Kirtland's warbler (Dendroica kirtlandii) COSEWIC assessment and status report: chapter 7

Population Sizes and Trends

Search effort

There have been few systematic surveys for the Kirtland’s Warbler in Canada until recently. This means that the species could be present in remote unsurveyed regions. For example, extensive early successional jack pine forests exist in Ontario and Québec, so the possibility exists that the species could breed in these locations undetected (Environment Canada 2006). The area actually surveyed for Kirtland’s Warbler is quite small compared to the area of potential suitable habitat.

At least four targeted surveys have been orchestrated by the Ontario Ministry of Natural Resources (OMNR) and the Canadian Wildlife Service (CWS) of Environment Canada.

First, following the discovery of a territorial male Kirtland’s Warbler on 4 July 1997 in Thessalon, surveys of potential Kirtland’s Warbler habitat were contracted out by CWS in the Algoma area (Knudsen 1999; B. Knudsen, pers. comm. 2007). Jack pine stands of 3-20 years were surveyed and identified according to GIS layers from the OMNR Forest Resources Inventory. However, no warblers were detected and it was noted that at least some of the stands were unsuitable for Kirtland’s Warblers being of > 20 years of age (Knudsen 1999; B. Knudsen, pers. comm. 2007).

Second, in association with the U.S. Kirtland’s Warbler Recovery Team, OMNR and CWS personnel carried out aerial surveys in the Sault Ste. Marie and Chapleau areas to determine priorities for monitoring (Bloom 2003). Reports of these surveys are not available.

Third, searches have been made in the Orillia area following the discovery of a singing male in 1986, as well as the Chapleau-Cartier area, Bruce Peninsula and Manitoulin Island. As yet no breeding pairs have been discovered, although only a small portion of the total potential breeding habitat for Kirtland’s Warbler has been surveyed in Ontario (Environment Canada 2006).

Fourth, targeted surveys of suitable habitat have been conducted in the Pembroke area. Searches were conducted by OMNR personnel in 2002 and CFB Petawawa and OMNR personnel in 2003, but Kirtland’s Warblers were not detected. Surveys have been expanded to other parts of Renfrew county by OMNR staff while CFB personnel have continued searches of suitable habitat at the base since 2003. In 2006, CFB Petawawa commissioned an inventory of species at risk, and as part of these efforts, suitable habitat for Kirtland’s Warbler was searched. Three singing males were found in 2006, two in 2007 (K. Tuininga, pers. comm. 2007) and an active nest, with a female and nestlings was found in 2007 (Canadian Forces Base Petawawa 2007).

Kirtland’s Warblers have been recorded only once on a Breeding Bird Survey (BBS) route in Canada (2007, B. McBride, pers. comm. 2007) and thus there are no data available to evaluate changes in numbers.

Targeted surveys of apparently suitable habitat have recently been conducted in Québec (M. Robert and F. Shaffer, pers. comm. 2008). Searches were conducted in Kazabazua, Île aux Allumettes, Île du Grand Calumet, and Parc de la Vérendrye in late-May 2007, where potential habitat for Kirtland's Warbler had been identified using forest stand attributes (tree species, age of stand, size of stand) from Québec government digital forestry maps. Kirtland's Warblers were not detected, and areas visited apparently contained little (if any) potential habitat.

Abundance

The current size of the Kirtland’s Warbler population in Canada is unknown. The largest number of singing males recorded at any one time from a specific area was the three singing males recorded at CFB Petawawa in the summer of 2006. Similarly, two males and a female were recorded from Petawawa in 2007. Given the availability of suitable habitat, the population is likely to be greater than three individuals, but is probably less than 10.

Fluctuations and trends

There is no information on population trends for Kirtland’s Warbler in Canada. The numbers of individuals reported from Ontario has been stable since 1990 (Appendix 1, Environment Canada 2006). Similarly, Point Pelee National Park has reported the species regularly since the mid-1990s, and annually on spring migration since 2001 (Appendix 2, Wormington 2008).

Kirtland’s Warblers have been increasing in the U.S. since the early 1990s (Figure 2). Surveys were begun in Michigan in 1951 and repeated in 1961, 1971 and then on an annual basis (Probst et al. 2003). The 1951 survey revealed 432 singing males, and the 1961 survey found 502; however, numbers fell to 201 in 1971 and an all-time low was reached in 1974 with only 167 males being counted. The population was relatively stable over the period 1971-1986 (Figure 2), despite aggressive cowbird control in almost all breeding areas. The greatest increases in population, however, occurred when cowbird populations were controlled and jack pine habitat was created through management and two large wildfires that occurred in 1975 and 1980. By 2006, the population had risen to 1,479 singing males (Figure 2; Byelich et al. 1985; Michigan Department of Natural Resources 2007).

Figure 2. Survey of singing male Kirtland’s Warblers in Michigan 1951-2006 (Michigan Department of Natural Resources 2007; T. Hogrefe, pers. comm. 2007).

Figure 2. Survey of singing male Kirtland’s Warblers inMichigan1951-2006 (Michigan Department of Natural Resources 2007; T. Hogrefe, pers. comm. 2007).

Rescue effect

Kirtland’s Warblers from Michigan (or possibly Wisconsin) could potentially breed in Canada. Large distances and extensive areas of open water do not appear to create a barrier to dispersal (Probst et al. 2003) and juvenile males have been known to disperse at least 350 km from their natal sites. Six banded males from the Lower Peninsula of Michigan were found in the Upper Peninsula and regular movements have been observed between the two populations (Probst et al. 2003). Moreover, there was a recent record of a Kirtland’s Warbler in Michigan within 25 km of Sault Ste. Marie. Together this information suggests that there is potential for rescue, although the population in the U.S. remains low, so rescue, if it occurred, would be limited.

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