Swainson's Warbler

Limnothlypis swainsonii

Order:
Passeriformes
Family:
Parulidae
Sections

Behavior

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Swainson's Warbler bathing.
© Will Stuart, North Carolina, United States, 14 June 2017
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Figure 5. Display of a Swainson's Warbler.

Birds spread their wings and tail and vibrate them, while walking up and down a branch and frequently turning around. This display is sometimes given during territorial disputes and also prior to copulation. Illustration by Shirley A. Briggs, used with permission of Nancy Greenspan.

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Swainson's Warbler precopulatory display.
© Liam Wolff, Georgia, United States, 5 May 2018

Locomotion

Walking, Hopping, Climbing, etc.

On ground, the typical gait is described as a rapid step—between a walk and a hop (178). Birds move hurriedly when foraging, changing direction often, sometimes reversing direction in single hop (1). Birds also exhibit “pattering” or rapid vibration of the feet when foraging, which may help flush prey items, and contributes to the impression of an erratic gait (7).

Flight

Fly directly from perch to perch instead of hopping through branches like most wood-warblers. May fly directly from one side of territory to the opposite side without stopping. Flight is often low within a territory; sometimes birds fly several meters at < 2 m above ground.

Swimming and Diving

Not known to occur.

Self-Maintenance

Preening, Head-Scratching, Stretching, Bathing, Anting, etc.

The following description is from Eddleman (58), unless otherwise noted. Head scratching occurs directly by raising the leg toward the front under a wing. Birds may preen for extended periods, especially on the breast. The order of preening was observed in an individual as follows: both wings were flexed, then the bird preened under one wing with the bill, alternating preening bouts with feather ruffling and tail and body shaking while holding the head still; the second wing was preened in a similar fashion, followed by preening under the tail, alternating preening bouts with feather-ruffling and body shaking. After feeding, food particles and dirt are removed from the bill by wiping it against a branch or twig before preening. Males bill-wipe between songs during intense song bouts.

Norris (188) reported that Swainson's Warbler preens and scratches more than any other wood-warbler, perhaps a necessity because of its foraging method. JDB observed a pair bathing together for ~3 min in a shallow muddy puddle, and then preening for ~5 min afterwards. No information on stretching or anting.

Sleeping, Roosting, Sunbathing

Females sleep on the nest and seem to alternate between periods of sleep and vigilance, often turning eggs during vigilant periods. In the final days of brooding, female may not return to the nest to sleep (TJB; N. Chartier, unpublished data). Away from the nest, birds apparently roost at night in dense cover low to the ground. J. Everitts (personal communication) radio-tracked 2 birds on 3 nights and found them roosting in an area with dense spicebush, greenbrier, and horsetail (Equisetum sp.) at ground level. Not known to sunbathe.

Daily Time Budget

Based on average lengths of incubation bouts and time away from the nest from video recordings (TJB), females spent an average of 68% of daylight hours on the nest during incubation. Meanley (1) observed that one female spent an average of 78% of daylight time on eggs. Another female, with 3-d-old nestlings, spent 53% of the time brooding (1). Video observations (TJB) throughout the brooding phase showed that females averaged 16% of daylight hours on the nest. As nestlings grow, the female spends more time foraging and less time brooding.

During the breeding season, males sing from approximately 30 min before dawn until there is sufficient light for foraging, then begin foraging and singing from the ground (185). The amount of time spent singing in relation to foraging depends on the stage of the breeding cycle, weather, time of day, and proximity of other males' territories (185).

Agonistic Behavior

Physical Interactions

Males chase each other during the breeding season, typically early in the season when setting up territories, and can be quite aggressive when defending a territory against conspecifics. Upon hearing an intruder's song, a male typically flies in and perches, then searches for the intruder. When the intruder is located, a chase occurs, and males may flutter about on the ground together and sometimes fly 1–2 m up from the ground while grasping at each other's bills (1, NMA).

