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) was created in 1977 as a result of a recommendation at the Federal-Provincial Wildlife Conference held in 1976. It arose from the need for a single, official, scientifically sound, national listing of wildlife species at risk. In 1978, COSEWIC designated its first species and produced its first list of Canadian species at risk. Species designated at meetings of the full committee are added to the list.

Environment Canada Environnement Canada

Canadian Wildlife Service Service canadien de la faune

The Canadian Wildlife Service, Environment Canada, provides full administrative and financial support to the COSEWIC Secretariat.

Introduction

The Least Bittern was first considered by COSEWIC in 1988 (Sandilands and Campbell 1987). It is the smallest member of the heron family nesting in Canada. It will be absent from large portions of its overall range as it breeds only locally in scattered patches of suitable habitat in North, Central and South America, as well as larger islands in the West Indies. In Canada, it breeds in extreme southern Manitoba, in southern Ontario, extreme southern Quebec and New Brunswick, and possibly occasionally in Nova Scotia.

It is a very quiet, retiring, and cryptic species, making it difficult to gather accurate population figures for. However, because it is confined to large patches of wetland habitat that had experienced tremendous losses in the past century, because most field observers felt it had continued to decline in numbers, and because the Canadian population was estimated to be in the order of only 1000 pairs, it was designated Rare, now Vulnerable, by COSEWIC in 1988.

Population Sizes and Trends

The Least Bittern is such a secretive and quiet bird that data on population trends and sizes is still contradictory and unclear (Gibbs et al. 1992). Between 1969 and 1993, breeding bird surveys from across the continent suggested an increase of 16.4%, but from 1984 to 1993 a significant decrease of 42.6% (Price et al. 1995). An overall nonsignificant decline was given as the continental trend by Dunn (1996), also based on BBS data. In fact, none of these trends can be taken as reliable since they are based on small sample sizes, for a species that is poorly suited to census by roadside counts.

In the maritime provinces, only 5 records were obtained during the breeding bird atlas surveys (1986 to 1990), and none were confirmed breeding. This indicates that breeding occurs sporadically at least, if not every year, and that total populations are likely in the low 10s at best (Erskine 1992).

In Quebec, during breeding bird atlas surveys (1984 to 1989) it was recorded in only 40 (1.6% of 2464) atlas squares and confirmed breeding in only 10 of those. The paucity of data meant no accurate estimates of population size are available from Quebec (Fragnier 1996). The bird has always been considered rare there and is classified as Vulnerable in the province. The overall impression is that the bird is in decline because of the continued loss of habitat there. Its breeding range is confined to the southernmost and most heavily settled parts of the province. It is designated S2 by the Nature Conservancy, indicating it is considered imperiled.

In Ontario, breeding bird atlas records from 1981 to 1985 came from 223 squares (12% of 1824), but breeding was confirmed in only 46 of those squares. These records come mainly from south of the Canadian Shield or on the southern fringes of the Shield, parts of the province with the greatest population and most heavily impacted by agricultural and developmental interests. The feeling of most contributors was that the species had certainly decreased in abundance in recent years in Ontario (Woodliffe 1978).

From abundance estimates provided to the atlas, Sandilands and Campbell (1978) suggested that the Ontario population was unlikely to exceed 1000 pairs. The Ontario Rare Breeding Bird Program, for three years 1989-1991, did not offer an alternative estimate, as coverage was not extensive enough on this species (Austen et al. 1992). However, the continued concern for a species that was probably still in decline because of habitat loss prompted a recommendation that the status be upgraded to threatened.

Although overall information on population trend is lacking, there has been a decline in the last few years in at least some marshes along the Great Lakes shorelines. Birds have all but disappeared from Long Point marshes, where they were considered common through the early 1980s (Ridout 1992, D. Sutherland, pers. comm.). They have also disappeared from Point Pelee marshes (D. Sutherland, pers comm.). It is possible that sustained high water levels on the Great Lakes in recent years, siltation and/or plant succession have been contributing to these declines (D. Sutherland, pers. comm.).

However, it is not known whether birds have actually declined throughout the Great Lakes marshes, or just some of them, or whether they are being pushed inland into smaller marshes. The Natural Heritage Information Centre still rates the Least Bittern as S3 (rare or uncommon) and it has not been considered by the Committee on the Status of Species at Risk in Ontario (D. Sutherland, pers. comm.).

In Manitoba, the Least Bittern was thought to nest in only 2 or 3 large marshes in the extreme south of the province (Sandilands and Campbell 1987). It now appears that it may be somewhat more dispersed there. The Conservation Data Centre, in consultation with local naturalists has listed it as S3, or estimated to occur at between 21 and 100 localities (J. Duncan, pers. comm.). Rather than an increase, however, this is probably the result of a more adequate and intense search for the birds. The overall population there is small.

