SPECIES

Canada Warbler Cardellina canadensis

Len R. Reitsma, Michael T. Hallworth, Marissa McMahon, and Courtney J. Conway
Version: 2.0 — Published May 7, 2020

Breeding

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Phenology

Pair Formation

May arrive paired; pairs also form quickly on breeding grounds. In one instance (1 out of 21 banded females) in New Hampshire, both male and female from the previous year returned on the same day, paired, and female began lining previous year's nest.

In northern Alberta, older males arrive first (30 May, on average) followed by young males (1 June), old females (2 June) and eventually young females (3 June) (128).

Nest-Building

Generally mid-May through early June in Vermont, Pennsylvania, Ontario, and New Hampshire (29, 97, Cornell Nest Record Card Program [CNRCP]; L. Reitsma et al., unpublished data). Can be as late as mid to late June if first nest fails in May or early June (L. Reitsma et al., unpublished data).

Most nests are built within 3–5 d; in rare instances when previous nest is re-used, within less than 2 d.

Only Brood per Season

Laying of First to Last Egg in Clutch

Primarily first week of June (1–20 June) in New York (11, 94), and 22 May–24 June in Pennsylvania, Ohio, North Carolina, British Columbia, Maine (95, 176, 182, 126, CNRCP).

Average date for first egg laid in New Hampshire (n = 62 nests): 1 June, ranging from 23 May to 7 June. Females lay one egg per day; initiate incubation on date last egg is laid; incubation averages 11–12 d (M. Goodnow et al., unpublished data).

Nestlings

See Figure 1. Third week in June to second week in July (range 5 June through first week in August; CNRP [n = 35 nests], 90, 182, 183, 155, 126).

Mean hatch date in New Hampshire (62 nests): 16 June (range 7–30 June). Nestling period averages 8.1 d in New Hampshire [n = 62 nests]. Nestlings fledge on average by 23 June (range 7 June to 7 July; M. Goodnow et al., unpublished data).

Nest Site

Selection Process

Females actively assess suitable sites during pair formation and select areas with dense shrubs of diameters up to 5 cm diameter-at-breast-height. Based on vegetative analysis in New Hampshire, higher concealment, coarse woody debris, and tree stem density are the main structural features associated with nest sites used by Canada Warblers (M. Goodnow et al., unpublished data).

Microhabitat

Well-concealed, often in thickets or areas with dense ferns (87, 91). In wet, mossy areas within forest among ferns, stumps, and fallen logs. Often in rhododendron thickets in southern part of range. Dense nest site cover is an important habitat requirement (176). Nest sites had significantly higher concealment (86.7% ± 1.32 S.E.) than random mock nest sites off territories (39.8 ± 1.92 S.E.) in New Hampshire (F = 95.36, df = 420, P < 0.0001) (M. Goodnow et al., unpublished data).

Site Characteristics

Typically on or near the ground, often on slopes, knolls, in earthen banks, or rocky areas (87). Often built within recessed hole of upturned tree root mass, rotting tree stump or sphagnum moss hummock. Less often within clump of grass, at base of tree stump, tucked under overhanging bank, beside fallen log, in rock cavity, at base of sedge tussock, under leaf on forest floor, at base of moss-covered logs/rocks, or in brush pile (184, 185, 186, 164, 87, CJC).

One unusual nest was 0.9 m off the ground in a crotch of a maple sapling (87). Often in dead fern hummocks in New Hampshire. Seldom on flat ground with no lip over nest, but occasionally at base of sapling that shields the cup (M. Goodnow, LR, MTH, personal observations).

Nest

Construction Process

Female generally builds nest but male seen carrying material to nest for female (M. Goodnow, personal observation). Carries grass, bark, and leaves to nest with bill. Lines nest with deer or moose hair.

Structure and Composition Matter

Bulky, loosely constructed. Cup with exterior made of some combination of grasses, bark strips, dead leaves, plant fibers, plant down, weed stalks, moss, pine needles, and twigs with deciduous leaves woven into outer wall (29, 11, 94, 2, 160, 87, 91). Lining often contains horse, deer, or other animal hair, and rootlets, deciduous leaves, and fine grasses (11, 94, 87).