Communicative Interactions

When confronted with an intruder in the territory, aggression is communicated vocally by chipping excitedly and by singing Incomplete Songs during and after territorial encounters. Meanley (1) observed extensive visual displays in which the wing and tail feathers were spread laterally and the tail was vibrated; birds walked sideways back and forth along a branch, frequently turning around (Figure 5). Brewster (25) reported that individuals also raise their crown feathers “in a loose crest” at the same time the display is performed. Tail-spreading is sometimes performed by a male that has invaded another male's territory and then is driven out (1). However, this display has also been given at other times during territorial encounters, often when female is present (NMA, TJB), and males also perform this display prior to copulation (1; See below; Pair Bond).

Spacing

Territoriality

Males defend territories against conspecific males during the breeding season. Habitat type and quality as well as phase of the breeding cycle are likely important determinants of territory size. Territory boundaries with neighbors appear to be relatively static within the breeding season, but breeding cycle fluctuations may influence how much of a territory or home range is used by a male. Meanley (1) suggested that the territory is largest when a male first arrives on the breeding grounds, however, territory boundaries are difficult to estimate during this early stage, and boundaries may change as new birds arrive. Meanley (1) also reported that the defended territory is smallest during the mating and nest-building periods when males are staying close to females, and territories may also be small during egg-laying and after fledging if males are traveling with recently fledged young, which have limited mobility (NMA). Territory size appears to increase during the nestling stage. Females are not territorial, and appear to generally move within their mate's territory, but more study is needed.

Territories appear large compared to values generally reported for other wood-warblers, although with recent use of radio-telemetry, it is apparent other wood-warbler species can use similarly large areas (189, 190). Anich et al. (191) studied territoriality (using telemetry points at which birds were singing) and found 95% fixed-kernel territory sizes averaged 6.48 ha (range 1.02–28.09, n = 28). Earlier studies reported smaller average territory sizes, but given the difficulty of following non-singing birds, these are presumably underestimates (191). Griscom and Sprunt (138) reported 4 territories to be 0.29–0.37 ha in size. Meanley (139) estimated 5 territories in Georgia, Virginia, and South Carolina to be 0.12–1.94 ha. Meanley (1) also reported a male using an area as large as 3.2 ha in July. Eddleman (58) estimated territory size in southern Missouri and Illinois to be 0.9 ha (range 0.4–1.8, n = 18). In South Carolina, Peters (192) estimated mean territory sizes of males (n = 64) in 2 forest treatments to be 1.58 and 2.32 ha, and Thompson (166) used territory mapping and estimated 95% kernel territory sizes of males averaged 1.56 ha (range 0.31–6.21, n = 51).

Individual Distance

During the breeding season, males are typically intolerant of incursions by other males and will chase them from territories. Females generally do not chase other females, but one paired female chipped constantly when another female foraged briefly near her and her mate (1). Females occasionally accompany their mates during territorial fights, and often give agitated Call Note (NMA, TJB). In other instances, females are apparently tolerant of other females; polygyny is thought to occur in some cases (see Mating System and Sex Ratio). During the courtship and mating period, the pair spends most of the day foraging within 10 m of each other, often < 1 m apart.

Interspecific Territoriality

In one instance of a presumed territorial conflict with another species, N. Chartier (personal communication) observed a Carolina Wren (Thryothorus ludovicianus) partially destroy a nest with two 6-d-old nestlings and both parents present. The wren attacked the female several times, but the female was quite passive and did not actively defend the nest. The nest collapsed the next day and the nestlings fell out of the nest. On another occasion (N. Chartier, personal communication), a Carolina Wren was observed pecking, lifting, and tossing a newly hatched nestling. When the female Swainson's Warbler returned, the wren left.

Isolated territorial interactions with many species have been recorded. A territorial male chased a singing Prothonotary Warbler from its territory in Illinois (58), and another male chased the male of a pair of Hooded Warblers, but ignored the female (188). Meanley (1) observed a Hooded Warbler chasing a Swainson's Warbler, and saw a male Swainson's Warbler give territorial display directed toward a pursuing American Redstart (Setophaga ruticilla). However, Norris and Hopkins (193) reported that a male Swainson's Warbler had no reaction toward a Hooded Warbler that perched within 4 m of the Swainson's Warbler's nest and within 1.5 m of the Swainson's Warbler itself.

