Kirtland's warbler (Dendroica kirtlandii) recovery strategy: chapter 1

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BACKGROUND

1.1          Species Assessment Information from COSEWIC

1.2          Description

The Kirtland’s Warbler is a medium-sized, omnivorous songbird in the Family Parulidae (North American wood-warblers). The adult male Kirtland’s Warbler has a bluish-grey head and back that is streaked with black, a lemon yellow front, and black streaks or spots on the sides. The eyelids are white, forming an almost complete eye ring, and indistinct whitish wing bars may also be evident. Adult females resemble males, but with duller plumage and paler underparts. Immature females are generally browner overall. Kirtland’s Warblers frequently pump their tails. The song of the male Kirtland’s Warbler is a loud, emphatic series of notes. Illustrations and further descriptions may be found in Mayfield (1992), Walkinshaw (1983), and standard field guides.

1.3          Populations and Distribution

The Kirtland’s Warbler is one of the world’s most critically endangered species, with a global rank of G1 [2] (NatureServe 2005). Its breeding range is confined mainly to the northern part of the Lower Peninsula in Michigan (Figure 1). It is ranked S1 [3] in Michigan. The Kirtland’s Warbler is considered an accidental migrant in six other eastern U.S. states. The Michigan population has steadily rebounded following a steep decline between 1961 and 1971, from a low of 167 singing males in 1974 to  1478 singing males in 2006 (Byelich et al. 1985; Michigan Department of Natural Resources 2005). This increase is mainly due to the creation of additional jack pine (Pinus banksiana) habitat by wildfire (Sykes 1997), the management of large areas of suitably aged jack pine plantations (Probst and Weinrich 1993), and active control of Brown-headed Cowbirds (Molothrus ater) (Kelly and DeCapita 1982).

The single breeding record documented for Canada is from the Midhurst (Barrie) area in Ontario in 1945 (Speirs 1984), although there is some ambiguity regarding this record (Natural Heritage Information Centre 2005). Kirtland’s Warblers may have been nesting in the Petawawa area in Ontario in the 1800s and early 1900s, and possibly elsewhere in Canada (Harrington 1939; COSEWIC 2000). One singing male was recorded on the Québec side of the Ottawa River, near Kazabazua, Québec, in 1978 (COSEWIC 2000).  Singing males were seen and heard regularly at Canadian Forces Base (CFB) Petawawa, near Petawawa, Ontario, during the summer of 1916, and a male Kirtland’s Warbler was also reported in the same area on June 5, 1939 (Harrington 1939). Singing males have been reported in early successional pine habitat in Ontario on at least eight  subsequent occasions, the most recent being the sighting of three individuals at CFB Petawawa in June 2006. However, none of these males was believed to be accompanied by a female. It is possible that birds in larger, less accessible patches in this area have gone undetected (COSEWIC 2000) (Figure 2).

(Multiple records at the same location are not shown)

Sightings have been reported in Canada from Minaki, Ontario, east to Kazabazua, Québec. The majority of the 76 records for this species in Canada, between 1900 and 2006 and principally in Ontario, are of spring migrants, with some summer reports (June to mid-July) of singing males and some fall migrants (Aird and Pope 1987; COSEWIC 2000; P.Aird pers comm. 2006; Petrucha and Sykes, 2006). Although there has generally been at least one sighting of the Kirtland’s Warbler each year in Ontario since 1990, there is no trend evident in the number of sightings of singing males. Kirtland’s Warblers were not documented as breeding in any region of Ontario during the recent five-year (2001–2005) Ontario Breeding Bird Atlas project (Ontario Breeding Bird Atlas 2005).

The global breeding distribution of the Kirtland’s Warbler is therefore confined to the United States. However, based on the expansion of the Michigan population and increasing numbers of singing males reported in Michigan and Wisconsin, some within 25 km of the Canadian border near Sault Ste. Marie, it is possible that breeding may be reported in Canada in the future.

1.4          Needs of the Kirtland’s Warbler

1.4.1      Habitat and Biological Needs

The Kirtland’s Warbler is a habitat specialist. In Michigan, essential habitat requirements include even-aged stands of jack pine over 32 ha in size (Mayfield 1953) and created by wildfire or specially designed plantations that mimic wildfires. Jack pine stands larger than 80 ha are optimal breeding habitats, indicated by improved nesting success in these stands. Anderson and Storer (1976) found that 90% of nests that fledged Kirtland’s Warblers were in stands larger than 80 ha. Optimal habitat also consists of dense stands of jack pine (minimum 3500 stems/ha) interspersed with small openings, which produces high foliage volume and 35–65% canopy cover (Probst 1988; Kepler et al. 1996). Studies suggest higher nest success in dense, scattered patches of trees 1.5–5 m tall (or 7–20 years old), which provide adequate branch cover near the ground for nests; dry, well-drained, sandy soils; and ground cover composed of plants such as blueberry (Vaccinium angustifolium, V. myrtilloides), bearberry (Arctostaphylos uva-ursi), bracken fern (Pteridium aquilinum), service-berry (Amelanchier spp.), sand cherry (Prunus pumila), sweet fern (Comptonia peregrina), grasses (Andropogon spp.), sedges (Carex spp.), and goldenrods (Solidago spp.).