Three nests from New Hampshire were dissected and, on average, were composed of: 48% leaves, 20% coniferous needles, 17% petioles and fern stalks, 8% deer and/or moose hair, 3% grasses, 3% sphagnum moss, and 1% rootlets (MTH, personal observation).

Dimensions

Outside diameter 9–14 cm; outside height 5.5–10.2 cm. Inside diameter 5.0–7.6 cm; inside depth 2.5–5.0 cm (14, 164, 87).

Microclimate

No quantitative information. Common component of preferred nesting sites appears to be relatively cool temperatures and high humidity (11, 187, 91).

Maintenance or Reuse of Nests, Alternate Nests

One nest built on top of nest from previous year (Western Foundation of Vertebrate Zoology [WFVZ] egg collection). Two nest locations re-used in New Hampshire and several instances where nest on a male's territory was built within a few meters of previous year's nest.

Nonbreeding Nests

No information, but probably none.

Eggs

Shape

Ovate or slightly short ovate (12, 14).

Size

From WFVZ: Length: 17.33 mm (range 15.99–19.50, n = 22 clutches, 86 eggs); breadth: 13.20 mm (range 12.00–14.05, n = 22 clutches, 86 eggs). All means are based on clutch averages, ranges on individual eggs excluding obvious runt and giant eggs. Also see Eaton (11), Forbush (12), and Bent (14).

Mass

Fresh egg mass 1.56 g (188); 1.65 g estimated using the equation mass = 0.548 × length × breadth2 (189). Eggs approximately 15% of adult female mass.

Color

Ground color brilliant, buffy, or creamy white, slightly glossy, well speckled with dots and small blotches of various shades and tints (brown, reddish brown, purple, chestnut, gray, purplish gray, lavender, and violet) in a wreath around the larger end (29, 12, 94, 14, 2, 160).

Surface Texture

No information.

Eggshell Thickness

No information. From WFVZ: Empty shell weight from sample drawn throughout North American range: 0.086 g (range 0.072–0.113, n = 22 clutches, 86 eggs).

Clutch Size

Usually 4 or 5 eggs per clutch. Mean 4.13 eggs (range 2–6, n = 23 nests) in Ontario; most (17 of 23) contained 4 or 5 eggs (87). Mean 4.36 eggs (range 3–5, n = 11 nests) in Michigan (90). Mean 4.45 eggs ± 0.70 SD (range 3–5, n = 75 nests) based on non-parasitized nests in 5 nest record schemes (NRS) and egg collections (CNRCP, Maritimes NRS, Quebec NRS, Royal Ontario Museum NRS, WFVZ egg collection); most (66 of 75) contained 4 or 5 eggs. Four-egg and 5-egg clutches laid with equal frequency in Vermont (97). One nest with 6 nestlings found in Ontario (87). Mean 4.8 eggs (range 4–6, n = 54 nests) in New Hampshire; most (42 of 54) contained 5 eggs (L. Reitsma et al., unpublished data).

Egg Laying

Female lays 1 egg per day until final egg is laid.

Incubation

Onset of Broodiness and Incubation in Relation to Laying

Incubation starts with laying of final egg.

Incubation Patch

Females caught in New Hampshire that were known to have active nests had brood patches. However, females caught prior to egg laying did not. Male is not known to incubate, thus no brood patch.

Incubation Period

In New Hampshire, 11–12 d, on average.

Parental Behavior

From Krause (160). Only the female incubates. One female spent 85% of time on nest. Based on 39.7 h of observation at a single nest in Michigan, average on-bout lasted 32 min (range 1–84), average off-bout lasted 7 min (range 1–17, median 4 min).

Considered close sitters, difficult to flush. One sitting female remained motionless on the nest for 10–20 min periods and appeared to doze off, broken by minor activity; yawning, swallowing, stretching neck or wings, rising up in nest, turning eggs, shifting position (160). Sometimes utters low chip as leaving nest. Return after off-bout slower and less direct than departure; includes perching and scanning if intruders present in area.

