Swainson's Warbler

Limnothlypis swainsonii

Order:
Passeriformes
Family:
Parulidae
Sections

Breeding

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Figure 2. Annual cycle of molt, breeding, and migration of Swainson's Warbler.

Data represent molt, breeding, and migration of Swainson’s Warbler in the southeastern United States; birds from higher latitudes and higher elevation begin breeding later in spring than southern birds do. Thick lines show peak activity, thin lines off-peak.

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Female Swainson's Warbler on nest.
© Nick Anich, Arkansas, United States, 18 May 2005
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Swainson's Warbler nest.
© T.J. Benson, Arkansas, United States, 25 July 2005
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Swainson's Warbler nest.

Nest collected at Mt. Pleasant, Berkeley County, South Carolina, on 9 June. Ruler is in cm; photographer Rene Corado.

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Swainson's Warbler clutch.

Eggs collected at Mt. Pleasant, Berkeley County, South Carolina, on 9 June. Ruler is in cm; photographer Rene Corado.

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Atypical Swainson's Warbler eggs in nest.

Swainson's Warbler eggs are generally white, but rarely are speckled, like this Swainson's Warbler clutch from St. Francis National Forest, Phillips County, Arkansas.

© Jason Sardell, Arkansas, United States, 25 May 2007
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Female Swainson's Warbler incubating eggs.
© T.J. Benson, Arkansas, United States, 3 June 2005
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Female Swainson's Warbler incubating eggs.

White River National Wildlife Refuge, Arkansas, 18 June 2007. Photo by Eric Huskinson.

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Newly hatched and hatching Swainson's Warbler young.

White River National Wildlife Refuge, Arkansas, 14 June 2007. Photo by Eric Huskinson.

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Swainson's Warbler nestlings and egg in nest.
© Jason Sardell, Arkansas, United States, 25 May 2007
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Swainson's Warbler nestling.
© Nick Anich, Arkansas, United States, 30 May 2006
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Swainson's Warbler nestlings in nest.
© T.J. Benson, Arkansas, United States, 3 June 2005
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Swainson's Warbler nestlings in nest.
© Jason Sardell, Arkansas, United States, 23 May 2007
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Adult Swainson's Warbler feeding young at the nest.

Honey Island Swamp, St. Tammany Parish, Louisiana. Photo by Walter Clifton.

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Swainson's Warbler nest with 2 Swainson's Warbler eggs (white) and 4 Brown-headed Cowbird eggs (speckled).
© Nick Anich, Arkansas, United States, 18 May 2005
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Brown-headed Cowbird nestling (right) in a Swainson's Warbler nest with Swainson's Warbler nestling (left).
© Jeremy Brown, Arkansas, United States, 7 July 2006

Swainson's Warbler nests are difficult to locate because of the dense, often thorny, habitat this species favors, and because nests are inconspicuous. In 50 years of research on this species, Brooke Meanley located only about 30 nests (5). More recently, researchers have had more success locating nests (e.g., over 100 nests in 4 separate study areas), expanding knowledge of the breeding ecology of this species (143, 22, 23, 43, 24, 158).

Phenology

Pair Formation

Pairs form soon (~1–7 d) after females arrive on breeding grounds; females typically arrive between several days and 2 weeks after males. Meanley (1) observed, for what he believed was the first time, a female entering the territory of an unpaired male; the female was chased for short distances, but was not driven beyond the territory boundary. Males stay close (within ~2 m) to females for much of the time during pairing and prior to nest building. Pair appears to maintain contact through short-range vocalizations during this period (see Sounds and Vocal Behavior). The pair bond may be broken after nest failure in some circumstances (see Behavior).

Nest-Building

Lasts 2–5 d, with construction entirely by the female (1, NMA, TJB). Nests may be completed several days before laying (43), and some nests may be abandoned after building, although this may be due to cryptic predation or observer influence (TJB, NMA).

First/Only Brood per Season

First clutches generally are laid around 2 weeks after male arrival—late April and early May for populations in the Mississippi Alluvial Valley, Lower Coastal Plain, South Atlantic Coastal Plain, and Gulf Coastal Plain, and likely somewhat later for populations in the mountains.