Plantations of jack pine and, rarely, red pine (Pinus resinosa) also provide suitable habitat for Kirtland’s Warblers on the breeding grounds (Weinrich 1994). In recent years, more than 90% of Kirtland’s Warblers nested in jack pine plantations specifically established for the species (P. Huber, pers. comm., 2006). Birds nesting in plantations can produce numbers of young comparable to those in naturally regenerated burn areas (Bocetti 1994).

With an average territory size estimated at about 15 ha (38 acres) per singing male, successful breeding of only 25 pairs is estimated to require the maintenance of roughly 375 ha (950 acres) of suitable habitat. Recovery efforts in Michigan are designed to provide a minimum of 15 200 ha (38 000 acres) at all times, involving rotational harvest management of approximately 76 000 ha (190 000 acres) of jack pine (Olson 2002). 

In spite of the specific requirements listed above, the required amount and quality of available habitat can be difficult to assess accurately. Areas of apparently high-quality habitat may not be occupied, and occupied habitat may not appear to be ideal, even to an experienced observer (Mayfield 1992). Other factors, including microclimate and specific structural features, may be more important than is currently understood. Further details on habitat requirements can be found in the literature (Wood 1904, 1926; Barrows 1921; Leopold 1924; Wing 1933; Mayfield 1953, 1960, 1962; Line 1964; Anderson and Storer 1976; Chamberlain 1978; Buech 1980; Harwood 1981; Ryel 1981; Wright and Bailey 1982; Probst 1986; Probst and Hayes 1987).

A study of the diet of the Kirtland’s Warbler through fecal analysis revealed the major food items as spittlebugs and aphids (Homoptera; in 61% of samples), ants and wasps (Hymenoptera; 45%), blueberry (Vaccinium angustifolium; 42%), beetles (Coleoptera; 25%), and moth larvae (Lepidoptera; 22%) (DeLoria-Sheffield et al. 2001). Presumably, sufficient quantities of these foods must be present for habitat to be suitable.

Sites surveyed in Ontario in the 1970s with potentially suitable habitat were not considered to be optimal habitat by Michigan standards: trees were taller than 6 m, and plant associations were different from those found in Michigan (Chamberlain 1978). However, work in 2003 in the Thessalon area in Ontario found plant associations and habitat very similar to those in Michigan’s jack pine barrens, with coarse sandy soils and ground cover dominated by low bush blueberry (V. angustifolium) and various grasses and sedges (Bloom 2003). Still, the actual amount of survey work completed in Ontario in relation to potential suitable habitat available is very small (P. Aird, pers. comm., 2006). In Ontario, singing males have been found in jack pine stands or plantations of more than 20 ha on well-drained sands or on shallow soils covering bedrock (Aird and Pope 1987). Kirtland’s Warblers have also been found in Scots pine (Pinus sylvestris) plantations on at least one occasion in Simcoe County, Ontario, on May 16–21, 1964 (Devitt 1967).

Kirtland’s Warblers are neotropical migrants that winter in the Bahamas, where they prefer areas of low, sparse vegetation (Mayfield 1972, 1996). Most of the islands are covered with broad-leaved scrub, and the northernmost islands have extensive pinelands. Most records were on islands that support open woodlands of Caribbean pine (Pinus caribaea) (Haney et al. 1998). Periods of degradation and recovery of pine ecosystems in the Bahamas may be related to periods of decline and recovery of Kirtland’s Warbler populations, and it has been suggested that winter habitat should be considered in conservation planning for the species (Haney et al. 1998). However, habitat availability on wintering grounds has not generally been thought to be a limiting factor (Sykes and Clench 1998).  

1.4.2      Ecological Role

The diet of the Kirtland’s Warbler includes a variety of insects representing a number of different orders (DeLoria-Sheffield et al. 2001), but there has been no research to determine the specific ecological effect of this warbler on populations of individual insect taxonomic groups. Blueberries (V. angustifolium) are also a major component of the Kirtland’s Warbler diet, and it is possible that birds assist in dispersing seeds into suitable habitat.