The Least Bittern is also a species considered to be facing serious problems in the adjacent northern U.S.A. It is listed as Endangered in Illinois and Ohio, Threatened or proposed Threatened in Pennsylvania and Michigan, a species of special concern in New York and Vermont, and ranked S1 in New Hampshire. However, the Marsh Monitoring Program begun in 1994, to monitor wetlands throughout the Great Lakes basin, has as yet not provided sufficient data to establish population trend (R. Weeber, pers. comm.). A more detailed analysis will be some time in coming, if in fact the census method employed proves to be adequate to reliably estimate change in this cryptic species.

Habitat

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).

Evaluation and Proposed Status

The clear perception among field observers is that the Least Bittern population in Canada is still declining. There has been an obvious loss of numbers in some Great Lakes marshes. It is not known if populations might simply be shifting inland until water levels decline. However, such a shift may result in a decline as smaller marshes, where some will end up, do not sustain viable populations. Following the Rare Breeding Bird program in Ontario, there was sufficient concern for this species that the recommendation was to upgrade it to threatened status (Austin et al. 1994).

The troubling aspect for this species is that it could all but disappear before we are aware of that fact. Without surveys specifically devoted to this species and the development of some reliable survey methods, we will remain in ignorance of its status. The Marsh Monitoring Program in the Great Lakes basin may be able to offer some insights in the future, but because of the behaviour of this species it is not certain yet that any clear population estimates or trends can be established. However, this is a species for which some form of monitoring must be attempted as long as pressures on wetland habitats are allowed to continue.

Technical Summary

Ixobrychus exilis

Least Bittern

Petit Blongios

Range of Occurrence in Canada: MB, NB,ON, QC

Extent and Area information

· extent of occurrence (EO)(km²) : 200,000

· area of occupancy (AO) (km²):100 ?

· number of extant locations : Few 100s at most

· habitat trend: specify declining, stable, increasing or unknown trend in area, extent or quality of habitat : Declining in quantity and quality

Population information

· generation time (average age of parents in the population) (indicate years, months, days, etc.) : No data; 1+y

· number of mature individuals (capable of reproduction) in the Canadian population (or, specify a range of plausible values) : 1000 or less

· total population trend: specify declining, stable, increasing or unknown trend in number of mature individuals : Declining inferred from habitat decline and local extirpation events

· is the total population severely fragmented (most individuals found within small and relatively isolated (geographically or otherwise) populations between which there is little exchange, i.e., <1 successful migrant / year)?No

Threats (actual or imminent threats to populations or habitats)

- destruction or degradation of habitat

- mortality due to collisions

Rescue Effect (immigration from an outside source) :Low

· does species exist elsewhere (in Canada or outside)?Yes

Quantitative Analysis

None available

Acknowledgements

Thanks to Colleen Hyslop for the opportunity to prepare this report and to the Canadian Wildlife Service, Environment Canada, for funding. Don Sutherland of the Ontario Natural Heritage Information Centre and Jim Duncan of the Manitoba Conservation Data Centre were most helpful with provincial information. Russ Weeber and Kathy Jones of the Marsh Monitoring Program of the Great Lakes area, coordinated by Bird Studies Canada, provided helpful discussion. Special thanks to the many unnamed volunteers who contribute to nest records schemes, breeding bird atlases, breeding bird surveys, the marsh monitoring program and American Birds/Audubon Field Notes, along with the many organizations who support such programs; their commitment is vital to whatever understanding we have of the status of this species.

Experts Contacted

Jim Duncan, Manitoba Conservation Data Centre, Dept. of Natural Resources, Wildlife Branch, Box 24, 200 Saulteaux Crescent, Winnipeg, MB. R3T 3W3.

Don Sutherland, Natural Heritage Information Centre, 300 Water Street, 2nd floor, North Tower, P.O. Box 7000, Peterborough, ON. K9J 8M5.

Russ Weeber, Bird Studies Canada, P.O. Box 160, Port Rowan, ON. N0B 1M0.

Literature Cited

Austen, M.J.W., M.D. Cadman, and R.D. James. 1994. Ontario birds at risk: status and conservation needs. Federation of Ontario Naturalists, Toronto, and Long Point Bird Observatory, Port Rowan.

Bardecki, M.J. 1981. The role of agriculture in declining wetlands. In A. Champagne, ed. Proceedings of the Ontario Wetlands Conference. Federation of Ontario Naturalists and Ryerson Polytechnical Institute, Toronto. Pp. 61-73.

Dunn, E. 1996. Trends in "other" waterbirds. Bird Trends 5:7-7.

Erskine, A.J. 1992. Atlas of breeding birds of the Maritime provinces. Nimbus Publishing and the Nova Scotia Museum, Halifax.