Second Brood per Season

Pairs routinely renest if broods fail; such attempts occur as late as mid-July; some females have 2 to 4 nests fail in a single season (4, 1, 195, 24).

Perry (4) and Wayne (201) thought that 2 broods were raised in a season because of nests with eggs found in July, but these may have represented renesting attempts after earlier failed broods. Bishop et al. (43) observed a bimodal distribution of nest initiation dates that may correspond to first and second broods and estimated that 21% of nesting attempts over a 2-year period were double-brooding attempts, including one possible case of triple-brooding. Chartier (24) observed 7 cases of double brooding. The interval between renesting or double-brooding attempts has been estimated to be as long as 12 d, ranging from 6–25 d (166), and as short as 2.5 d, ranging from 1–9 d (24).

Nest Site

Selection Process

Nest-site selection may take 3–4 days and both sexes are often together during this period (202). Griscom and Sprunt (138) reported that the nest site is selected by the female alone, but Meanley (202) suggested that the male chooses the nest site. Our observations indicate that the male initially selects and defends the area that eventually will contain the nest, and he subsequently accompanies the female during the period of nest-site selection, but inspection of potential nest substrates appears to be done primarily by the female (NMA, TJB). The male is generally less responsive to song playbacks during this nest-site selection, unless the female is also responsive (TJB, NMA).

Microhabitat

Nests are usually placed near, at the edge of, or within dense growth of giant cane, vines, shrubs, or other dense understory vegetation, often associated with canopy gaps (143). Nests are sometimes situated near water, but generally not over water, unless flooding occurs during the nesting period. Within bottomland forest, nests are usually on the relatively higher elevation sites that are dry in most years and free from the effects of flooding. Initially thought to be located near the edge of a territory, placement is likely related more to the availability of preferred nest-site habitat. Griscom and Sprunt (138) reported that nests are sometimes located outside a male's territory, but Meanley (1) suggested that these estimates of territory size may have been made during incubation stage when males avoid the nest site. Although males do sometimes appear to have nests near the edge of or outside of their territory, these early impressions are likely influenced by underestimates of space use in this species (191). Of 21 nests examined in Arkansas, 10 nests were located within the male’s 55% core area (162).

Site Characteristics

Nests are usually placed at relatively low heights (range 0.4–4.1 m) (27, 5, 203). Mean height of nests in Arkansas 1.39 m (range 0.41–3.30, n = 212) (195), 1.42 m in South Carolina (n = 110) (43), 2.00 m in the mountains of South Carolina (n = 12) (151), 1.83 m in Alabama (n = 33) (203), and 0.85 m in North Carolina (n = 109) (24).

Nests are placed in understory vegetation suspended by several thin vines or supported by giant cane, small trees, or shrubs, especially within giant cane leaves near the plant's stem, or where stems from more than one cane plant intersect. Although nests may be placed in the predominant understory vegetation, some plant species, notably giant cane, may be selected even when rare in the understory of a territory (143). Plants commonly used to support nests include giant cane, greenbrier, grape, and hardwood saplings; other used substrates include eastern hemlock, Japanese honeysuckle (Lonicera japonica), sweet pepperbush, spicebush, palmetto, myrtle (Ilex myrtifolia), rhododendron, laurel, poison ivy (Rhus toxicodendron), Virginia creeper (Parthenocissus quinquefolia), and numerous other vine and shrub species (130, 1, 151, 143, 43, 24, 158).

If the nest is not placed within a thicket, then dense growth of protective cover is generally nearby, and nest sites have generally denser vegetation than surrounding areas (130, 143). In addition to dense understory vegetation, nest sites are generally characterized by a high density of small-diameter woody stems (e.g., < 2.5 cm diameter) including giant cane, vines, or shrubs, high total canopy cover, little ground cover of green vegetation, and abundant leaf litter (159, 142, 145, 143). The overstory trees at the nest site appear to be similar in composition to those throughout the bird's home range (see also Distribution, Migration, and Habitat: Habitat).

