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On the basis of observation of both caged adults and hand-reared chicks, American Redstarts have 3 stretching postures, usually after period of rest (Ficken 1962b): (1) sideward (outward) stretch of a wing along with 1 leg and the tail, (2) upward stretch of both wings unfolding above head, and (3) extension of each leg. Scratches head with leg over drooped wing; often associated with preening activity. Bathes by flicking water sideways with bill, then by shaking water over body with wings, while breast is immersed in water; also bathes by fluttering wings rapidly against wet foliage, often at dawn (e.g., leaves wet with dew during dry season in Jamaica; TWS). Preens by "nibbling" at more than one feather; behavior appears within several days after hatching, apparently to break pin-feather sheaths (Ficken 1962b). Preening also coats feathers with oil from the uropygial gland, soon after bathing, starting with outermost primaries and progressing from base to tip (Ficken 1962b). Anting has not been reported.
Fluffs feathers while resting, and points bill upward while closing eyes for brief periods. In sleeping posture, places bill on top of scapulars and then tucks it into feathers, a behavior pattern similar to that of many other passerines (Ficken 1962b).
No daily time budgets available, but foraging behavior changes diurnally (Holmes et al. 1978a) and seasonally (Sherry 1975, Lovette and Holmes 1995) in ways that indicate dramatic changes in activity patterning, depending on constraints of reproductive season and of food resource demand versus availability.
Agonistic behavior consists of diverse vocal displays (see Sounds and Vocal Behavior: Vocalizations) coupled with both aerial and nonaerial displays (Ficken 1962a). The most distinctive aerial display is Circling, a ritualized contest at territorial boundaries. Circling begins when 1 bird flies, often with stiffened wings, in horizontal semicircular or circular flight just in front of (occasionally behind) a second bird, coming to within 30 cm of it before continuing the arc back to initial position. As first bird is thus "retreating," second bird often initiates identical flight toward first, sometimes well before first bird lands, and so on, alternately, for up to tens of minutes duration (≥ 15 circles; Ficken 1962a, TWS). In the most intense displays, a bird does not land before pursuing a retreating individual (TWS). Mated males (occasionally females) usually perform Circling (Hickey 1940, Ficken 1962a), almost exclusively before nest completion (Ficken 1962a, TWS). During overwintering period, males and females, including hatch-year and older individuals, engage in Circling (Holmes et al. 1989, RTH, TWS).
Gliding is another aerial display used during the breeding season. This display involves the male holding wings outstretched so that they undergo low-amplitude movements, and spreads tail, while traveling 3–7 m (Ficken 1962a). Males generally use while departing from another male at territorial boundary, and generally solo, although 2 males occasionally glide close together, with one bird close behind the other.
Chasing is a third type of ritualized aerial display; tends to occur at or near territory boundaries (Ficken 1962a). Chaser usually stops at boundary, at which time roles often reverse. Sounds accompanying either Circling or Chasing are rare, generally bill snaps or rarely Chip notes. Fight may be aerial, with 2 birds flying first upward, then toward ground, while striking at each other and sometimes even grappling with beaks and feet, almost invariably without physical harm. Fights may involve either sex (but usually same sex); often accompanied by bill snaps.
In the Head-forward Display, a bird faces horizontally, head outstretched toward opponent, mouth agape, wings in normal or slightly drooped position, and body plumage sleeked (Ficken 1962a). This display usually accompanied by bill snapping; given by female approached by mate. Males in prenesting season occasionally give Neck-Withdrawn Gape Posture from below; hunched neck points up toward female and mouth is agape. In close approach during boundary disputes (within 1–2.5 m of each other), male may adopt Wings-out Display by projecting wings upward and outward to enhance patches of color at bases of primary feathers, and by adopting horizontal posture with sleeked feathers (Ficken 1962a). Tail-spread Display is component of various other displays, with diverse contexts, but degree to which tail is spread horizontally or "cocked" upward is greatest in context of courtship and territorial boundary disputes (Ficken 1962a).