Fragnier, P. 1996. Least Bittern. In Gauthier, J. and Y. Aubry, eds. The breeding birds of Quebec: atlas of the breeding birds of southern Quebec. Association québécoise des groupes d'ornithologues, Province of Quebec Society for the Protection of Birds, Canadian Wildlife Service, Environment Canada, Quebec Region, Montreal. Pp. 240-241.

Gibbs, J.P., F.A. Reid, and S.M. Melvin. 1992. Least Bittern. In A. Poole, P. Stettenheim and F. Gill, eds. The birds of North America No. 17. Academy of Natural Sciences, Philadelphia, and American Ornithologist's Union, Washington, D.C.

Lands Directorate. 1986. Milieux humides le long des fleuves Saint-Laurent, 1950-1978. Environment Canada, Quebec Region. Working Paper no.45.

McCullough, G.B. 1981. Wetland losses in Lake St. Clair and Lake Ontario. In A. Champagne, ed. Proceedings of the Ontario Wetlands Conference. Federation of Ontario Naturalists and Ryerson Polytechnical Institute, Toronto. Pp. 81-89.

Peck, G.K. and R.D. James. 1983. The breeding birds of Ontario: nidiology and distribution. Vol. 1: nonpasserines. Life Sciences Misc. Publ., Royal Ontario Museum.

Price, J., S. Droege, and A. Price. 1995. The summer atlas of North American birds. Academic Press, New York.

Ridout, R. 1992. The nesting season -- Ontario region. American Birds 46:1131-1133.

Sandilands, A.P. and C.A. Campbell. 1987. Status report on the Least Bittern, Ixobrychus exilis. COSEWIC, Ottawa.

Snell, E.A. 1987. Wetland distribution and conservation in southern Ontario. Working paper No. 48. Inland Waters and Lands Directorate, Environment Canada.

Tiner, R.W. Jr. 1984. Wetlands of the United States: current status and recent trends. U.S. Fish and Wildlife Service, National Wetland Inventory, Washington, D.C.

Woodliffe, A.P. 1987. Least Bittern. In Cadman, M.D., P.F.J. Eagles, and F.M. Helleiner, eds. Atlas of the breeding birds of Ontario. University of Waterloo Press, Waterloo. Pp. 48-49.

The Author

Ross James is a Departmental Associate and former Curator of Ornithology at the Royal Ontario Museum in Toronto, Ontario. He studied the foraging behaviour of vireos in southern Ontario, and the ecological and behavioural relationships of Blue-headed and Yellow-throated Vireos for master’s and doctoral research at the University of Toronto. He has also conducted bird population studies in boreal forest and southern woodlands and wetlands. He is interested in the status and distribution of birds in Ontario, authoring an Annotated checklist of Ontario Birds, and coauthoring two volumes on the Breeding Birds of Ontario. He was a committee member for and a contributor to the Atlas of Breeding Birds of Ontario, and a coauthor of Ontario Birds at Risk. He is an author of two accounts for the Birds of North America, and has published more than 80 papers on birds. He spent more than a decade as chair and co-chair of the Birds Subcommittee of COSEWIC. In this capacity he was familiar with previous status reports and the status of this species.

Addendum
2001 Addendum to the 1999 Status Report on the Least Bittern

As indicated in the report, a Marsh Monitoring Program was initiated in Ontario in 1995. Its ability to provide accurate information on the status and trend of Least Bittern in Canada is not confirmed due to the secretive nature of the bird and the remoteness of breeding sites. Nonetheless, the results for the period 1995-2000 are presented below. The results were provided by Steve Timmermans (Bird Studies Canada-Études d’oiseaux Canada) in an e-mail to Bob Milko (CWS) dated 14 November 2001.

Figure 1 presents observations pooled for all sub-basins within the Great Lake region (i.e., Lake Ontario, Lake Erie, Lake Michigan and Huron). Data suggest a slow decline (% per year = -6.8; 95% confidence interval = -16.4 to +3.7), but the trend is not statistically significant (p=0.1965). Trends calculated individually for each sub-basin were not significant.

Figure 2 presents observations pooled for all sub-basins within the Great Lake region (i.e., Lake Ontario, Lake Erie, Lake Michigan and Huron) but separated according to whether a route occurred at a coastal or inland location. Regression analyses suggest a decline in each data set (coastal wetland % per year = -13.6, 95% confidence interval = -26.2 to +1.3; inland wetland % per year = -1.1, 95% confidence interval = -14.5 to +14.5), but the trends are not statistically significant (p=0.066 and p=0.8784 respectively). Trends calculated individually for each sub-basin were not significant. Figure 2 also shows variation in mean lake level (IGLD85) in metres. Overall population trends appear to be responding to fluctuating lake levels, but the pattern is not consistent among individual sub-basins (data not shown).

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