Breeding

Pair Formation

In central Alberta, most females paired within 1 d of their arrival (CC).

Nest-Building

Little information; in Utah, “soon” after arrival of females (Frydendall 1967); in central Alberta, about 2–3 d after their arrival (CC).

First/Only Brood Per Season

See Figure 3 . Late May to mid-Jun. Egg dates: Texas records: 17 May–13 Jul (Oberholser 1974); Ontario nest records: 15 May–17 Jul (n = 967), most of these 2 Jun–15 Jun (n = 483; Peck and James 1987); s. Manitoba study: clutches initiated between 26 May–7 Jul, with 50% of clutches initiated within 2–8 d interval (10–11 Jun 1974, 3–8 Jun 1975, 28 May–4 Jun 1976; Goossen and Sealy 1982); for n. Manitoba, 14 Jun–10 Jul (Briskie 1995); for central Alberta: 30 May–21 Jun, most (115 of 127) 2–13 Jun (CC); for British Columbia: 10 May–16 Aug, most (250 of 455) 7–23 Jun (Campbell et al. in press); for se. Alaska, 24 May–26 Jun (n = 20; Rogers 1994). One brood normally reared; second broods only rarely attempted (Goossen and Sealy 1982).

Selection Process

No information.

Site Characteristics

Nest built in upright fork of bush, sapling, or tree. In British Columbia, nest height 0–14 m, most (235 of 385) between 1.2–2.8 m (Campbell et al. in press). In central Alberta, nest height ranged from 0.5–4.20 m, most (113 of 211) between 1.2 and 2.2 m; all in willows (Salix spp.; CC). In Ontario, nest height ranged from 0.3–6.0 m (n = 367), most (n = 183) between 0.9 and 1.5 m; most frequent supporting plants (of 349 nests) were hawthorns (Crataegus spp., 78), willows (52), raspberry and other Rubus spp. (22), northern white cedar (Thuja occidentalis, 21), dogwoods (19), honeysuckle (Lonicera spp., 16) and Spiraea spp. (15; Peck and James 1987). In Colorado, nest height 0.5–2.0 m in bushes (Knopf and Sedgwick 1992). In Illinois, mean nest height 2.5 m (range 0.3–10.7, n = 56; Graber et al. 1983). In n. Manitoba, mean 0.6 m ± 0.18 SD (range 0.24–1.20 m, n = 36; Briskie 1995). As high as 50 ft (15 m; Trautman 1940).

Construction Process

Built by female in about 4 d (n = 3); intervals between construction visits about 4 min but may be as long as 20 min (Schrantz 1943). In n. Manitoba, nests build in 6.2 d ± 1.1 SD (range 4–10 d, n = 5; Briskie 1995). In central Alberta, nest built in 3 d (n = 1) or 4 d (n = 6); in one case of renesting, material of depredated nest used to construct new nest in 2 d (CC).

Structure And Composition Matter

Deep cup built of grasses and strips of bark; outside covered with plant down and fine fibers. Description of nest structure based on Mico’s (1998) work with nests from willow community at Delta Marsh, s. Mani–toba. Nests without buried cowbird eggs were either 3-layered (n = 87) or 2-layered (n = 26). The 3 layers comprised (1) base constructed primarily of nettles (Urtica dioica), (2) frame composed mainly of grass fibers, and (3) liner composed mainly of deer (Odocoileus virginianus) hair, feathers, and fibers of airborne seeds (mostly Eastern cottonwood [Populus deltoides], dandelion [Taraxacum officinale], willow [Salix spp.], and cattail [Typha spp.]). All listed materials found in all layers but proportions differ. Two-layered nests included only base and liner. Nests with buried cowbird eggs (n = 27; see Brood parasitism, below) were more complex, with up to 6 layers (3 above and 3 below buried egg); only liner of bottom nest always present. Nests in forks of willow generally deep, V-shaped and usually 3-layered; wider nest sites accommodated nests with flat (U-shaped) bottoms and usually 2-layers. Mean mass of 3-layer nest (n = 87): base, 3.40 g ± 1.52 SD; frame, 1.96 g ± 1.04 SD; and liner, 1.06 g ± 0.52 SD. Spider webs on outside of nest and around supporting branches. In Labrador, one nest described as “almost entirely of white rabbits’ fur” (Todd 1963).