Male frequently joins female when foraging during off-bout, typically remaining silent several feet away from female; male appears to be following female rather than vice versa. One Michigan male sometimes perched near nest for 1–10 min periods during on-bouts (at least 3% of incubation during 39.7 h of observation). One male approached nest 9 times during 39.7 h of observation, in all cases he gave chip, and female left nest immediately prior to his arrival. Male often displays anticipatory feeding behavior prior to hatching (127). One male called female off nest and “brought food” to unhatched eggs 9 times during 39.7 h of observation over 7 d, and as early as 8 d prior to hatching; male gave food to incubating female only once and female initially refused food. A male in New Hampshire fed an incubating female who accepted.

Hardiness of Eggs Against Temperature Stress; Effect of Egg Neglect

Eggs appear to be hardy in comparison to other wood warbler species. During prolonged bouts of rainy cool weather (about 10°C as a low) in the White Mountains, New Hampshire, few eggs failed, compared to high weather-induced failure in the Black-throated Blue Warbler (Setophaga caerulescens).

Hatching

More synchronous than laying—usually all 5 eggs hatch within 24 hours. Eggs that fail to hatch after this may be infertile; up to 2 unhatched eggs found in one nest (M. Goodnow, personal observation).

Young Birds

Early growth asymmetry can be significant. In one case, a 2 day-old fledgling was twice the mass of another, but this most often evens out by fledging. Using an equation developed by Ricklefs (190), {W(d) = A/[1 + e – K(d – T)]}, where Wd = the nestling mass (g) at days of age, d = age of nestlings in days, A = asymptotic mass (g),
K = growth rate, and T = ln(A/W-1)/K, 90% of the nestlings' growth (g) was obtained in 5.7 days. Most of the feather development occurs after day 4 and pin feathers on the wings are not exposed until day 5 or day 6. Nestlings reached their asymptotic mass (9.6 g) in about 6 days. Tarsus length asymptotes at about 5.5 days.

Parental Care

Brooding

No information.

Feeding

Roles of Parents

Both parents feed nestlings. Male carried food to one nest in Illinois twice as often as female did when nestlings were approximately 6 d old (164). Male took dominant role in feeding young at one Michigan nest (160), but at another, female fed more than male (191). May result from differences between sexes in response to observers; male appeared bolder than female in feeding young at one nest (male fed 14 times and female twice during 2 hours of observation; 192).

In New Hampshire, however, females visited the nest at higher rates than did males (P = 0.043; 31 hours of observation); duration of each visit (~20 s, n = 169 visits) was not significantly different between the sexes (P = 0.49). Also in New Hampshire, females more aggressive than males in defending nests and fledglings (M. Goodnow, LR, MTH, personal observation). Female observed gleaning insects from leaves very near nest (164).

Method of Feeding

Direct; carries food in bill and places it in opened mouths of young. May regurgitate food when nestlings are young (1–3 days of age).

Food of Young: Kinds and Size of Items

Little information. Observed feeding nestlings yellow grubs, lepidopteran larvae, brown measuring worms, moths, and mosquitoes (14). Crane flies (Tipulidae) brought to young in New York (187). Dipterans also key food in New Hampshire: most of food delivered was adult dipterans (45.4%) and lepidopteran larva (34.0%) (M. Goodnow et al., unpublished data). Male brought larger insects than female at a nest in Michigan; female fed little wooly aphids that were very abundant (191).

Rate of Feeding

Little information. During 230 min of observation at 1 nest, male fed 3- 4-d-old nestlings 30 times, female fed 49 times (191). Another pair fed at 3–6 min intervals (extremes, 1–20 min; C. J. Stanwood in Bent [14]). No information on amount of food brought to nest or apportionment among nestlings.

Nest Sanitation

Both parents remove fecal sacs (14).

Cooperative Breeding

There is one record of a female feeding young at the nest of another female, and another record of a male that was not part of the pair bond delivering food to nestlings (M. Goodnow et al., unpublished data). In the latter case, the male was chased by the male associated with that nest (LRR, personal observation).

Brood Parasitism by Other Species

Identity of The Parasitic Species

Brown-headed Cowbird (Molothrus ater).