Nest

Construction Process

Built entirely by the female. Male may accompany the female to the nest as she builds, but does not help build and only stays briefly (1). Nest-building requires 2–5 days; most construction takes place before 12:00 noon (139). Meanley (202) observed a female working on a nest over a 3-day period from 07:00 to 11:00 and from 16:00 to 17:00; she took 100–125 trips to get nest material in the morning, but ≤ 6 trips each evening; she spent an average of 24 s at the nest with each new addition of nesting material, with a range of 9–70 s.

Structure and Composition of Matter

The outward appearance of the nest is inconspicuous, similar to an unorganized clump of leaves caught in the branches of giant cane, vines, or shrubs (i.e., aerial leaf litter). The nest may be cup-shaped or appear like a disorganized leaf clump containing a nest cup, and is built from materials gathered relatively close to the nest site. Some have hypothesized that the Swainson's Warbler places its nest in areas where natural accumulations of leaves are common in potential nest substrates, and that these clumps may serve as nest decoys (142, 151).

The outer layer is loosely constructed of dried and partially skeletonized leaves, with few sticks, and some vines and tendrils. The next layer forms the outer shell of the nest cup and is more compactly built, largely using the stems of skeletonized leaves, very small twigs, or needles (1). The nest is usually lined with only a few of the following types of material: red maple (Acer rubrum) pedicels, pine needles, bald cypress (Taxodium distichum) twigs and needles, Spanish moss (Tillandsia usneoides), hair, rootlets, grass, and fern stems (199, 204, 1, 205). Meanley (139) disassembled 2 nests in Arkansas that had 418 and 323 plant pieces respectively. The mean dry weight of nests was 34.5 g (n = 4, range 24.3–47.7), with nests from renesting attempts being lighter than initial nests (139). However, nest size does not necessarily decrease between attempts and nests from the same female may appear similar (TJB, NMA).

Dimensions

Largest nest of all North American wood-warblers (Parulidae) that nest above ground (1); generally similar in size to a Northern Cardinal nest, although Swainson's Warbler nests have much smaller cups and are constructed using few sticks. Average outside diameter 14.02 cm, inside diameter 5.95 cm, outer depth 8.75 cm, and inner depth 4.29 cm (n = 89–92 nests) (43).

Microclimate

Nests are sometimes concealed from above by vegetation such as leaves of giant cane, shrubs, saplings, vines, or palmetto fronds (1), and side concealment is generally much greater due to greater shrub-level vegetation density than in surrounding areas. Quantitative microclimate data has not been recorded.

Maintenance or Reuse of Nests, Alternate Nests

Not known to reuse nests or build alternate nests. When renesting or double-brooding, females build new nests. Nests generally do not survive until the next breeding season; nests that persist between years are obviously damaged, often not recognizable as a nest, and unlikely to be mistaken for a nest from the current season (TJB, NMA). However, nest sites may be in close proximity across years, occasionally in the same plant (TJB, NMA).

In some areas, golden mice build domes over old Swainson's Warbler nests and use them for nests and feeding platforms (142); this has been observed across the range, in Louisiana (142), Arkansas (NMA, TJB), and North Carolina (N. Chartier, personal communication).

Non-Breeding Nests

Not known to construct nonbreeding nests; however, females may build nests that are not used for breeding if abandon nests before egg-laying; in general, this appears to be because of disturbance at the nest site. However, a large proportion of nests that are found, even early in the breeding season, are empty, although this is likely due to high rates of nest predation (22).

Eggs

Shape

Elliptical.

Size

Length: mean 19.39 mm (range 17.99–21.76); breadth: mean 14.95 mm (range 14.14–15.57); n = 20 clutches, 69 eggs (Western Foundation Vertebrate Zoology [WFVZ]).

Mass

Empty shell weight: mean 0.130 g (range 0.113–0.154); n = 20 clutches, 69 eggs (WFVZ). No data on whole-egg mass.

Color

Eggs are usually white and unmarked and can appear bluish or pinkish, although this pinkness appears to be dependent on incubation stage. Rarely, eggs are faintly spotted with reddish brown (206, 207); out of at least 112 laying-stage or incubation-stage nests observed in Arkansas, only 2 had these spots—clutches likely laid by the same female (they occurred within the same general area in the same year; NMA, CR, TJB).

Surface Texture

Smooth, slight gloss (208).