Dimensions

For nests from central Alberta, mean outside height 7.65 cm ± 1.85 SD (range 5–14, n = 134), inside diameter 4.74 cm ± 0.87 SD (range 3.0–7.9, n = 118), inside depth 3.61 cm ± 0.74 SD (range 1.60–6.00, n = 112; CC). For 30 nests from s. Manitoba, outside diameter 7.2 cm ± 0.77 SD, depth 6.6 cm ± 0.99 SD, inside diameter 4.7 cm ± 0.43 SD, depth 4.0 cm ± 0.49 SD; in n. Manitoba, nests with thicker walls (2.0 cm rather than 1.3 cm; n = 30 for both groups) giving larger outside dimensions (Briskie 1995). For 15 Ontario nests, outside dimensions ranged from 4.5–9 cm diameter and 4.5–11 cm depth and inside dimensions of 3.5–6.4 cm diameter and 3–5 cm depth (Peck and James 1987).

In response to cowbird parasitism, Yellow Warblers frequently add to nest, covering cowbird egg(s)—and any of its own—producing nest of ≥2 tiers and of greater height. In s. Manitoba, burial nests larger in bottom measurement (mean 4.49 cm ± 2.38 SD [n = 27]) and heavier (mean dried mass 8.12 g ± 2.72 SD [n = 27]) than 3-layered nonburial nests (mean 6.40 g ± 1.74 SD [n = 87]; 3.60 cm ± 1.38 SD [n = 87]). Two-layered nests smaller in size (especially at the bottom) and lower mass than 3-layered nests (Mico 1998).

Microclimate

No information.

Maintenance Or Reuse Of Nests

Does not reuse nests but will reuse material from old nests when building new nests (Kammeraad 1966, CC). Pair seen to obtain nest material from nearby, active American Redstart nest (Robbins 1990).

Nonbreeding Nests

No information.

Shape

Usually ovate.

Size

Mean length and breadth (D. p. aestiva populations): 70 eggs from nw. Iowa: 17.32 mm (range 15.5–20.5) × 13.29 mm (range 12.3–16.0; Schrantz 1943); 85 eggs from s. Manitoba: 16.80 mm ± 0.74 SD × 12.60 mm ± 0.34 SD (Sealy 1992).

Mean length and breadth from Western Foundation of Vertebrate Zoology (WFVZ) data based on clutch means: D. p. aestiva, 92 eggs from 22 clutches, 16.57 mm (range 15.22–18.24) × 12.70 mm (range 11.71–13.39); D. p. amnicola, 65 eggs from 15 clutches, 16.99 mm (range 15.53–18.10) × 12.87 mm (range 11.72–13.78); D. p. morcomi, 85 eggs from 22 clutches, 16.76 mm (range 14.88–18.35) × 12.60 mm (range 11.33–13.45); D. p. rubiginosa, 69 eggs from 15 clutches, 16.88 mm (range 15.40–18.04) × 13.25 mm (range 11.88–13.39); D. p. sonorana, 30 eggs from 8 clutches, 16.53 mm (range 14.73–18.20) × 12.53 mm (range 11.69–13.11).

Mass

For eggs from s. Manitoba: mean 1.43 g ± 0.37 SD, n = 85 (Sealy 1992); about 15% of non-laying female mass. Mass increases with laying sequence for eggs within a clutch (Hébert and Sealy 1993a).