Documented predators of Kirtland’s Warbler adults, nestlings, and eggs in Michigan include Blue Jay (Cyanocitta cristata), thirteen-lined ground squirrel (Spermophilus tridecemlineatus), raccoon (Procyon lotor), striped skunk (Mephitis mephitis), domestic cat, and garter snake (Thamnophis sirtalis)(Walkinshaw 1983). It is also suspected that red squirrels (Sciurus vulgaris) and crows (Corvus spp.) may predate Kirtland’s Warbler eggs and nestlings in Michigan, but there has been no need to control predators to date (Huber et al. 2001).

1.4.3      Limiting Factors

Some characteristics of Kirtland’s Warbler biology noted from Michigan populations that could limit recovery in Canada include:

·       an extremely narrow preference for early successional and densely stocked jack pine. Suitable jack pine habitat created by wildfires due to fire suppression necessitates continued management (e.g. specially designed plantations). Suitable early successional habitats are dispersed and are possibly limiting in some parts of Ontario;

·       a preference for nesting territories within expansive tracts of suitable habitat;

·       temporary occupation of habitat (8–15 years) due to early successional habitat preferences (Probst 1986);

·       a high level of susceptibility to parasitism by Brown-headed Cowbirds in nesting areas (Mayfield 1977; Harwood 1981); and

·       dispersal characteristics. Young of the year typically disperse widely in search of new territories. Although dispersal may establish birds on new territories, dispersed singing males may have little chance of finding a mate in areas where the density of this species is very low (Mayfield 1983).

1.5    Threats

There are three main threats to the Kirtland’s Warbler. These are all well-demonstrated threats to the Michigan population. Further survey work is necessary to determine the extent to which fire suppression and forest succession and lack of suitable habitat are factors in Ontario. Cowbird parasitism is a significant problem in Michigan’s Lower Peninsula, but control has not been necessary in the Upper Peninsula (S. Sjogren, pers. comm., 2006). If a population is detected in Ontario, control may be necessary only in the south, where cowbirds are more common and the habitat is more fragmented.

1.5.1      Fire Suppression and ForestSuccession

Jack pine cones normally remain closed until exposed to heat from wildfires, and stand densities are higher following fire than following standard forest harvest practices (Olson 2002). Natural regeneration by wildfire also creates thickets and openings, favoured by breeding female warblers for nest site selection (Bocetti 1994). Optimal habitat was probably most extensive in the late 19th century when forest fires frequently followed extensive logging (Mayfield 1960). Fire suppression in the 20th century has greatly reduced available habitat for the Kirtland’s Warbler in both Ontario and Michigan (COSEWIC 2000).

A similar habitat structure can be mimicked by specific rotational harvest prescriptions and natural jack pine regeneration, with or without direct seeding or establishment of jack pine plantations (S. Sjogren, pers. comm., 2006). However, occupation of successional habitat by this species is still limited to 8–15 years (Probst 1986). Rotational harvest of large patches within a very large total area is therefore required to provide long-term occupancy of the Kirtland’s Warbler in an area (see section 1.4.1). A broad and potentially complex ecosystem approach that considers not only forestry practices, but also physical site factors, including microclimate and moisture, will be needed to manage large areas of habitat for the species (Kashian and Barnes 2000).

1.5.2      Lack of suitable habitat

Sufficiently large tracts of high-density, early successional jack pine forest with optimal understorey structure and composition may be limiting the establishment of Kirtland’s Warblers in some parts of Ontario. However, patches of suitable habitat have been documented, and there are large tracts of jack pine habitat across Ontario that need to be surveyed, including,Thessalon, the Petawawa area, the area between Cartier and Lake Wanapitei, the region between Chapleau and Gowganda, Manitoulin Island, and the Bruce Peninsula (Austen et al. 1993; Bloom 2003; P. Aird, pers. comm., 2005). There is now considerable evidence that a lack of suitable habitat was limiting the small but stable Michigan population prior to a vast wildfire in the 1980s (Probst and Weinrich 1993; Kepler et al. 1996). It has also been demonstrated that the pairing success rate of the Kirtland’s Warbler is lower in habitats of marginal quality (Probst and Hayes 1987).

1.5.3      Brood Parasitism by Brown-headed Cowbird

Brood parasitism by the Brown-headed Cowbird has been shown to reduce both hatchling and fledgling success in Michigan populations (Kelly and DeCapita 1982; Walkinshaw 1983). Through the 1960s and 1970s, a steep decline in the Kirtland’s Warbler population led to the confirmation that more than 70% of warbler nests were parasitized, reducing the production of young to fewer than one young per pair per year (Ryel 1981). Cowbird control began in 1972 and reduced parasitism to about 3% or negligible levels (Kelly and DeCapita 1982) and increased productivity to an average of nearly three fledged young per pair per year (Kelly and DeCapita 1982; Walkinshaw 1983).