Frequency of Occurrence, Seasonal or Geographic Variation

Considered to be fairly regularly parasitized by cowbirds in suitable localities (193, 127), although little information. Needs more detailed study. Parasitized nests reported in Saskatchewan, Ontario, New Brunswick, New York, Indiana, Michigan, and Minnesota (194). Five of 25 nests (20%) parasitized by cowbirds in Ontario; 3 of these nests contained 3 cowbird eggs each (87). In Michigan, 3 of 14 nests (21.4%) parasitized (90), and 6 of 11 nests parasitized (54.5%; 195, 196, 192). None of 72 nests parasitized in New Hampshire (L. Reitsma et al., unpublished data).

Proportion of observed nests with cowbird eggs underestimates frequency of parasitism if hosts abandon parasitized nests or reject cowbird eggs, or if parasitized nests suffer higher probability of depredation than non-parasitized nests.

Timing of Laying in Relation to Host's Laying

No information.

Response to Parasitic Mother, Eggs, or Nestlings

No information, but at least some individuals accept and successfully hatch and fledge cowbirds.

Effects of Parasitism on Host

Little information. Of 3 nests with a cowbird nestling in Michigan, 2 nests had 1 unhatched host egg and the other had only 1 host nestling (195, 192), suggesting removal of host eggs and/or reduced survival of host young. The only remaining unhatched host egg in one of these nests was partially encased in the broken shell of the cowbird egg, and the cowbird was ready to fledge (192). One nest in Ontario was abandoned after cowbirds laid 3 eggs and broke or removed the 3 host eggs (148). Of 11 parasitized nests in nest record schemes (CNRCP, Maritimes NRS, Quebec NRS, Royal Ontario Museum NRS), 2 (Nova Scotia, Minnesota) fledged a cowbird but no host young, 1 was depredated, 1 failed due to human activity; outcomes of others not determined. Fewer host eggs in parasitized nests (mean 2.5 eggs ± 0.85 SD [range 1–4 eggs, n = 10]) compared to non-parasitized nests.

Fledgling Stage

Departure from the Nest

Average age at departure for 58 nests in New Hampshire: 8.1 d. At departure, young are barely able to fly but are occasionally vocal. Five young jumped the nest when approached by a human; only 2 days earlier these young were unable to open their eyes (164). Thus vulnerable to force-fledging; such fledglings most often survive if ≥ 6 d old (M. Goodnow et al., personal observation). Telemetry showed young birds (n = 4) to stay relatively close to the nest site for a few days (L. Reitsma, unpublished data). Hop from stem to stem usually lower to the ground in thick bracken fern (Pteridium aquilinum), balsam fir (Abies alba) patches, or small thick shrubs, often times not moving for hours.

Growth: Mass, Proportions, Structures

Post-fledging growth is rapid. Fledglings capable of flight within 2–3 d after fledging and have full-length tail and flight feathers within 1–2 wk. Faint necklace visible by about 3 weeks of age (or 2 weeks from fledging).

Association With Parents or Other Young

Parents continue feeding young after they leave the nest (127). Male observed feeding fledgling a few days out of nest (197).

When the adults feed fledglings they tend to chip softly until the fledgling answers, or they just return to the fledgling's previous location. Parents may divide up fledglings: much of the time, same parent feeds a specific fledgling, especially after fledglings disperse away from siblings (M. Goodnow, personal observation).

Parents still on territory feeding fledged young 1 wk after fledging in Minnesota (185). Duration of feeding varies, as does distance of dispersal from nest among fledglings. One fledgling in New Hampshire was found 370 m from male parent's territory only 6 d after fledging; another stayed within 50 m of nest site for at least 12–18 d (M. Goodnow et al., unpublished data).

Ability to Get Around, Feed, and Care for Self

Fully self-reliant within 2 wk of fledging.

Immature Stage

Little information. Skull ossification complete 15 October through December (19).

Recommended Citation

Reitsma, L. R., M. T. Hallworth, M. McMahon, and C. J. Conway (2020). Canada Warbler (Cardellina canadensis), version 2.0. In Birds of the World (P. G. Rodewald and B. K. Keeney, Editors). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.canwar.02