Color

See note at beginning of Appearance, below, regarding color designations. Background color grayish white or greenish white, occasionally bluish white or soft, pale green. Spots and blotches form wreath around large end and may show much overlap; spotting ranges from tiny flecks to approximately 6 mm diameter with variety of colors; Bent (1953) mentions fuscous (5YR 3.5/1.0; 21), olive brown (10YR 3.8/2.0; 28), citrine drab (5Y 5.2/4.0; 51), buffy brown (10YR 5.5/4.0; 24), buffy olive (5Y 5.0/4.0; 51), light brownish olive (2.5Y 4.6/3.5), raw umber (7.5YR 3.5/3.0; 23), metal bronze and tawny olive (1Y 5.2/4.5; 26).

Surface Texture

Slightly glossy.

Eggshell Thickness

No information. Mean mass of dry, empty shell from WFVZ data based on clutch means: D. p. aestiva, 92 eggs from 22 clutches, 0.076 g (range 0.059–0.087); D. p. amnicola, 65 eggs from 15 clutches, 0.078 g (range 0.059–0.096); D. p. morcomi, 85 eggs from 22 clutches, 0.073 g (range 0.059–0.090); D. p. rubiginosa, 69 eggs from 15 clutches, 0.076 g (range 0.058–0.087); D. p. sonorana, 30 eggs from 8 clutches, 0.073 g (0.058–0.088). No data comparing DDT and post-DDT eras.

Clutch Size

Four or 5 eggs are common number. In 3-yr Manitoba study, overall mean clutch 4.5 eggs ± 0.60 SD (n = 144; Goossen and Sealy 1982); more extensive data from same area give mean clutch 4.45 eggs ± 0.02 SD (n = 1005) for unparasitized nests; 4.15 eggs ± 0.04 SD (n = 216) for parasitized nests (Sealy 1992). See Table 1 .

Egg-Laying

Eggs laid 1/d, approximately 24 h apart (McMaster et al. 1999); occasionally days skipped in laying sequence (Smith 1943), but this more likely early in season and early in laying sequence. At one nest, first egg laid before 06:30 on day after nest finished (Schrantz 1943). On average, females enter nests to lay within 10 min after sunrise (McMaster et al. 1999). At 10 nests in Manitoba, egg-laying occurred between 05:15 and 06:15, mean time 05:52 (sunrise times 05:2305:19 CDT; Sealy 1992). Female on nest average 25.5 min ± 12.0 SD (range 13–50, n = 10) for laying bout (Sealy et al. 1995), actual egg-laying occurred midway in this interval and took about 2 min (McMaster et al. 1999).

Of >1,500 clutches monitored in Manitoba, intraspecific egg dumping indicated at 4 nests that received >1 egg/d during egg-laying and possibly at 7 others with large clutches (6 with 6 eggs and 1 with 7 eggs; Sealy et al. 1989).

Onset Of Broodiness And Incubation

By female alone (Schrantz 1943).

Incubation Patch

Female only.

Incubation Period

Begins before clutch is completed. For eggs in incubator, 37.5°C, 50–60% relative humidity, 11.9 d ± 0.39 SD (n = 15; McMaster and Sealy 1998). Measured as interval between last egg laid and last egg hatching: 11 d (n = 1; Bigglestone 1913); 11 d (range 11–12, n = 6; Schrantz 1943). For s. Manitoba, 11.3 d ± 0.47 SD (n = 22); for n. Manitoba, 11.7 d ± 0.81 SD (range 11–13 d, n = 15; Briskie 1995); for central Alberta, 10 d (n = 6), 11 d (n = 16), 12 d (n = 6) or 13 d (n = 1; CC); for Colorado, 10.5 d ± 1.2 SD (n = 16; Ortega 1998).

Incubation attentiveness (measured as time on nest per 0.5 h-observation period = 1,800 s) increased during laying sequence; in 4- and 5-egg clutches attentiveness increased from 381.92 s ± 502 SD (n = 26) for day ante-penultimate egg laid (i.e., egg 3 in 5-egg clutch), to 922.13 s ± 671 SD (n = 29) for day penultimate egg laid, to 1,241.38 s ± 578 SD (n = 31) for day last egg laid (Hébert and Sealy 1992).