Agonistic displays of the American Redstart have been discussed in ethological, ecological, and evolutionary contexts. Ficken 1962a inferred motivation for agonistic displays ethologically (i.e., resulting from conflicting drives to attack, escape, remain). For example, Circling was considered to be threatening, frequently leading to fights, but also accompanied acts with high escape tendency, suggesting motivation by nearly balanced, probably oscillatory, attack and retreat drives. Importance of competitive rather than sexual motivation for this display is suggested by (1) use of display by both sexes to defend territory boundaries in both summer and winter, (2) ease of provoking such displays with model birds at both times of year, and (3) the different agonistic behaviors associated with territorial versus nest space (Ficken 1962a, Holmes et al. 1989, Sliwa and Sherry 1992). Aggression levels also appear to be linked to habitat: more aggressive males and females tend to occupy mangrove habitat on the overwintering grounds (Marra 2000).
Breeding Period. Individuals of both sexes use variety of vocalizations, aerial displays, postures, and combinations thereof to defend territorial boundaries in summer (see Agonistic Behavior). Primarily involves male (Hickey 1940; reviewed by Ficken 1962a), which responds vigorously to intrusion by other male, as well as to simulated intrusion using song playback (TWS). Female occasionally defends territory using similar agonistic behavior, primarily against other females, sometimes even before nest is built (Sturm 1945, Ficken 1962a, TWS). Both sexes feed and complete most reproductive activities within territory, except for nest-building female, who occasionally wanders into territories of adjacent males (Ficken 1962a, TWS), and males seeking extra-pair mating opportunities. Female precipitates fights among males during these transgressions of territory boundaries, probably in part because female is sexually fertile at this stage and because extra-pair copulations are frequent (see Sexual Behavior). Female also appears to control location of male’s territory to some extent, because male sometimes shifts territory to surround nest of mate when she builds at the periphery (or off) his territory (Ficken 1962a). These observations suggest that males and females may have different territories or home ranges from each other, a topic that needs further research.
Size of breeding territory is generally < 1.0 ha (Hickey 1940, Ficken 1962a, Sherry and Holmes 1989; references cited by Cyr and Darveau 1996), but ranges from < 0.1 ha (Sturm 1945; see also Demography and Populations: Population Status: Numbers) to several hectares in years when population density is low (TWS; but see Ficken 1962a). Average territory size in Quebec ranged from 0.39 to 0.64 ha (studies cited by Cyr and Darveau 1996). Secondary territories, in the case of polyterritorial polygynous males (see Sexual Behavior), tend to be isolated from primary territories by hundreds of meters in some cases, and are probably smaller than the former because of use only for courtship, but too few polyterritorial males have been studied to warrant broad generalizations.
Yearling male territoriality similar to that of older male, particularly if yearling attracts mate. A yearling male that remains unmated, however, typically sings continuously (advertising, repeat-mode song; see Sounds and Vocal Behavior: Vocalizations) and defends small territory on any given day, but moves around in area of up to several hectares over several-day period, and occasionally visits nest sites of females mated to other males (TWS). If it remains unmated, yearling typically disappears from particular territory after about 2 wk (Ficken 1962a), and moves as far as several kilometers away to establish new territory (TWS). Older males appear to be competitively dominant to yearlings, constraining where yearlings are able to settle (Ficken and Ficken 1967, Morse 1973, Sherry and Holmes 1989, Lemon et al. 1996, Lozano et al. 1996, TWS). Thus, unmated yearling males are often “floaters” in the sense that they move widely and often do not breed (Hickey 1940, Howe 1974b, Sherry and Holmes 1989, TWS).
