Main Foods Taken
In the breeding season, American Redstarts feed mostly on insects, including leafhoppers and planthoppers (Homoptera), flies (Diptera), small and mostly parasitic wasps (Hymenoptera), beetles (Coleoptera), and moths and their caterpillar larvae (Lepidoptera; Bent 1953b, Robinson and Holmes 1982; K. Judson and TWS, unpublished data). Some small berries and fruits in late summer (Bent 1953b). During overwintering period, also insectivorous, feeding mainly on small flies, leafhoppers and planthoppers, bark lice (Psocoptera), beetles, small wasps and ants, and beetles (Sherry 1985; K. Judson and TWS, unpublished data; RTH).
Microhabitats for Foraging
Breeding Period. Forages from ground to near top of canopy (Williamson 1971, Robinson and Holmes 1984, TWS). In northern hardwood forest at Hubbard Brook Experimental Forest, New Hampshire, males and females foraged at similar heights (10.9 m ± 0.8 SD [n = 1,111] and 11.3 m ± 4.6 SD [n = 510], respectively), obtaining 64–75% of prey from foliage, 23–31% from air, 1–4% from twigs and branches, and < 1% from ground (Holmes 1986). In New Brunswick, however, female tends to forage lower than male, often near (but not on) ground (R. E. Lemon, personal communication), possibly because females nest lower there than at Hubbard Brook (see Breeding: Nest Site). Forages in all parts of trees, but tends to take prey from twigs, branches, and leaves (Williamson 1971, Robinson and Holmes 1984, Holmes 1986, TWS). Sometimes concentrates temporarily on taking adult moths and other prey from tree trunks, presumably when such prey are relatively abundant on such substrates (TWS). Differs from most other wood-warblers by taking more flying prey, but overlaps and competes with other fly-catching species, such as Least Flycatcher, for same kinds of insect prey (Holmes et al. 1979b, Sherry 1979, Sherry and Holmes 1988).
In New Hampshire, captured arthropod prey about equally from upper and lower leaf surfaces (42% and 47%, respectively), in contrast to several species of Setophaga (previously Dendroica) warblers, which take prey mainly from lower leaf surfaces (Holmes and Schultz 1988). However, this depends on tree architecture when controlling for prey type (Whelan 2001): For example, in experiments with mealworm pieces as prey, redstarts predominantly captured prey from lower surface of typically raised leaves (above the branch = perch) of sugar maple (Acer saccharum) but from upper surface of planar yellow birch (Betula alleghaniensis) leaves at the bird’s foot, in both cases using energetically most economical foraging tactics (gleans rather than hovers). When individuals attack less accessible lower surfaces of yellow birch, they tend to use aerial hover tactics, much as when attacking prey on upper surfaces of sugar maple. Aerial attacks are also used more frequently to attack prey on lower leaf surfaces, and prey at greater distances from the bird. Different species of Setophaga, despite overall similarities in leg and wing morphology, attack prey differently compared to redstarts, and to other species when confronted with different foliage structure (Whelan 2001).
While in molt, tends to forage using relatively economical behaviors (hops, gleans, infrequent sallies, and few aerobatic pursuits of prey or long flights), and tends to stay in relatively dense vegetation, sometimes high in canopy, and to give infrequent (soft) songs (TWS), perhaps because reduced wing and tail surface area from temporarily lost remiges and rectrices either decreases efficiency of aerial foraging attacks and/or increases vulnerability to predators.
Overwintering Period. Similar to breeding season. Forages over full vertical range of woodland or forest habitats (RTH, TWS), but concentrates at middle heights in vegetation (e.g., 6–12 m in rain forest and tall mangroves, 36 m in secondary scrub woodland; Bennett 1980). Prey taken most frequently from air (40–70%), or foliage (28–50%), but also occasionally from tree trunks (Jeffrey-Smith 1972) or ground (Lack and Lack 1972, Bennett 1980, Lovette and Holmes 1995), particularly in open habitats such as in pneumatophores of mangrove trees (TWS; P. P. Marra, personal communication). Females shift from near-perch (glean, reach, and hang) to more energetically expensive aerial tactics (hawk, hover, hover-glean) over the overwintering period in poorer (drought-prone, deciduous) thorn scrub habitat, but not adjacent black mangrove habitat in Jamaica, either because of changes in the availability of prey types or increasing leaf drop allowing better detection of flying and more distant prey (Powell et al. 2015). These foraging behaviors are also related to body condition (see Demography and Populations: Population Limiting Factors).