Parental Behavior

Female remains on nest overnight with “quiet” periods of 10–20 min interrupted by some movement; during day, female became active about 12 min after sunrise (04:53) until 9 min before sunset (19:56). In between these times, attentive periods average 35.7 min, off-time 3.0 min (Kendeigh 1952). Male may feed incubating female; male feeding rate 5.6 times/h based on 4.5 h observation over 7 d (Kendeigh 1952).

Hardiness Of Eggs Against Temperature Stress; Effect Of Egg Neglect

Little information. Experimental removal from territory of 5 incubating females included holding their eggs (unknown stage of incubation) 24 h at 28°C; 2 clutches later hatched, other clutches deserted or depredated (Weary et al. 1994).

Preliminary Events

No information.

Shell Breaking And Emergence

For 1 egg, emergence <4 min; egg observed to move in nest and shell bulged as shell cracked and young pushed its way out; eggshell eaten by both adults (Bigglestone 1913). Time of day: at one nest, last egg hatched at 05:30 (Bigglestone 1913); of 119 eggs, 9 hatched in afternoon, 110 during early morning or at night (Schrantz 1943).

Hatching spread is interval between hatching of first and last eggs in clutch. For s. Manitoba, this value averaged 36.7 h ± 9.6 SD (n = 19) in 4-egg clutch; and 50.5 h ± 9.2 SD (n = 21) in 5-egg clutch (Hébert 1993). For n. Manitoba, hatching spread 1.3 d ± 0.51 SD (range 1–2 d, n = 6) for 4-egg clutches; 1.2 d ± 0.40 SD (range 1–2 d, n = 20) for 5-egg clutches; 77% of all (n = 26) clutches hatched within 24 h (Briskie 1995).

Condition At Hatching

Altricial and nidicolous. At hatching, 1.32 g ± 0.21 SD (n = 112; Hébert 1993). Natal down light smoke gray (Smith 1943), skin reddish (Kammeraad 1964).

Growth And Development

Mean daily mass, based on collective data from Schrantz (1943: 380), Biermann and Sealy (1982: 339), Biermann and Sealy Weatherhead (1989: 364), Briskie (1995: 538) and Ortega (1998: 196): at age 0 d, 1.5 g; 1 d, 2.2 g; 2 d, 3.2 g; 3 d, 4.8 g; 4 d, 6.1 g; 5 d, 7.5 g; 6 d, 8.5 g; and 7 d, 9.0 g. Growth rate for surviving young shows no statistically significant differences due to brood size (see Schrantz 1943, Biermann and Sealy 1982, Briskie 1995) or to presence of Brown-headed Cowbird young (Weatherhead 1989). Specific growth rate constant (K) similar for different brood sizes (based on composite growth rate for each brood): 0.546, 0.551, and 0.541 for broods of 3, 4, and 5 young, respectively (Biermann and Sealy 1982). Specific growth rate (based on growth rates calculated individually), K = 0.667 ± 0.174 SD (n = 157 Yellow Warbler young in 37 nests without cowbird young) and 0.636 ± 0.164 SD (n = 66 Yellow Warbler young in 23 nests with cow-bird young; Weatherhead 1989).

See note at beginning of Appearance, below, regarding color designations. All feather tracts visible by second day; at 2 d, bill Isabella color (2.5Y 5.8/4.5) with commisure olive ocher (5Y 6.5/6.5); at 7 d, head and back citrine drab (5Y 5.2/4.0; 51), wings dark olive (5Y 3.5/2.0; 30), wing-bars dark olive buff (5Y 6.2/4.5), throat citrine drab, breast and belly olive ocher (Smith 1943). Eyes open when about 3 d (Kammeraad 1964).

Brooding

By female. At 1 nest observed in nw. Iowa, brooding periods ranged from 1–12 min with few even longer (23, 24, and 32 min); female would “fluff out the under coverts against the rim of the nest and bring the wings down, just inside, so as to effectually close the nest. [There were] different brooding attitudes for the varying circumstances. [Posture described used] against the cold . . . [and] rains. During the heat of midday she usually stood in the nest with wings spread, shielding the young, but without shutting off the circulation of the air. On the contrary, at times she gently flapped her wings, as if fanning the young” (Bigglestone 1913: 58; passage repeated in Bent 1953: 168); brood periods averaged 22.7 min and off time 4.8 min based on measurements as determined by thermocouple recordings (Kendeigh 1952).