Males often stay on territory from arrival in spring to beginning of fall migration, but behavior changes, as does territory size. In New York, territory was smaller during nestling-feeding period (0.07 ha) than during “self-maintenance” period (0.41 ha) before hatching of young; 1 pair observed while feeding fledglings defended area similar in size to that measured during self-maintenance period (Yarrow 1970). In Alberta, Canada, males spent about 18% of their time off-territory, usually making trips to visit neighboring territories with nest-building (and thus possibly fertile) females, but they invested less time off-territory when local breeding was highly synchronized, probably to protect against cuckoldry (Churchill and Hannon 2010). Intruding males tended to have more adjacent females and sang more, but whether these intrusions facilitate extra-pair paternity is not known. Response to male intruders also appears to reach greatest intensity at time of peak female fertility during nest-building and egg-laying activities, on basis of frequency of male-male aggression in New Hampshire (TWS). Just before southward migration, male shows slight increase in aggressiveness; even during migration, males sometimes engage in agonistic encounters such as observed in breeding territorial disputes (Ficken 1962a, TWS).
Overwintering Period. Vigorously defends overwinter territory (Schwartz 1964, Rappole and Warner 1980, Holmes et al. 1989, Sliwa and Sherry 1992, Sherry and Holmes 1996a). Defends small area against intruder redstarts; many, if not most territorial individuals remain on territory through the overwintering period in Jamaican studies, particularly in favorable habitats like black mangrove swamp and wet limestone forest (Cooper et al. 2015, Peele et al. 2015), but not so much in Venezuela, where tends to remain only about 2 mo (Lefebvre et al. 1994a); and returns to same site in subsequent years (Holmes et al. 1989, Holmes and Sherry 1992, Johnson et al. 2006b, Peele 2015). Overwinter territories of females are less studied than those of males, but are known to overlap those of other females and males; overwinter territories of individual males overlap 3–6 other males’ territories. Based on a novel 3-dimensional analysis, territory overlap was around 10% less than in 2-dimensional space, indicating subtle spatial defense and avoidance at small spatial scales (Cooper et al. 2014).
Uses similar agonistic behaviors during overwintering and breeding periods, including Circling, Head-forward Display, Wings-out Display (probably same as “Wing Droop”; Rappole and Warner 1980), Tail-spread Display (see Agonistic Behavior), Pivoting (see Predation), and frequent Chip call notes (see Sounds and Vocal Behavior: Vocalizations), but not song. Circling can last for almost 1 h, involve all age and sex combinations, and resume on subsequent days (Holmes et al. 1989, RTH, TWS). Other evidence for overwinter territorial behavior is strong response of both males and females to song and playback of Chip call notes: For example, 82% of playbacks elicited appearance of 1 bird, 22.3% elicited 2 or more birds, 7.4% Circling, 5.3% songs, and 3.2% fights (Holmes et al. 1989); and responding birds were significantly more even than random in spatial dispersion across range of habitats and regions in Jamaica (Sliwa and Sherry 1992). Territory size during overwintering period (generally < 0.25 ha) was initially characterized to be about the same as that in summer (Holmes et al. 1989), but may be much smaller, at least in some circumstances; e.g., males in black mangroves in Jamaica had an average territory size of just under 3,000 m2 (i.e., 0.3 ha; Cooper et al. 2014) corresponding to 3-dimensional volumes of 10,715 + 7,886 m3 (Cooper et al. 2015); and densities of individuals in this same habitat can exceed 15 individuals per ha, although this includes males, females, and transient individuals (Peele et al. 2015). In a female-dominated dry forest habitat in Puerto Rico, where territories were more dispersed, territory size still averaged 0.39 ha (Toms 2011).
A Venezuelan population dominated by females tended to be less territorial, containing more floaters or transient individuals than the other overwintering studies just cited, perhaps because of greater seasonal variability of food resources (Lefebvre et al. 1994a). Toms (Toms 2011, Toms 2013) also noted abundant transient (“vagrant”) individuals in a female-dominated (Toms 2011, Dugger et al. 2004) Puerto Rican population, particularly in a site dominated by the interspecifically competitive, socially dominant Adelaide’s Warbler (see Demography and Populations: Interspecific Competition). Surprisingly, Peele et al. 2015 compared point count density estimates versus mapped territories to show that transient individuals may comprise up to 50% of individuals in Jamaican sites such as black mangroves and adjacent logwood scrub habitat where prior research has focused largely on territoriality. Moreover, these Jamaican transients tended to be female or second year birds compared with territorial individuals in the same sites; they were rare in an inland, higher elevation wet limestone forest; and their frequencies varied among years in coastal habitats, peaking in a year following a hurricane that made landfall on the south coast (Peele 2015, Peele et al. 2015). Thus, transient individuals may be widely important in some overwintering redstart populations. It is not known when during the overwintering season that transients are most frequent, when and where they arise (or why, except for hurricanes damaging habitat), or what population consequences they have (but see Peele 2015).
