Behavior

Walking, Hopping, Climbing, Etc

Does not walk; hops on branches, etc., probably hops on ground.

Flight

Not described.

Swimming And Diving

Not known to swim.

Preening, Head-Scratching, Stretching, Bathing, Anting, Etc

Preening movements not specifically described but similar to those of other passerines.  One female observed 5 m from nest preening for about 30 s; head stretched forward and on sides; preening concentrated on breast and underparts (CC). Probably scratch head overwing (as noted for 9 species of Dendroica; Yellow Warbler not mentioned among 3 species listed as exceptions; Nice and Schantz 1959). No reports of anting.

Sleeping, Roosting, Sunbathing

No information.

Daily Time Budget

No known quantitative studies.

Physical Interactions

Fighting encounters between males observed in which birds hit each other and may sometimes grapple (“lock”) together. Of 49 nonvocal encounters by unmated males, 12 included fighting (Ficken and Ficken 1965).

Communicative Interactions

Circle Flight. Termed “circle flight” by Burtt (1986); called “circling” by Ficken and Ficken (1965). Male flies from perch towards neighboring male or female at ter-ritory boundary, then turns away; flight path horizontal and semicircular; performed early season during territory establishment. Of 49 nonvocal encounters by unmated males, 16 included circle flights (Ficken and Ficken 1965).

Gliding. Male glides 3–7 m with wings and tail spread, occasionally with few wing-beats; usually away from opponent (Ficken and Ficken 1965, Burtt 1986).

Moth-Flight. Slow, stalling flight by male away from female with large amplitude wing-beats about twice normal speed and head held high above back; during territory conflicts and courtship (Ficken and Ficken 1965, Burtt 1986).

Chase. Bird flies toward and pursues conspecific (rarely another species), which flees. Of 49 nonvocal encounters by unmated males, 21 included Chases (Ficken and Ficken 1965, see also Burtt 1986).

Tail-spread. Tail held in a spread position; used together with Wings-out (Ficken and Ficken 1965).

Wings-out. Wings and/or tail momentarily spread; during territory conflicts and possibly in courtship. Body feathers sleeked, body held horizontal facing opponent and wings lifted from body; tail held parallel to body axis and feathers fanned (Ficken and Ficken 1965, Burtt 1986).

Sleeked Postures. Body feathers compressed; sometimes preceding but more commonly following agonistic encounters (Ficken and Ficken 1965).

Males approaching other males sing Type II Songs. As male becomes close, songs often become muted, and if interaction escalates to chasing or close encounters, male often switches to use of call notes (DAS).

Level of aggression by males varies with variation in breast streaking (which ranges from 20–55% of breast covered with chestnut streaking). Brighter males, defined as having greater percentage of breast with chestnut streaking, showed greater aggressive response towards mounts than did duller males with less streaking; brighter Yellow Warbler mounts elicited greater aggressive response than did duller mounts (Studd and Robertson 1985a).

Territoriality

On breeding grounds, Yellow Warblers defend multipurpose territories. Territory interactions are dynamic and continue throughout the breeding season; thus, overlaps of used areas and small shifts in boundaries are not rare. Terri–tories established as soon as males arrive (Frydendall 1967). In nw. Iowa, territory is about 150 ft (46 m) diameter or 0.4 acre (0.2 ha), but adults observed to travel 1,600 ft (490 m) distant to forage (Kendeigh 1941). In Manitoba, mean territory size 0.043 ha (n = 6); territory of polygynous male, 0.047 ha, divided unequally between 2 females (Hobson and Sealy 1989d). In Michigan, territories of polygynously mated males (mean 0.78 ha; n = 3) larger than territories of monogamously mated males (mean 0.21 ha ± 0.05 SD, n = 20; DellaSala 1986). Some male Yellow Warblers maintain, at least briefly, 2 breeding territories; some of these males are successful in having female and nest on each territory (Ford 1996, Spector 1991). In nonbreeding areas foraging territories are usually maintained. Resident Mangrove Warblers are territorial; however, territorial defense directed to other Mangrove Warblers; migrant Yellow Warblers (aestiva group) within these territories usually tolerated (Wiedenfeld 1992).