Feeding

At one nest in nw. Iowa observed entire nestling period (144 h 53 min), male made 813 feeding visits (first 7 d only) and female 1,560 visits (Bigglestone 1913). In Manitoba, broods with 2-d old young selectively fed geometrid larvae; broods of 8-d old young selectively fed chironomid, geo-metrid, and other Lepidoptera larvae (Biermann and Sealy 1982). Feeding rates vary with size and age of brood (increasing with both) and sex of parent (male more than female). For broods of 2-d-old young: Male feeding rates varied from 1.9 trips/30 min ± 1.4 SD (n = 48) for broods of 3 young, to 3.1 trips/30 min ± 1.8 SD (n = 36) for broods of 5 young; female feeding rates ranged from 0.9 trips/30 min ± 1.8 SD (n = 36) for broods of 5 young, to 1.6 trips/30 min ± 1.1 SD (n = 32) for broods of 4 young. For broods of 8-d-old young: Male feeding rates varied from 3.8 trips/30 min ± 2.8 SD (n = 32) for broods of 3 young, to 5.2 trips/30 min ± 2.8 SD (n = 31) for broods of 4 young; female feeding rates ranged from 2.7 trips/30 min ± 2.2 SD (n = 31) for broods of 4 young, to 5.6 trips/30 min ± 2.8 SD (n = 32) for broods of 5 young (Biermann and Sealy 1982; see also Lozano and Lemon 1995, 1998). Each nestling received about 6.2–7.2 mg protein/h at 2 d of age and 13.2–19.4 mg protein/h at 8 d (Biermann and Sealy 1982). At one nest observed the whole day when first of 3 young fledged, male and female made 61 feeding visits between 05:00 and 06:00; male stopped coming to nest after 06:00 when first young left; female made 518 additional feeding visits between 06:00 and 19:43 (Finley 1917, cited in Graber et al. 1983). For 15 h observation at 8 d after hatching, males averaged 11.1 trips/h ± 5.8 SD (range 8.3–16.3); females 8.3 trips/h ± 4.6 SD (range 4.3–11.3; Briskie 1995).

Which young is fed by adults determined more by height reached by begging chicks—not simply nestling size, begging intensity, or species, even in nests with Brown-headed Cowbird young (Lichtenstein and Sealy 1998).

Quality of paternal care reported to vary inversely with amount of breast streaking (Studd and Robertson 1985a, 1985b, 1988), but a more recent study has failed to confirm this finding (Lozano and Lemon 1996).

Nest Sanitation

At one nest observed for entire (10 d) nestling period with 4 young for first 2 d and 3 young for remainder of nestling period (Bigglestone 1913), 363 fecal sacs produced; male ate 18, carried off 70; female ate 76, carried off 189; fecal sacs fell to ground 8 times; unknown outcome for 2 fecal sacs.

Not known to occur.

Identity Of Parasitic Species

Brown-headed Cowbird (Molothrus ater) and Shiny Cowbird (Molothrus bonariensis).

Frequency Of Occurrence

Yellow Warbler abun-dantly recorded host of Brown-headed Cowbird (Friedmann 1963; see Table 2); a consequence, in part, of the warbler’s own abundance and broad sympatry with Brown-headed Cowbird.

Golden Warbler parasitized by Shiny Cowbird (see Table 2).

Timing Of Laying In Relation To Host’S Laying

For 255 cowbird eggs laid in nests in s. Manitoba, 105 laid before any warbler egg (and of these 66 abandoned), 89 on same day as warbler’s first or second egg, 35 on day of third or later egg; and 25 after clutch completed (Sealy 1992).