Function. Likely functions of territoriality in summer (for males) include protection of paternity, defense of habitat within which mate may find appropriate nesting microsites (Ficken 1962a), defense of food resources, and/or defense of site in which to attract one or more mates (Secunda and Sherry 1991). For females, possible functions of intersexual aggression include preventing mate from seeking extra-pair broods on which to divide parental care (Secunda and Sherry 1991), and defending suitable nesting microsites. During the overwintering period, members of each sex independently defend site that will maintain food resources throughout the period, e.g., during seasonal droughts or otherwise inclement-weather periods (Holmes et al. 1989, Marra et al. 1993, Parrish and Sherry 1994), although many females may be transient and not defend territories at all (see above). Similar function of breeding and overwintering territoriality is suggested by communication using similar agonistic behaviors, and by competition for limited resources in both breeding (age-specific competition; Sherry and Holmes 1989) and overwintering periods (sex-specific, maybe also age-specific, competition; Marra et al. 1993, TWS, RTH). However, possible differences are that territories are primarily male in summer and may help protect paternity, while during the overwintering period they may help individuals have enough food to depart breeding grounds early and in good physiological condition—which can have important reproductive consequences (e.g., Marra et al. 1998, Norris et al. 2004a, Reudink et al. 2009a, Tonra et al. 2011, Tonra et al. 2013, Cooper et al. 2015, Rushing et al. 2016).
Solitary during breeding and overwintering periods; generally does not tolerate close approach of conspecifics, at least of same sex. In captivity, fledglings tolerate close approach and contact until 2 mo old, after which individual distance expands to a few centimeters around an individual (Ficken 1962a). In the same study, sexual maturation was implicated in individual distance, insofar as a male used agonistic displays, including Head-forward Display (see Agonistic Behavior) and Snarl (see Sounds: Vocalizations), to establish dominance over cohabiting female. In wild, female may defend small area of several-meter radius around nest, both from mate and from other species of birds (Ficken 1962a).
Predominantly monogamous, with 1 female per territory, but a New Hampshire population had 5–30% polygynous males, depending on year (Secunda and Sherry 1991, Omland and Sherry 1995, TWS, RTH) and a New Brunswick population had 0–4% polygyny (S. Perreault, R. E. Lemon, and U. Kuhnlein, unpublished data). In the New Hampshire study, polygyny tended to be “polyterritorial”—i.e., males advertised secondary territories up to 500 m away from, and out of earshot of, a primary territory. Males usually advertised to secondary females after primary female had completed clutch and begun incubation; males reverted to advertising, repeat-mode song type on secondary territories, with the effect that their song modes changed, sometimes abruptly (i.e., within minutes), as they shuttled between primary and secondary territories. One male had tertiary and quarternary territories, and 3 simultaneously active nests (Secunda and Sherry 1991), although only secondary polygynous territories found in this species in a study in Ontario, Canada (Reudink et al. 2009c). Two cases of polygyny observed in which secondary nest was within 25 m of primary nest; 1 of these involved yearling male (TWS). Overall, this species is variably and facultatively polygynous, involving mixed polyterritorial and monoterritorial polygyny by 0–30% of predominantly older males. More study of these behaviors is needed to clarify norms and ranges of social behaviors involved, and relevant ecological constraints and population consequences.
Sex ratio at time of fertilization, hatching, fledging, and sexual maturity has not been quantified, but existence of abundant unmated yearling males (Ficken 1962a, Ficken and Ficken 1967, Morris and Lemon 1988b, Omland and Sherry 1995) suggests that sex ratio when breeding birds reach sexual maturity (i.e., operational sex ratio) is skewed toward males. Polygyny explains some of mate shortage (particularly for yearling males), but skewed operational sex ratio may also be exacerbated by adult female mortality at nests (see Predation) and possibly during overwintering periods due to male social dominance over females (Marra 2000, Marra and Holmes 2001).