Eggshell Thickness

Eggshell thickness index values (from Ratcliffe [209]; eggshell weight in mg/[length in mm × breadth in mm]) of 0.45 based on the average WFVZ values above (n = 69 eggs), and 0.39 (n = 2 eggs) from Georgia (210).

Clutch Size

Clutches are generally 3 or 4 eggs, although clutches of 1 to 7 eggs have been observed. Mean clutch sizes reported throughout the breeding range include 3.43 in Arkansas (range 2–4, n = 68) (22), 3.30 in Louisiana (range 2–4, n = 84) (158), 3.65 in Missouri (n = 17) (159), 3.19 in South Carolina (range 1–7, n = 69) (43), 2.98 in North Carolina (range 1–4) (24), and 3.3 from nest records and collections from throughout the range (range 2–5, n = 90) (Cornell Nest Record Card Program, WFVZ).

Egg-Laying

The first egg is often laid 2–3 days after nest completion (average 2.64, range 1–6 d; 43), then one additional egg is laid daily until the clutch is complete (1). Eggs are laid in the early morning; at one nest in Virginia, all eggs were laid before 07:00 (202). Chartier (24) observed the average laying time as 07:49. Clutches lost early in the breeding season may be replaced with a new nest and clutch, and up to 3 or 4 consecutive clutches of eggs may be laid by one female in different nests in a breeding season (1, 166, 195). Eggs lost to cowbird nest parasitism or predation are not replaced, and females may abandon nests after parasitism or predation during the laying phase. Intraspecific egg dumping is not known.

Incubation

Onset of Broodiness and Incubation in Relation to Laying

Incubation begins when the last egg in the clutch is laid (1).

Incubation Patch

Females have a ventral brood patch, males have none.

Incubation Period

Generally 14 d, ranges from 11–15 d (211, 202, 166). Chartier (24) estimated as 13.22 d in North Carolina, whereas Bishop et al. (43) estimated as 13.85 d based on birds at their South Carolina site.

Parental Behavior

Only the female incubates. Female sits tightly on the eggs and if flushed will drop to the ground and perform a distraction display (see Behavior: Predation, Response to Predators). Male may feed the female during this stage as they forage on the ground together, and may occasionally feed her at the nest (e.g., up to 1–2 times per day; 1, CR, TJB). Based on video data from nests in Arkansas, average female incubation bouts lasted 58 min (range 17–126, n = 18 nests). Trips away from the nest averaged 27 min (range 13–68). Meanley (1) found that one female spent 78% of daylight hours incubating and averaged 70 min/incubation period and 19 min/foraging period. Another female also spent 78% of time incubating, averaging 54-min periods on the nest and 15-min periods off the nest (212). In Arkansas, females incubated throughout the night for an average of 10 h 46 min, generally starting between 17:00 and 20:00 and leaving in the morning around 06:00 (TJB).

Hardiness of Eggs Against Temperature Stress; Effect of Egg Neglect

Most eggs hatch (166) despite extremely warm temperatures during the breeding season. At a video-monitored nest in Arkansas, a female failed to incubate for one night in late May and the sole Swainson's Warbler egg that remained in the nest did not hatch in the following 18 d, although a single Brown-headed Cowbird egg did hatch (TJB).

Hatching

Preliminary Events and Vocalizations

No information.

Shell-Breaking and Emergence

The first egg in a clutch in Virginia was observed hatching at 06:30 (202), although eggs may hatch at any time of day (TJB). In North Carolina, Chartier (24) estimated the average hatch time to be 10:21. The eggs usually hatch on the same day or sometimes 1 day apart (166, TJB); 56% of North Carolina nests had eggs hatch on the same day, while 44% hatched over 2 days (24). The hatching rate for eggs in non-depredated nests is high; in Louisiana and South Carolina, the average brood sizes, 2.9 and 2.6 (n = 70 and 55, respectively), were only slightly lower than the mean clutch sizes of 3.3 and 3.1, respectively (158, 166). Bishop et al. (43) estimated hatching success at 81% for eggs that survived through the incubation period, whereas Chartier (24) estimated this value as 84% in a different population.

Parental Assistance and Disposal of Eggshells

The female often consumes eggshells after hatching, although the male or female occasionally leave the nest with the shell (CR, TJB).