In Utah, mean territory size 0.14 ha (range 0.05–0.29, n = 12; Frydendall 1967). In the aspen parkland in central Alberta, warbler territories included ≥1 patches of willow or aspen (intervening mixed grassland habitats unused); mean size of territories including >1 patch, 0.45 ha (range 0.10–1.03, n = 25); territories within 1 patch, mean 0.28 ha (range 0.16–0.59, n = 14); overall mean size of territory 0.39 ha (n = 39); willow patches averaged 0.29 ha ± 0.362 SD (range 0.001–231, n = 281); mean distance between patches, 26.6 m ± 21.09 SD (range 5–188, n = 297); mean distance between patches included in same territory, 33.53 m ± 22.75 SD (range 7–119, n = 80; CC).

Males sometimes enter other males’ territories in apparent efforts to obtain extra-pair copulations (Ford 1983) or to observe activities at a nest (DAS); males and females sometimes enter other birds’ territories for locally scarce resources such as puddles for bathing or sources of nest lining material (Golden Warbler; DAS). During 148 h observation of 6 territories in Manitoba, 249 intrusions observed: 162 by neighboring male, 14 by nonneighbor male, 73 by male of unknown identity; intruder chased by male 225 times, by female 15 times, not chased 9 times (Hobson and Sealy 1989c).

In a playback experiment, males showed greatest aggression towards a dummy Yellow Warbler male during incubation period (Frydendall 1967). In experimental presentation of Yellow Warbler mounts placed at nests of Yellow Warblers, females responded first and were more agitated towards female mounts, especially during their egg-laying period (Hobson and Sealy 1989b).

Nonbreeding territories averaged 0.052 ha (n = 12) and included 6.7 ± 1.2 SE trees in cattle pasture in Chiapas, Mexico (Greenberg and Salgado-Ortiz 1994); averaged 0.5 ha ± 0.83 SD (n = 11) in Guatemala coffee plantations (Greenberg et al. 1996); averaged 0.16 ha (n = 9) in Panama second growth scrub (Neudorf and Tarof 1998).

Individual Distance

No information.

Mating System And Sex Ratio

Primarily monogamous, but occasional polygynous matings (Ford 1983, 1996, DellaSala 1986, Reid and Sealy 1986, Hobson and Sealy 1989d, Spector 1991). Sex ratio unknown.

Pair Bond

Little information. Pair bonds appeared to be maintained through fledging of young at a James Bay, Ontario, breeding site, after which mated pairs ceased to associate (Rimmer 1988). Males generally remained on breeding territories throughout Prebasic molt; most females disappeared from the area. Apparent mate fidelity varies among sites. At n. Ontario site none of 15 males and 13 females that returned in at least one subsequent year were documented to renest with a former mate (CCR), but at Massachusetts site the majority of males and females returning nested on the same territory as in previous years and remated with a previous mate if available (DAS).

Courtship Displays

Males initially exhibit Chase behavior toward females around territory; Chase frequency decreases over 2–4 d; males also per-form Moth-Flight, Tail-spread, and Wing-out during interactions with females; Zeep and Chip calls apparently serve as contact calls; copulation oc-curs near end of nest building (Ficken and Ficken 1965).

Solicitation display of female similar to that of many other passerines, with crouched body, quivering wings and often soft vocalization (Ficken and Ficken 1965).

Extra-Pair Copulations

Mated males trespass into neighboring territories, most frequently during nest building stages (1.76 intrusions/h); intrusions noted as observation of resident male chasing intruding male from vicinity of female or nest; some interactions between intruding male and female suggest attempts of forced copulation (Ford 1983). For 55 observations of male intruding into territory, 39 were by neighboring male, 7 by nonneighboring male, 9 unknown male; intruder chased by resident male 44 times, by resident female 9 times, twice intruding male mounted female (and cloacal contact once; Hobson and Sealy 1989c). During nest building and egg-laying periods, males quietly follow their mates in apparent mate-guarding (Ford 1983, DAS). Using multi-locus DNA fingerprinting, 70 of 130 families in s. Ontario, involving 160 of 484 offspring, included young sired by extra-pair copulations (Yezerinac et al. 1996, Yezerinac and Weatherhead 1997b); 7 of 53 young, from 3 of 12 nests, were sired by extra-pair copulations near Churchill, MB (Yezerinac et al. 1999). Amount of streaking on breast of male and likelihood of fathering extra-pair young show significantly positive regression (Yezerinac and Weatherhead 1997b); extra-pair activity by male reduced during mate’s fertile period (Yezerinac and Weatherhead 1997a).