From Ficken 1963a, except where noted. Using ethological observations, Ficken recognized 3 stages of courtship: (1) pair formation, requiring only a few hours; (2) pre-nest-building stage of 12 h to several days needed to choose nest site; and (3) nest-building period of 3–10 d, leading up to copulation near time of nest completion. Various graded displays, with much flexibility of components, contribute to formation of pair bond and courtship, but these can be grouped roughly into those with Fluff Display versus those with Bow. In Fluff Display, erects feathers on breast and back, sometimes also scapulars, head, and rump. Bow consists of raising head vertically while tipping breast toward ground, usually spreading tail, and usually posturing wings either outward or upward, and/or waving them. Many of these courtship postures, including Bow, appear to have evolved as ritualized flight intention movements. Bow given only in context of courtship (unlike some other postures sometimes associated with Bow and Fluff displays), and appears to have strong sexual motivation in preparation for mounting the female.
Pair formation begins as soon as female arrives at breeding grounds, and usually takes place before female has visited many males. Typically, female approaches male within few meters, he flies toward her as though to attack, and she responds by circling away and returning to nearby spot, rather than attacking male or leaving area, as would most males. Female uses behaviors such as Tail-spread Display (see Agonistic Behavior) and Harsh Chip (see Sounds and Vocal Behavior: Vocalizations) in response to being chased, and rarely fights with male. Pair formation per se involves just the array of behaviors and postures associated with agonistic behavior, and no new courtship displays.
During pre-nest-building period, pair explores territory and examines nest sites. Sexual motivation by both birds remains weak; both male’s attack tendency and female’s escape tendency weaken. Male typically follows female during pair formation, as female begins to explore male’s entire territory. When female approaches male, he often leaves vicinity with Gliding Display (seen only during this stage of courtship). When he chases her, female typically flies increasingly lower toward ground, sometimes reaching ground. If she inadvertently takes horizontal posture at this time, male may try to mount and copulate, but rarely, if ever, succeeds.
During nest-building period, male typically follows female closely, presumably a form of mate-guarding (TWS). A male sometimes “tries out” nest sites, by fitting his breast and abdomen into a spot and turning his body. Female tries out many sites, but she generally rejects sites shown her by her mate. Sexual displays begin for the first time during nest-locating and -building: Male assumes Fluff Display posture and female occasionally performs Solicitation Display (see below), but pair does not copulate yet. Male is rarely aggressive toward female, especially near nest site. Fluff Displays, which represent weak sexual drive and reduce female’s attack tendency, become gradually replaced by Bows and other displays with sleeked body feathers, which have opposite effect. Displays and copulations become increasingly frequent during nest-building, up to time that first egg is laid, when all sexual activity ends quickly.
On basis of relatively few observations in wild, pair copulates only 1 to a few times per reproductive effort; copulation is short (a few seconds); male mounts female from rear and touches his cloaca to hers. Birds copulate on horizontal tree limb about 6 m above ground (this height may depend on habitat structure), usually about 6–10 m from nest (Ficken 1963a, TWS). Successful copulations almost invariably immediately follow female’s Solicitation Display, which involves horizontal and crouched posture, quivering wings, elevated tail, and a soft eeee vocalization (Ficken 1963a). Unsuccessful copulations, such as male mounts without cloacal contact, are frequent because of inappropriate behaviors by male or female. Atypical sexual activity includes mounting inappropriate objects (by caged male) and female’s mounting of male (“reverse mounting” or “pseudomale” behavior; Ficken 1963a). Sperm competition is apparently intense in this species, and could occur either via copulation by multiple males at peak period of fertility or via storage of sperm tubules earlier in season (S. Perreault, R. E. Lemon, and U. Kuhnlein, unpublished data). In older males, at least, cloacal protuberance increases in size, measured as width, in the two weeks after arrival to breed, despite total circulating androgen declining over this period, perhaps because territorial behavior is most important when males arrive, whereas reproductive opportunities increase with time as females arrive and come into breeding condition (Tonra et al. 2011).