Young Birds

Condition at Hatching

Altricial and nidicolous; primarily naked, eyes closed. Oberholser (28) described the natal plumage as being “dark, rather brownish-drab.”

Growth and Development

Bishop et al. (43) provided information on average daily developmental milestones for nestlings. Eyes begin to open with pin feathers emerged on wings at day 4, lesser and median coverts are out of feather sheaths at day 8, and most wing feathers are out of sheaths with head fully feathered and having 2 pronounced tufts of down at day 9 when the nestling is ready to fledge.

Parental Care

Brooding

Only the female broods the young. Meanley (1) reported that one female was brooding or standing on the edge of nest 53% of time during a 7-hour observation period when the chicks were 3 days in age. Based on video observations in Arkansas (TJB), the mean duration that females spent brooding per bout was 26 min, the average minimum and maximum values for bout length were 7 and 61 min, respectively (n = 14). The average length between brooding bouts was 137 min, and the average range was 84 to 244 min (n = 15). When the nestlings are near fledging, the female often stands on the nest rim during the day rather than sitting on the nest cup (TJB, NMA). Females brooded during the night until the nestlings were near fledging, generally arriving at the nest around 20:00 and departing around 06:00 for an average of 9 h 50 min on the nest each night (n = 11).

Feeding

Both parents feed the young. During a 7-hour midday period at a Virginia nest, Meanley (139) observed that nestlings at 3 days of age were fed 14 times, 8 times by the male and 6 by the female; intervals between feedings ranged from 9 to 59 min. On average, parents deliver food to non-parasitized nests 2 to 3 times per hour depending on brood size; this rate increases with nestling age and when Brown-headed Cowbird nestlings are are present, especially when > 1 cowbird is present (213). In parasitized nests, Swainson's Warbler young receive 12.8 and cowbird young receive 21.9 feedings per day (TJB, unpublished data). Based on video data from nests in Arkansas, adults mostly brought soft-bodied larvae and the number and size of prey were variable (CR). There is no information on the volume or identity of prey brought to nestlings.

Nest Sanitation

Both parents may remove or consume fecal sacs (1).

Parental Carrying of Young

Not known to carry live young. Dead young are removed from the nest by one of the parents (TJB).

Cooperative Breeding

Not known.

Brood Parasitism

Identity of Parasitic Species

The Brown-headed Cowbird is the only known brood parasite.

Frequency of Occurrence

Brood parasitism may be common in some areas, but was not known in this species until 1917, when Kirn (197) reported that 2 of 6 nests found in Oklahoma contained Brown-headed Cowbird eggs. Other reported rates of brood parasitism: 3 of 5 Arkansas nests (1), 3 of 18 West Virginia nests (130). Brood parasitism has also been reported in Louisiana (214), Mississippi (215), North Carolina (216), South Carolina (217), and Texas (REB); no nests were observed to be brood parasitized in Virginia (n = 11) (218).

Brood parasitism likely occurs in all parts of the breeding range. In recent studies, no brood parasitism was observed in Missouri (n = 17) (159), 3 to 8% parasitism was reported for Louisiana (n = 138) (142), 6 to 10% of nests were parasitized in South Carolina (n = 17, 79) (145, 166), and about 12% of nests (n = 109) were parasitized in North Carolina (24), with about 3% of nests receiving multiple cowbird eggs. The highest rates reported thus far are from Arkansas, where 36% of nests were parasitized and 10% of nests received multiple cowbird eggs (n = 135) (22). The number of nests parasitized by cowbirds has probably increased because of habitat fragmentation and the expanding range of the Brown-headed Cowbird (219).

Timing of Laying in Relation to Host's Laying

Meanley (218) suggested that in Virginia Swainson's Warbler may avoid or reduce brood parasitism by nesting late in the season when most cowbirds have finished laying eggs. In Arkansas, the parasitism rate declines substantially throughout the breeding season, and persistent renesting appears to be important to Swainson's Warbler reproduction (195, 22).