Degree Of Sociality

Territorial in breeding season; many individuals territorial on wintering grounds. During migration found in single- and mixed-species flocks.

Play

No information.

Nonpredatory Interspecific Interactions

On small islands off the coast of Maine, 11 of 19 aggressive interactions (attacks, fights, chases) between Yellow Warbler and American Redstart initiated by Yellow Warbler; islands <1 ha had either Yellow Warbler or American Restart but not both (Morse 1973). Yellow Warblers were initiators in 10 of 12 interactions with Parula Warbler (Parula americana), and in 3 of 7 interactions with Yellow-rumped (Myrtle) Warbler (Dendroica coronata coronata; Morse 1973). Frequently observed to interact aggressively with Magnolia Warbler (D. magnolia), Chestnut-sided Warbler, and American Redstart on Maine mainland (Morse 1966, 1977). Yellow Warbler and American Restart males observed countersinging with very similar songs in Montana (NKK). Aggressive interactions with Least Flycatcher (Empidonax minimus) also documented (Goossen and Sealy 1982). At nest watched continuously throughout nestling period, one or both adults chased away several species (Bigglestone 1913): Eastern Kingbird (Tyrannus tyrannus), Blue Jay (Cyanocitta cristata), House Wren (Troglodytes aedon), Black-capped Chickadee (Poecile atricapilla), Brown Thrasher (Toxostoma rufum), Red-winged Blackbird (Agelaius phoeniceus), and Brown-headed Cowbird (Molothrus ater), but not Gray Catbird (Dumetella carolinensis). Observed chasing Blue-winged Warblers (Vermivora pinus) and American Goldfinch (Carduelis tristis; DAS). Other observers, however, portrayed Yellow Warblers as more tolerant of other species; e.g., Smith (1943) noted Yellow Warblers showed scarcely any “territorial exclusiveness” to Chestnut-sided Warbler that nested <2 m away from own nest. In central Alberta, transient Yellow Warblers observed foraging in mixed flocks with Yellow-rumped (Myrtle) Warblers at beginning of breeding season (CC).

Male American Redstart helped at Yellow Warbler nest with young over at least 4 d period; redstart fed both female and young and would displace male Yellow Warbler from nest area (Mannan 1979).

Yellow Warbler may defend nonbreeding territories; observed aggression directed to 37 species, primarily (90 of 314 chases) to Magnolia Warblers (Greenberg and Salgado-Ortiz 1994). Young HY birds not territorial but observed to join in flocks with Tennessee and Bay-breasted (Dendroica castanea) warblers in Panama (Morton 1976).

Distraction display performed to observer: Singing individual (presumed male) appeared to fall from tree and float to ground, then fluttered along ground away from tree with wings quivering over body (J. Carroll in Bonney 1988; see also description of “hunching” display, Hobson and Sealy 1989a; identified as “broken wing display” by Studd and Robertson 1985b). Females also seen to perform this display in response to observer approaching nest (CC) and to model and real snakes near nest (Studd and Robertson 1985b).

Kinds Of Predators

Predators of adults likely to be similar as those of other small birds in same habitat. Known or suspected predators on adults include long-tailed weasel (Mustela frenata; Batts 1958); nest predators include garter snake (Thamnophis sirtalis; Bigglestone 1913); red squirrel (Tamiasciurus hudsonicus; Goossen and Sealy 1982); Blue Jay, raccoon (Procyon lotor), and blue racer (Coluber constrictor; Batts 1958); weasel (Mustela spp.), red fox (Vulpes fulva), arctic fox (Alopex lagopus), Gray Jay (Perisoreus canadensis), and American Crow (Corvus brachyrhynchos; Briskie 1995); striped skunk (Mephitis mephitis; CCR) and domestic cat (Felis domesticus; DAS).

Response To Predators

Meager information. Twice observed to join mixed species flock mobbing mount of Great Horned Owl (Bubo virginianus; Lowther et al. 1987). For 86 presentations of rubber snakes at 27 warbler nests, females gave “agitated vocalizations” and were quickly joined by males. Females commonly responded with distraction displays in front of snake; other details not given. Responses to models same as response observed to live black rat snake (Elaphe obsoleta; Studd and Robertson 1985b).