Ficken 1962a observed copulations of females with neighboring males, and noted that males were attracted to females using soliciting vocalizations. Ficken also speculated that females are not able to recognize their own mate (and vice versa), which would help explain high frequency of genetically successful extra-pair copulations (see below), possible mate-guarding behaviors, and intensity of territorial behavior (often accompanying such intrusions) toward end of nest-building period (TWS), when females are presumably most fertile. However, doubt is cast on this idea that birds cannot recognize their mate, based on humans’ ability to recognize individual males by plumage and vocalizations (see Appearance).
On basis of DNA-fingerprinting of blood tissues collected from locally contiguous family groups, 59% of broods (19 of 32) and 40% of nestlings (n = 108) were sired outside of social pair bond (i.e., extra-pair young [EPY]; Perreault et al. 1997). This study also found that males inexperienced with a site, i.e., breeding there for first time, were significantly more likely than experienced males to be cuckolded. Inexperienced males included both yearlings and presumptive 2-yr-olds; latter were assumed to be breeding for first time in particular locality. Of 6 broods in which no young were sired by resident male, 2 (33%) were of mixed paternity—i.e., at least 2 different males had copulated with that female. However, mixed paternity within broods was generally less than predicted, more consistent with “good genes” than “genetic diversity” hypotheses for these extra-pair matings. This study also showed that 70% of genetic fathers involved in extra-pair copulations were immediate neighbors of the cuckolded male, an observation consistent with observations of copulations with neighboring males (Ficken 1962a).
In another study of extra-pair copulations (McKellar et al. 2014), 40% of nests (n = 100) contained EPY, and 23% (n = 313 offspring) of young tested were EPY. This study was novel by showing that the relative contribution of EPY (as a fraction of total reproductive fitness including EPY and within-pair young) to male’s fitness was density-dependent, higher in years with fewer males in the population, and higher within years where local density was lower. These patterns suggest that either the benefits of EPY to males increase at low density and/or the costs of these behaviors increase. A plausible mechanism for this pattern is higher rates of intrusion by males into neighbor males’ territories at high density, constraining males to spend proportionately more time trying to protect their own paternity on the home territory (Churchill and Hannon 2010, McKellar et al. 2014).
American Redstarts tend to occur as pairs during breeding season; males are highly territorial, females occasionally so (see Spacing; Secunda and Sherry 1991). Yearling male that has not mated may follow and sing to female mated to another male, particularly if other male is busy feeding fledglings (M.-A. Villard, personal communication); yearling male may also visit nest sites of other males (TWS).
During the overwintering period, tends to occur solitarily because of territoriality, certainly within a sex and sometimes between sexes (Holmes et al. 1989, Sliwa and Sherry 1992, Sherry and Holmes 1996a).
Least Flycatchers are aggressive towards many bird species, including American Redstart, and in experimental studies even reduced species richness primarily by reducing settlement probability of other small, migratory bird species (Fletcher 2008). Redstart is also socially subordinate to Least Flycatcher (Sherry 1979, Sherry and Holmes 1988), a status mediated in part by song: the flycatcher approaches source of redstart song, whereas the latter moves less in response to the former’s song (Martin et al. 1996a). In experiment involving broadcast song vocalizations, flycatcher habitat selection in Montana, was positively influenced by redstart song and conspecific song, particularly strongly at intermediate population densities, whereas redstart selection was negatively influenced by flycatcher song, all these results consistent with the asymmetric interspecific competition for resources (Fletcher 2007). Redstarts settle preferentially in response to conspecific song vocalizations (Fletcher 2007), particularly newly arriving older (After-second-year) males (Hahn and Silverman 2006; see also McKellar et al. 2015).
