Canada warbler COSEWIC assessment and status report: chapter 6

Biology

Reproduction

The Canada Warbler usually nests in wet forested areas, often in dense ferns or fallen logs (Conway 1999). It has a typical clutch size of four to five eggs, with one clutch produced annually (Peck and James 1987; Curson et al. 1994; Conway 1999). Incubation lasts approximately 12 days (Conway 1999) and chicks remain in the nest for about 10 days (Conway 1999). The post-fledging dependent period probably lasts two to three weeks, as it does in other species of wood warblers (Curson et al. 1994).

The generation time is estimated at two to three years, taking into account the species’ age at first breeding (one year; Conway 1999) and maximum life span (eight years) (Klimkiewicz et al. 1983).

Survival

The maximum recorded age in North America for the Canada Warbler is seven years, 11 months (Klimkiewicz et al. 1983).

Survival estimates for the Canada Warbler in Canada come primarily from the Monitoring Avian Productivity and Survivorship program (MAPS; Michel et al. 2005). The objective of MAPS is to determine the productivity and survival rate of North American landbirds using the capture-mark-recapture method from a network of constant-effort monitoring stations across North America (Michel et al. 2005). The proportion of adults recaptured annually at these stations provides a measure of the apparent survival rate. The survival rate of the Canada Warbler was estimated between 1992 and 2003, for Alaska and boreal Canada (three stations) and the northeastern United States (seven stations; Michel et al. 2005). The apparent adult survival rate for Alaska and boreal Canada was 0.482±0.063 and for the northeastern United States 0.374±0.116; both are considered low (Michel et al. 2005).

Productivity

The only data on Canada Warbler productivity come from the MAPS program between 1995 and 2001 (Michel et al. 2005). Productivity is estimated at each MAPS station, taking into consideration the ratio of the number of young captured to the number of adults captured (productivity index). The results show a downward trend in the productivity index for the Canada Warbler between 1995 and 2001 (Michel et al. 2005).

Movements and dispersal

Data gathered at the LSLBO banding station in Alberta suggest that the fall migration begins around 10 July and ends around 20 September (Jungkind 2001). The species arrives at its breeding grounds between 12 May and 14 June (Jungkind 2001).

Adult birds captured in several Atlantic coast states (i.e., Connecticut, New Jersey and Maryland) during spring migration were recaptured, probably on their breeding site, several years later in the St. Lawrence Valley in Quebec (n = 3) and southern New Brunswick (n =1; Brewer et al. 2000). Two adults captured in Michigan in the spring were recaptured in southern Ontario (Brewer et al. 2000)

Food and feeding habits

The Canada Warbler feeds primarily on flying insects (i.e., Diptera and Lepidoptera) and spiders in the shrub layer (Conway 1999). Although not considered a spruce budworm specialist, it may also feed heavily on this insect during outbreaks (Crawford and Jennings 1989; Patten and Burger 1998). It uses a variety of foraging techniques, including flycatching, sallying, hover gleaning, foliage gleaning and ground gleaning (Conway 1999).

During the breeding season, the species forages in both conifers and hardwood trees. In Wisconsin, it concentrates its foraging effort in conifers (Sodhi and Paszkowski 1995), whereas in northern British Columbia, it forages primarily in red osier dogwood (Cornus sericea) and young white birch (Betula papyrifera; Enns and Siddle 1996).

During migration, the Canada Warbler forages in the shrub layer at heights of less than 7 m (Keast 1980). On its wintering grounds, it often feeds within mixed-species flocks (Ridgely and Gwynne 1989).

Interspecific interactions

Like most forest passerines, the Canada Warbler is territorial during the breeding season. However, it can occur in small groups within mixed-species flocks during dispersal, migration and on wintering sites (Bent 1953; Ridgely and Gwynne 1989). Agonistic interactions with other warblers during foraging in the breeding season are also reported (Conway 1999).

No specific information is available on interactions with predators (Conway 1999).

Home range and territory

Telemetry studies in Vermont have shown that males establish their territories in late May (Chase 2005). Average territory sizes, based on this study and another in the Fort Liard area, ranged from 0.4 ha (n = 29 pairs; Chase 2005) to 0.75 ha ± 0.36 SD (n = 40 territories; Machtans 2006), respectively. The average home range size, based on the Vermont study, was two ha (Chase 2005).

Behaviour and adaptability

The Canada Warbler requires a well developed shrub layer in its breeding habitat. Given this, it exhibits a certain degree of adaptability to human disturbances such as forest harvesting. The species occupies forest regenerating following harvesting, particularly stands between 6-20 years post-harvest in the east (Lambert and Faccio 2005) and 20-30 years post-harvest in the west (Hobson and Schieck 1999; Schieck and Hobson 2000). This is presumably because the shrub layer peaks during early regeneration stages (Sodhi and Paszkowski 1995; Norton and Hannon 1997; Tittler et al. 2001). The Canada Warbler also occupies stands recovering from fire (Hobson and Schieck 1999; Schieck and Hobson 2000), although the species is less abundant in stands regenerating from fire than those regenerating from harvest (Hobson and Schieck 1999).

Canada Warblers also appear to be relatively tolerant of the habitat fragmentation that results from forest harvesting (Schmiegelow et al. 1997). This may be because of the regeneration, and thus a well-developed shrub layer, that occurs adjacent to the fragments following harvesting (Schmiegelow et al. 1997). On the contrary, there is evidence to suggest that the species is relatively intolerant of fragmentation associated with agriculture (Robbins et al. 1989; Hobson and Bayne 2000c).

On its wintering grounds, the Canada Warbler demonstrates a certain degree of adaptation to changes occurring in its habitat (Terborgh 1989). In the Andes, the species made relatively heavy use of coffee plantations (Terborgh 1989; Finch and Stangel 1993), although a certain percentage of forest cover must be maintained above the plantations (Terborgh 1989).

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