Response To Parasitic Mother, Eggs, Or Nestlings

Yellow Warblers identify and respond aggressively to Brown-headed Cowbirds in areas of sympatry, as indicated by experimental presentation of mounts (Folkers and Lowther 1985, Briskie et al. 1990, Gill and Sealy 1996, Gill et al. 1997), but allopatric pop-ulations do not (Briskie et al. 1992, Briskie 1995). Young (second-year [SY]) females may simply utter Seet alarm calls, but older females sit in nest as if incubating and extend wings (“nest-protection behaviour”; Burgham and Picman 1989, Hobson and Sealy 1989b). In natural encounter, first male warbler, then female, approached and followed cowbird (cowbird sex not specified; Kammeraad 1964).

Yellow Warbler frequently responds to cowbird parasitism by building over the parasitized clutch making multi-tiered nests. Ontario nest cards recorded 29 two-tiered nests, 7 three-tiered, 2 four-tiered and 1 five-tiered nest out of 399 parasitized nests (Peck and James 1987). One warbler nest with 6 tiers contained 11 cowbird eggs, distributed as follows: 1 cowbird egg within original base of nest (laid during nest construction), 3 cowbird eggs in first tier, 1 in second tier, 2 in third tier, 2 in fourth tier, 1 cowbird plus 1 warbler in fifth tier and 1 cowbird in top; nest was 14.6 cm tall (Berger 1955).

Yellow Warbler more likely to desert or bury cowbird egg if cowbird egg appears before any warbler egg or appears early in laying sequence. For nests at which warblers responded to parasitism (buried cowbird egg or deserted), mean number of warbler eggs present in nest at time of parasitism was 0.93 (n = 27) for burying nests and 1.08 (n = 13) for deserting nests. At nests with accepted cowbird eggs, mean number of warbler eggs present at time of parasitism was 2.33 (n = 46). For 51 cowbird eggs laid in Yellow Warbler nests, 14 eggs accepted, 9 eggs hatched, 2 young left (Weatherhead 1989). Response to cowbird parasitism by yearling and older females similar (Sealy 1995).

For 1,885 Yellow Warbler nests in s. Manitoba, 396 were parasitized, receiving 1 (354 nests), 2 (38 nests) or 3 (4 nests) cowbird eggs (Sealy 1992). Of 109 nests in Ontario, 45 parasitized: 28 cowbird eggs in 20 nests buried, 12 cowbird eggs in 10 nests deserted, 16 cowbird eggs in 12 nests accepted; cowbird egg acceptance more likely if ≥2 warbler eggs in nest at time cowbird lays (Clark and Robertson 1981).

Effects Of Parasitism On Host

Nests abandoned or covered over with a new lining may involve loss of warbler eggs (see above). Clutch size is reduced because of host egg removal by cowbird (see Table 1; Weatherhead 1989; warbler clutch reduced by 0.32 eggs, Sealy 1992). Within-nest competition between cowbird and warbler young usually detrimental to warbler, but sometimes warbler young do survive with cowbird. In s. Manitoba, 58 parasitized nests included 235 warbler eggs, of which 95 hatched and 39 survived to leave the nest; overall warbler production 0.7 young/parasitized nest, significantly lower than 1.8 young/unparasitized nest (Goossen and Sealy 1982).

Success Of Parasite With This Host

Yellow Warblers are often successful in rearing Brown-headed Cowbird young (see Table 2). The apparent high success warblers show in rearing Shiny Cowbird young in Puerto Rico may be because difference in cowbird mass (about 34 g for Shiny Cowbird on Puerto Rico vs. about 45 g for Brown-headed Cowbird in e. U.S.).

Departure From The Nest

Young leave nest 8–10 d after hatching; in s. Manitoba, mean 8.2 d ± 0.80 SD (n = 12); in n. Manitoba, mean 8.5 d ± 0.64 SD (n = 14; Briskie 1995).

Growth

No information once young leave nest.

Association With Parents Or Other Young

Young still with adults at 17 d and possibly 21 d after leaving nest (Smith 1943).

Ability To Get Around, Feed, And Care For Self

No information.

No information.