Response to Parasitic Mother, Eggs, or Nestlings

Adult Swainson's Warblers appear not to respond strongly to adult cowbirds. At one nest in Arkansas, a female cowbird was observed to drive the female Swainson's Warbler away from the nest prior to laying (TJB). Cowbird eggs and nestlings are generally accepted, although parasitized nests sometimes appear to be abandoned during the egg-laying stage; female Swainson's Warblers occasionally appear to press cowbird eggs into the nest lining during this period (TJB, NMA, JDB). In Arkansas, a nest found with 1 host and 2 cowbird eggs had 1 intact cowbird egg on the nest rim, and 1 intact cowbird egg on the ground during the next nest visit, and the female was incubating (B. Reiley, personal communication). Swainson's Warbler is largely viewed as a “naïve” host (219) that has only been exposed to cowbirds relatively recently and apparently has few defenses against nest parasitism. Provisioning rates at parasitized nests are generally greater than at non-parasitized nests (213).

Effects of Parasitism on Host

Cowbirds directly reduce the success of individual nests by puncturing Swainson's Warbler eggs (197), by removing eggs from the nest, or by removing nestlings (23, 24). Clutch size for parasitized nests in Arkansas was reduced to 2.57 for singly parasitized and 1.45 for multiply parasitized nests (22). Egg removal occurs both during the egg-laying and incubation stages. Removal of eggs during the incubation phase or nestlings after the eggs have hatched are likely to induce renesting. Nestling cowbirds compete with nestling Swainson's Warblers for food, resulting in decreased size of host nestlings, increased probability of starvation, and likely decreased size of fledged host young (TJB). In Arkansas, parasitism decreased fledgling production by >2 young per parasitized nest (22). In South Carolina, parasitism was also observed to have a large negative impact on both hatching success of eggs (decrease from 81 to 55%) and production of fledglings (43).

Success of Parasite with this Host

Swainson's Warblers can successfully raise cowbirds to fledging (216), but production of Swainson's Warbler fledglings was reduced from 2.75 to 0.60 young per nest at parasitized nests in Arkansas (22), and parasitized nests in South Carolina produced 0.28 host young per nest (43). Nests with multiple cowbird young have not been observed to successfully produce Swainson's Warbler young, likely because of the larger size and competitive abilities of cowbirds and resulting unequal distribution of food in the nest (22). Because of their relative rarity, Swainson's Warblers are probably not important in the overall nesting economy of cowbirds (220).

Fledgling Stage

Departure from the Nest

The young generally leave the nest from 8–12 d after hatching, averaging around 10 d (130, 1, 138, 43, 24), although 5-day or 6-day-old nestlings may fledge early during nest predation attempts (22). The mean number of fledglings from successful nests is around 2.12 in Missouri, 2.50 in South Carolina, 2.06 in North Carolina, and 3.0 in Louisiana (159, 43, 24, 158). In Arkansas, successful nests produced 2.13 fledglings, although when restricted to only non-parasitized nests the number was 2.75 (22).

Growth: Mass, Proportions, Structures

Not known.

Association with Parents or Other Young

The fledglings follow parents and are fed for up to 4 weeks after leaving the nest (166). Parents and young may be found together considerably later than 4 wk after fledging, although the extent of parental care at that stage is unknown (TJB, NMA).

One or both parents may care for the fledglings, parents may split broods during the post-fledging period, and males may take the entire brood when females are attempting a second brood (166, NMA, TJB). Fledglings make chip notes to inform feeding parents of their location, and they increase the intensity of chipping as parents get closer (1). Young 2 days out of the nest may remain close to each other (1.5–3 m) or may be farther apart (15–30 m) (185) and generally stay close to the nest (166). Begging may become more intense when the young are older and follow parent birds while foraging (166).

Ability to Get Around, Feed, and Care for Self

Meanley (1) reported that birds 2 days out of the nest were not able to fly far and mostly hopped along the ground for locomotion; they were unable to feed themselves and were fed by the female an average of once per 15 min. When they are older, fledglings attempt to forage between parental feeding trips (166).

Immature Stage

No information.

Recommended Citation

Anich, N. M., T. J. Benson, J. D. Brown, C. Roa, J. C. Bednarz, R. E. Brown, and J. G. Dickson (2019). Swainson's Warbler (Limnothlypis swainsonii), version 3.0. In The Birds of North America (P. G. Rodewald, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bna.swawar.03