Redstart is also interspecifically aggressive, typically the aggressor with Cerulean Warbler in multiple geographic areas including Ontario, and the Mississippi Alluvial Valley (Buehler et al. 2013). Is involved in kleptoparasitic behavior over nesting material with Cerulean Warbler (Buehler et al. 2013), documented as perpetrator towards Cerulean Warbler on four different occasions in Tennessee, although conclusions about initiator may have been biased by focus on nests of latter species (Jones et al. 2007b). Spider webs, an important adhesive in nest construction, may have been the focus of some of these interactions (Jones et al. 2007b, Buehler et al. 2013).
Joins mixed-species flocks early in breeding season, especially during inclement weather (May in New Hampshire; TWS) and again after breeding season (August–September; Morse 1970b, TWS). Rarely flocks with > 1 other American Redstart.
May join mixed-species flocks or aggregations during the overwintering period (Eaton 1953, Ewert and Askins 1991, Lefebvre et al. 1994a, Latta and Wunderle 1996b). Most individuals (95%) in Hispaniolan pine (Pinus occidentalis) forest occurred in mixed-species flocks (Latta and Wunderle 1996b). An older male redstart once followed closely behind a Jamaican Vireo (Vireo modestus) for > 1 h in nearly leafless, dry-season thorn scrub habitat, presumably eating scarce arthropods disclosed by vireo’s activity (TWS).
Competes interspecifically during overwintering period with multiple socially dominant species (see Demography and Populations: Interspecific Competition).
An older male redstart fed nestlings at a Yellow Warbler nest and dominated the attending male Yellow Warbler; the cause of this unusual social arrangement was not determined (Mannan 1979).
Foraging adults taken by raptors, such as Sharp-shinned Hawk (Accipiter striatus) (Scott 1921; P. H. Yaukey, personal communication; TWS). Nesting adults are vulnerable to climbing mammals, such as red squirrel (Tamiasciurus hudsonicus), on basis of remains (flight- and tail-feathers, and headless carcass) of adult female redstarts found either on nest, or on ground at base of nest tree (n = 11 cases; TWS). Daily survival rate of female adults associated with nests at Hubbard Brook, New Hampshire, estimated to be 0.9957 (on basis of 11 inferred deaths, 2,531 nest exposure days at 268 nests), which extrapolates to survival probability of 0.92 throughout typical 20 d of incubation plus nestling periods (TWS). Female presumably experiences greater mortality risk than male while nesting, because of her predictable and closer association with nest, and this likely contributes to the male-biased sex ratio at breeding, and shortage of mates for yearling compared to older males (see Demography, Other: Life-history Traits: Delayed Plumage Maturation and Sexual Selection). Migrating adults are vulnerable to jaegers (Stercorarius spp.; Griscom and Sprunt 1957) and hawks (D. Aborn, personal communication), but precise risk has not been quantified.
Documented nest (eggs, nestlings) predators in Alberta include red squirrel, northern flying squirrel Glaucomys sabrinus, Yellow-bellied Sapsucker (Sphyrapicus varius), Cooper’s Hawk (Accipiter cooperii), Sharp-shinned Hawk, northern goshawk (Accipiter gentilis), Northern Saw-whet Owl (Aegolius acadicus), Blue Jay Cyanocitta cristata (McCallum and Hannon 2001, Hannon et al. 2009), and Gray Jays (Perisoreus canadensis; S. Hannon, unpublished data). Eggs and/or nestlings at Hubbard Brook, New Hampshire, are known to be depredated by Blue Jay, Common Raven (Corvus corax), and red squirrel (TWS). Sheet-metal baffles (downward-opening cones and sheathes around tree trunk) significantly reduced frequency of predation at active nests < 10 m above ground at Hubbard Brook, New Hampshire, indicating importance of climbing mammals as nest predators, probably both red squirrel and eastern chipmunk (Tamias striatus; Sherry et al. 2015). Also at Hubbard Brook, cameras at artificial nests baited with quail eggs, and placed in tree crotches to simulate American Redstart nests, documented visits by red squirrel, fisher (Martes pennanti), eastern chipmunk, black bear (Ursus americanus), flying squirrel (Glaucomys spp.), and Blue Jay (n = 42; Sloan 1991), thus implicating all these species as potential nest predators