If there is an undetected breeding population of the Kirtland’s Warbler in Canada, it is possible that cowbird parasitism is a threat to nesting success, and an assessment may eventually be needed.

1.5.4      Other

Forest management practices that do not consider the Kirtland’s Warbler’s specialized habitat requirements may inadvertently cause jack pine forests to become unsuitable. Conversion of jack pine to less preferred species, fragmentation of jack pine by harvesting in small blocks when large blocks are available, and planting unsuitable stocking densities may all threaten occupation by the Kirtland’s Warbler.

1.5.5      Actions already completed or under way

A long-term, multiagency recovery program has been extremely successful in Michigan (Solomon 1998). With over 800 published works on the Kirtland’s Warbler, sustained census, management, and research activities have been well documented in the literature (Mayfield 1992). In 2003, a joint recovery meeting was held with members of several U.S. recovery teams and representatives from the Canadian Wildlife Service (CWS). Opportunities for collaboration and joint recovery efforts for the Kirtland’s Warbler were discussed (R. Bloom, pers. comm., 2005).

In Canada, recovery actions have been limited mainly to survey work. There have been several targeted search efforts to detect breeding of the Kirtland’s Warbler in the past decade. In response to a report of a Kirtland’s Warbler sighting on July 4, 1997, in the Thessalon area, a survey for the species was conducted in this vicinity in 1999, but without success (Knudsen 1999).

This general area was later used to develop a site prioritization scheme using spatial data. Areas of suitable jack pine habitat in northern Ontario are potentially very large, and a prioritization method was developed to help focus future survey effort. Spatial information (e.g. pine cover, successional stage, soils) was used to evaluate sites for likelihood of dispersal and to set priorities for monitoring. Sections of this area were then surveyed by air by CWS staff, accompanied by American researchers (Bloom 2003). Although the project was not finalized, the mapping to date could be used to intensify aerial and ground surveys, initiate a monitoring program, or ground-truth sites for habitat suitability using established field methods (e.g. Bocetti 1994).

Targeted search efforts have also been undertaken in the Pembroke area. In 2002 and 2003, staff from the Ontario Ministry of Natural Resources (OMNR) and CFB Petawawa searched suitable habitat unsuccessfully. Since 2003, survey work has been continued by CFB Petawawa, and OMNR staff have begun to focus search efforts on other areas of suitable habitat in Renfrew County. Suitable habitat on CFB Petawawa was searched as part of a larger inventory of species at risk commissioned by the Base in 2006. Three singing males were located in June 2006 and one was eventually banded and released.

Searches have been undertaken annually in suitable habitat near Orillia, where a singing male was located in 1986, and over the past decade in the Chapleau–Cartier area, on the Bruce Peninsula, and on Manitoulin Island. Still, of the entire range of jack pine across Ontario, only a very small percentage has been surveyed. To date, no breeding pairs have been located (P. Aird, pers. comm., 2005).

Periodic sightings of migrant Kirtland’s Warblers by Ontario birders are generally reported to one or more of the following: the Ontario Field Ornithologists listserv ( ontbirds@hwcn.org), Ontario Bird Records Committee, the Ontario Natural Heritage Information Centre, Bird Studies Canada, the Kirtland’s Warbler Recovery Team, or the Ontario Breeding Bird Atlas project.

1.5.6      Knowledge Gaps

Despite substantial research on this species, several knowledge gaps remain for the global population of the Kirtland’s Warbler. These include population responses to habitat change and foraging ecology, migration routes, winter habitat requirements, and the potential role of global climate warming on jack pine habitats (Woodby et al. 1989; Olson 2002).

The most urgent knowledge gaps in Canada are related mainly to distribution, location, and availability of suitable habitat. If the Kirtland’s Warbler is reconfirmed as breeding in Canada, the following areas will also require research:

·       the extent of cowbird parasitism;

·       habitat characteristics (including a comparison with habitat in Michigan);

·       recruitment levels (nesting and fledging success);

·       dispersal tendencies;

·       site fidelity;

·       competing species and predators; and

·       possible management requirements.

[1]One singing male was also recorded in Quebec.

[2]A global rank of G1 indicates that the species is extremely rare globally, usually with very few remaining individuals.

[3]A subnational rank of S1 indicates that the species is extremely rare at the state or provincial level.