Canada Warbler Cardellina canadensis
Version: 2.0 — Published May 7, 2020
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Very capable flyer in thick habitat; typically active and alert, gregarious (2). Often observed in low vegetation with tail cocked and wings flicked (16, 5). Hops and climbs along branches while moving through thick vegetation. Flight is direct. Agile in thick vegetation while foraging using a varied repertoire of foraging maneuvers.
After leaving nest, one incubating female sometimes perched for 1–2 min to yawn, rub mandibles on branch, defecate, ruffle and shake feathers, stretch wings and legs, or preen (sometimes vigorously) breast, wrist, or underwing before foraging (160). Unlike Hooded Warbler (Setophaga citrina) and most other passerines, scratches head directly by bringing foot forward and underwing, rather than over wing (169). Some males seen preening for just under 10 consecutive min (LR, personal observation).
No information on sleeping, roosting, sunbathing, or daily time budget.
During winter near Popayán, Colombia, 3 hostile intraspecific interactions (no details provided) and no hostile interspecific interactions observed during 206 foraging observations (174). In Ecuador, hostile interactions between males were also observed (MTH, personal observation). During the breeding season, Morse (175) observed 5 hostile intraspecific interactions and 9 hostile interspecific interactions during 86 foraging observations (more hostile interactions than 3 sympatric warblers). Exhibits Wing Display, in which male faces male opponent with contour feathers sleeked, body held horizontal, and wings lifted horizontally out from body (28).
Many aggressive interactions observed among neighboring territorial males in New Hampshire, especially early in the breeding season when territory boundaries are disputed; involved physical contact with sparring in flight and onto the ground. Breeding territory boundaries persist in being disputed throughout the relatively short breeding season with neighbors perching alongside each other, drooping tails and wings, visually assessing each other (LR, MTH, M. Goodnow, personal observation).
Two color-marked females seen fighting aggressively for > 1 hr at beginning of 2008 breeding season in New Hampshire (LR, personal observation). These two believed to be paired to the same male in previous season (both nests on that male’s territory) and that male was present during the aggressive interaction but not continuously.
Breeding territories are distributed where habitat is most suitable and consequently appear as neighborhoods in heterogeneous habitat mosaics (L. Reitsma et al., unpublished data). Territories can overlap to some degree, but such boundaries often remain in dispute. Male arrives in breeding territory before female and establishes and aggressively defends territory by singing (176). In one case in New Hampshire, a pair arrived on same day and female began lining previous year's nest later that day (this pair fledged 6 young from that nest, the most of any of 117 nests over a 6-yr period). Males may defend an area larger than the area in which they sing (e.g., the singing area for a male in New York was 0.24 ha, but after nesting began he ranged over a 0.8 ha—it is these larger areas that are apparently defended as territories against intruders ). For details on individual spacing and territory size, see Population Spatial Metrics .
Both sexes and age classes join mixed-species foraging flocks singly or as pairs during the nonbreeding season (MTH and LR, personal observations). Flocks are likely territorial. Individuals also persist on home ranges when flocks have moved through.
Mating System and Sex Ratio
Although considered socially monogamous, extra-pair fertilizations occur among individuals monitored in a long-term breeding population study in New Hampshire (177). Additional observations suggest considerable fitness asymmetry among males in this population based upon high variation in male behavior, but extra-pair paternity data are still forthcoming. Some evidence that birds maintain pair bond year-round, although recent work in Ecuador does not support this (M. Hallworth et al., unpublished data). However, on Barro Colorado Island in Panama the species occurs as male-female pairs during spring and fall migration (18 of 30 birds observed in fall and 64 of 81 birds observed in spring were in bright-dull pairs, the other 12 were solitary birds; 6).
Returning females in New Hampshire documented pairing with males that were previous neighbors, even if females were successful with a mate the year before (L. Reitsma et al., unpublished data). Females assess males by slowly moving through territories close to ground, sometimes probing specific ground-level substrate. Males follow females during these forays. Males decrease singing frequency during active pair formation (while following females), but renew singing if female leaves territory.
Of 61 nests and 185 nestlings monitored from 2005–2010 in New Hampshire, 41.6% of nestlings were extra-pair young (EPY) and 57.4% of nests had at least one EPY (177). Thus the majority of nestlings were fed by their biological fathers but majority of nests had mixed male parentage. Males were as likely to be cuckolded by non-neighbors as neighbors and a few accounts of cuckoldry were 1–2 km between territories. The number of nests a male cuckolded in one season was inversely proportional to direct evidence of a male successfully fledging his own young suggesting a possible trade-off. Males that were 3-yr old or older had a higher proportion of EPY than of the younger age classes. One-year-old males sired 17% of EPY and of the 83% sired by mature (after-second-year) males; 58.5% were sired by males 3–7 yr old. This population has had two documented 1-yr-old males, caught as second-year birds and returned for 8 subsequent years (L. Reitsma, unpublished data).
Social and Interspecific Behavior
Degree of Sociality
Mostly solitary or with mate during breeding season. Often singly or in small groups within mixed-species feeding flocks during the nonbreeding season.
Nonpredatory Interspecific Interactions
Observed mixing with other species of wood warblers in early summer prior to territory formation (94). Observed in flocks and interacting with other species during nonbreeding season; attends army ant swarms during overwintering period (14, 60, 16).
Two separate aggressive encounters (no details given) observed between male Gray Catbird (Dumetella carolinensis) and migrant Canada Warbler at catbird nests (178). In Quebec, a male Chestnut-sided Warbler chased away a male Canada Warbler that had come near its nest (179). No apparent conflict with Yellow Warbler (Setophaga petechia) nesting only 9 m away from nest in New Brunswick. Can nest within meters of Northern Waterthrush (Seiurus noveboracensis) in New Hampshire without aggressive interactions (LR, personal observation).
Kinds of Predators
Parents of at least two nests observed exhibiting highly agitated behavior toward Blue Jays (Cyanocitta cristata) on the same day. These nests were within 40 m of each other and failed that same day (L. Reitsma, personal observation).
Telemetry data on fledglings included one case in which the fledgling was eaten by a milk snake (Lampropeltis triangulum), two days post-fledgling (L. Reitsma, personal observation).
Manner of Predation
Blue Jays observed moving slowly through Canada Warbler territories looking in multiple directions at each perch, and often near the ground; readily respond to distress calls of adult Canada Warblers. Observations are consistent with visually astute predatory tactics that focus on adults during the feeding phase of the nest cycle, but no direct observations of this (L. Reitsma, personal observation).
Response to Predators
When disturbed at the nest, female often gives distraction display on the ground, feigning injury with breast feathers ruffled, tail fanned, and wings half-cocked or fluttering above the back (164, 127); male may do likewise (180). Females also stay on nest until threat approaches to close distance, presumably to camouflage the eggs (M. Goodnow, personal observation). Female imitates fledglings along the ground in presence of predators (human and snake) shortly after fledging (MTH, LR, and M. Goodnow, personal observation).
Becomes agitated when a human observer enters territory (more so than many warblers), chipping loudly and flying from shrub to shrub (94, 181, 91, 92). Pishing or squeaking on territory often elicits agitated behavior; wing flipping, bill cleaning, and sharp chip calls (97). Females exhibit intense distress when humans are near fledglings, as do males (but to a lesser degree). Same is true when humans approach nest, especially during nestling phase (M. Goodnow and LR, personal observation).