Overwintering Territoriality

Swainson's Warblers appear to be territorial on the overwintering grounds. Graves (6) found that overwintering birds responded to song playbacks in Jamaica, and playback has been used to capture birds overwintering in Guatemala (N. Chartier, A. Savage, and J. Gerwin, personal communication). A. Brunner (104) found much overlap between neighboring home ranges in Jamaica and suggested that overwintering home ranges are larger than defended areas. Banded individuals have returned to the same sites in Jamaica, Puerto Rico, Guatemala, and the Dominican Republic (P. Marra, J. Faaborg, A. Brunner, C. Robbins, personal communication; 106).

Sexual Behavior

Mating System and Sex Ratio

Generally behaviorally monogamous. However, Graves (5) noted one case of probable polygyny in Louisiana: 2 nests in the incubation stage were located 2.8 m apart within the territory of one male, and each nest was attended by a different female. J. Gerwin (personal communication) and colleagues have also observed 2 instances in which 2 females had nests within a single male's territory at the same time, suggestive of polygyny, and a similar case was reported by Henry et al. (158). Sex ratio is unknown, due to the difficulty of detecting females.

Pair Bond

Courtship-feeding has not been observed. However, during incubation period male brings food to the incubating female at the nest, up to once or twice a day, though she sometimes refuses it (TJB). Meanley (1) observed the precopulatory display in which the male, perched 1 m from ground, extended his quivering wings and raised his head, tail, and rump feathers (Figure 5); when the female approached, the male vocalized with a quiet twee-twee-twee. Copulation is usually accomplished with a struggle on the ground. The male flies to the female foraging on ground, pounces on her, pecks at her rump, and struggles with her on the ground with very rapid movements (1). When copulation occurs in a tree or bush, the male often grasps the female's crown feathers with his bill (1). Duration of the pair bond varies. Within years, after a successful nesting attempt, pairs have been observed to raise a second brood with each other (194, 166, 195, 24). However, pairs have also been observed renesting with different partners (J. Gerwin, personal communication, TJB). Two different pairs were reported nesting with the same partners over multiple years but in many instances birds were found nesting with different partners the following year (J. Gerwin, personal communication; TJB).

Extra-Pair Copulations

Extra-pair copulations have been observed (J. Gerwin, personal communication) and mate-guarding behavior of males seems to reflect the risk of extra-pair copulations. Male accompanies the female closely before eggs are laid and male home ranges were largest while the female was on the nest and feeding young (153), suggesting male attempts to protect his paternity with the current female, and once nesting has begun, may attempt to leave the territory in search of additional copulations. In one instance, 2 males were seen feeding the same fledged young (J. Gerwin, personal communication); perhaps both males copulated with the female, or perhaps a neighboring male sought to copulate with the female for a renesting attempt. Although singing incursions into territories of neighboring males are infrequent, silent intrusions into neighboring home ranges are more frequent (11% of radio-telemetry locations; 191), and males may be searching for extra-pair copulations. Females have also been found to range widely into territories of neighboring males when foraging (J. Gerwin, personal communication). Based on genetic comparisons of nesting females and nestlings, Chartier (24) found evidence of extra-pair paternity in 3 of 23 broods (and no evidence of conspecific brood parasitism).

Social and Interspecific Behavior

Degree of Sociality

Although Swainson's Warblers have been reported to nest in “colonies” during the breeding season (139, 166), this is likely due to the patchy distribution of high-quality habitat for this species, rather than a need to be gregarious; Anich et al. (191) indicated that males defend discrete territories, and we discourage referring to high-density breeding areas as “colonies”.

During the overwintering period, Eaton (180) reported that birds in Cuba were generally found singly, whereas Kirkconnell et al. (100) observed Swainson's Warblers in Cuba often foraging in close association with other species, particularly Ovenbird, Worm-eating, Hooded and Kentucky warblers, and suggested that Ovenbirds may help flush prey for Swainson's Warblers.

During migration, Swainson's Warblers may move in small groups, based on tower kills (135, 137). B. Sargent (personal communication) reported good numbers of migrant Swainson's Warblers (up to ~30 birds in one day) on the Alabama coast in early April, and a foraging flock of 18 migrants in Belize on 12 March.