there. Mobs Blue Jay vigorously, as well as Northern Saw-whet Owl (TWS), Common Grackle (Quiscalus quiscula) and Cooper’s Hawk (Ficken 1962a). Other known nest predators of redstarts include fox snake (Elaphe vulpina gloydi) that took eggs (Sturm 1945; see also Stevenson and Anderson 1994b), house cats (Bent 1953b), and Red-bellied Woodpecker (Melanerpes carolinus) that preyed on a nestling redstart (Watt 1980). At a low elevation site in Ontario, primary nest predators identified as American Crow (Corvus brachrhynchos), eastern chipmunk, American red squirrel, gray squirrel (Sciurus carolinensis), Blue Jay, Broad-winged Hawk (Buteo platypterus) and Sharp-shinned Hawk, and black rat snake (Elaphe obsoleta) (McKellar et al. 2014). Snakes are almost certainly important predators in southern North America, and TWS witnessed a pair of redstarts with day-old fledglings intensely mobbing a ratsnake, probably a Texas rat snake (Elaphe obsoleta lindheimeri) in shrubbery, Bogue Chitto National Wildlife Refuge, southeastern Louisiana. Two other likely nest predators at this Louisiana site (TWS) include Barred Owl (fledged owl family group feeding diurnally at same time and place as redstart nests became inactive) and Swallow-tailed Kite (Elanoides forficatus) based on this raptor’s abundance in this habitat and its habit of carrying nest and nestlings of other small, open-cup nesting songbirds like canopy-nesting Blue-gray Gnatcatcher (Polioptila caerulea) back to its own nestlings (Coulson 2001).
Besides mobbing responses to particular predators mentioned above, responds to predators using several postures. A young bird (8–11 d or so posthatching) may Crouch, i.e., retract its head, flatten its body against nest, and remain motionless for minutes at a time (Ficken 1962a). Incubating female may also Crouch as low in nest as possible to avoid detection, e.g., in response to human observer approaching nest (TWS). In Freezing Posture—e.g., in response to aerial predator—bird sleeks feathers, elongates neck, makes body horizontal, and bulges eyes for up to many minutes (Ficken 1962a). Alert Posture, often initiated by disturbance such as loud sound, consists of upwardly stretched neck and sleeked body feathers, spread tail, and constant peering in all directions (Ficken 1962a). Both sexes approach predator closely, i.e. “mob” it, near nest, and give frequent Chip notes (Root and Yarrow 1967, TWS). Pivoting—spreading tail and alternately rotating body (but not feet) through 60° arc—accompanies mobbing, usually at a distance > 6 m from targeted animal (Ficken 1962a). Frequency and intensity of mobbing with call notes increase as nestlings approach fledging age (TWS), although some of this increase may be learned response to repeated intrusions by humans.
Performs captivating Distraction Display in context of threats by potential predators to eggs, nestlings, or recent fledglings (Bent 1953b, Ficken 1962a, TWS). Displaying bird, often a female leaving her nest at approach of intruder, typically flies to ground, crouches in horizontal posture, flutters (vibrates) wings rapidly and shallowly, spreads wings and tail so as to display conspicuously the plumage’s contrasting colors, and twitters in high-pitched vocalization. When intruder turns away from displaying bird and back toward nest, parent often doubles back into position near intruder as if to entice it away from nest vicinity (TWS). An older male performed Distraction Display along branch several meters away from 10-m-high nest when Blue Jays approached (TWS).
Female occasionally gives Tsip call note from nest when startled by nearby human observer (TWS). Male often sings during nestling period when carrying food to nest, but typically with decreasing volume while accumulating a load of food in bill; ceases to sing entirely for up to several minutes just before visiting chicks; shortly after leaving nest, male sings several loud song phrases, but not until he has moved a distance of 10–20 m away (TWS). In general, male does not sing from nest, except sometimes while female builds.
At other times of year (e.g., while on stopover sites during migration), forages deeper into leafy shrubs in response to greater density of hunting Sharp-shinned Hawks, indicative of an ability to perceive risk level (Cimprich et al. 2005).