Play

Not known to occur.

Non-Predatory Interspecific Interactions

Not likely to occur in multispecies flocks on overwintering grounds (180). Other similar species (Louisiana Waterthrush, Yellow-throated Warbler, Worm-eating Warbler) have initially flown in to Swainson's Warbler song or chip-note playback. However, these species appear to quickly lose interest when it becomes clear that the song in question is of Swainson's Warbler (JDB, TJB, NMA). See also Interspecific Territoriality.

Nest cameras have shown several passerines, including Worm-eating Warbler, Hooded Warbler, Yellow-breasted Chat (Icteria virens) near Swainson's Warbler nests, and the species sometimes investigate the nest (CR, TJB, N. Chartier, personal communication). Wild Turkey (Meleagris gallopavo), Pileated Woodpecker (Dryocopus pileatus), White-eyed Vireo, Prothonotary Warbler, Kentucky Warbler, Hooded Warbler, Carolina Wren, Blue-gray Gnatcatcher, Wood Thrush (Hylocichla mustelina), Northern Cardinal (Cardinalis cardinalis), Indigo Bunting (Passerina cyanea), Eastern Towhee (Pipilo erythrophthalmus) and other species have been known to nest within Swainson's Warbler territories (1, TJB).

Predation

Kinds of Predators; Manner of Predation

Predators of adults are largely unknown, though evidence of adult Swainson's Warblers killed at nests has been found (24, 158, TJB).

Many predators of eggs and young have been recorded (Table 3). Across the range, ratsnakes (Elaphe spp.) are among the most common predators, reported by 4 investigators (23, 43, 24; M. Revels, personal communication). Collier (196) reported a milksnake (Lampropeltis triangulum) taking eggs from a nest. Female Brown-headed Cowbirds have been recorded on video removing eggs and young from Swainson's Warbler nests on many occasions (23, 24). Brown-headed Cowbirds also destroy or remove eggs in Swainson's Warblers' nests when they deposit their own eggs (197), and may be responsible for significant reduction in annual recruitment (195; see Breeding: Brood Parasitism). Video cameras on nests in North Carolina revealed a raccoon (Procyon lotor), a golden mouse (Ochrotomys nuttalli), a Red-shouldered Hawk (Buteo lineatus), and an Eastern Screech-Owl (Megascops asio) depredating nests (24). Video cameras on nests in Arkansas (23) revealed the Red-shouldered Hawk as a more frequent predator, and single instances of Barred Owl (Strix varia), Blue Jay (Cyanocitta cristata), Broad-winged Hawk (Buteo platypterus), and Eastern Screech-Owl as predators of eggs or young. These cameras also revealed one white-tailed deer and one American black bear (Ursus americanus) trampling nests, apparently by accident. The cryptic nature of nests has contributed to accidental tramplings even by humans; e.g., Brooks and Legg (149) reported that the eggs had been spilled from a Swainson's Warbler nest by “a bootlegger who had been hiding whiskey in the thicket.” In 2 unexpected occurrences, several harvestmen (Arachnida: Opiliones) attacked and killed Swainson's Warbler nestlings (198).

Response to Predators

When startled at the nest, adults freeze and remain motionless for extended periods (130). Incubating females remain motionless until danger has passed, and males at the nest slip off quietly when the risk of detection is gone (130). Females sit tightly on the nest, allowing predators to approach closely before flying away; the brown dorsal plumage of the incubating female undoubtedly camouflages the plain white eggs, which would be conspicuous against dark nest lining if the female were to leave the nest. Incubating females often allow approach within several centimeters (197, 199, 193, 5), and Grimes (200) reported that one female actually had to be pushed off the nest in order to allow inspection of its contents, and later even straddled his fingers trying to get back into the nest. When a female leaves the nest in the presence of a potential predator, she drops to the ground and flutters away like a crippled bird with her tail and one or both wings drooped while making soft chipping vocalizations (5, REB). This distraction display, similar to displays given by Black-throated Blue Warbler (Setophaga caerulescens) and Black-throated Green Warbler (S. virens), is probably intended to lure predators away from the nest.

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