Food Capture And Consumption
Forages alone, although members of pair sometimes forage near one another and occasionally join mixed-species flocks in nonbreeding season (Morse 1970b, Ewert and Askins 1991, Latta and Wunderle 1996b, TWS).
Breeding Period. Both male and female move rapidly while foraging (Williamson 1971, Robinson and Holmes 1982, Lovette and Holmes 1995). Spreads and displays (“flashes”) wings and tail, presumably to flush prey (Root 1967), particularly while provisioning nestlings and fledglings, but also frequently during the overwintering period (Lovette and Holmes 1995, TWS). In yellow birch, compared with other tree species in northern hardwood forest, responds with increased attack rate, and with increased frequencies of flush-chase tactics (i.e., prey is startled into flight and pursued) and tail fanning, presumably to help flush relatively abundant leafhoppers (Cicadellidae) found in yellow-birch foliage (Robinson and Holmes 1984). Searches different northern hardwood-tree species approximately in proportion to availability (Holmes and Robinson 1981). When actively foraging, hops and flies rapidly between perches, changing perch 27–30 times/min on average (Williamson 1971, Robinson and Holmes 1982). Gleans prey, makes short hovering maneuvers to attack prey on leaves, often tumbles aerobatically while chasing prey flushed from foliage, and makes mostly short sallies (up to about 5 m) into air to catch flying prey (Ficken 1962b, Robinson and Holmes 1982, TWS). Unlike some tyrannid flycatchers, rarely returns to same perch from which it initiates aerial attack (TWS). Its mean search radius (distance over which it perceives and attacks prey) is 53 cm ± 5.3 SD (n = 522) with little variation among tree species; attacks prey at average rate of 5.0–5.6/min (Robinson and Holmes 1982, Robinson and Holmes 1984, Lovette and Holmes 1995). Whacks large prey items forcefully against perch to subdue prey, in the case of caterpillars to remove gut contents, and to remove wings of large moths (Ficken 1962b, TWS). An adult that is provisioning nestlings feeds much of the day, uses energetically demanding tactics, and captures prey at a high rate, suggesting that food demand is high relative to availability (Lovette and Holmes 1995). Female may find incubation particularly demanding in northern breeding latitudes, on basis of relatively high rate of foraging (Sherry 1975) and on the very small prey items taken every few seconds then (TWS), presumably because females have little time to forage while maintaining constantly high egg temperature for rapid embryo development in a cool temperate climate.
Overwintering Period. Foraging tactics and movements similar to those in summer, but aerial sallies and flush-chases are proportionately more common (Bennett 1980, Sherry 1985, Lovette and Holmes 1995). On average, rate of movement while foraging is faster in winter than in summer, proportion of time spent foraging is greater, and rate of attacks on prey is lower (2.4–2.8/min), observations consistent with food being less abundant or less accessible in winter than in summer (Lovette and Holmes 1995). Can catch up to 8–13 insects/min during overwintering period (Jeffrey-Smith 1972).
In general, foraging behavior is flexible, depending on individual and time of day (Holmes et al. 1978a, Ficken 1962b), season (Lovette and Holmes 1995, TWS), habitat (Maurer and Whitmore 1981, Woodrey 1995, Powell et al. 2015), tree species and strata within a forest (Robinson and Holmes 1984), and tree architecture (Whelan 2001). Opportunistic response of redstart to unusual opportunities is reinforced by anecdotes: holding a territory around a garbage container in Mexico (S. Bennett, personal communication) and around latrines and chicken coops in Jamaica (source of a common name “latrine bird” there; K. Levy and L. Douglas, personal communication); and feeding at night on insects attracted to a window (Bakken and Bakken 1977). Individuals also forage actively in rainforest interior of Cocos Island, Costa Rica, a habitat rarely used by other migrant warblers, and facilitated by ability to flush and chase the proportionately abundant planthoppers (Hemiptera, Fulgoroidea) there (Sherry 1985). Diet opportunism supported (Sherry et al. 2016) based on diets in shaded coffee plantations in Jamaica: guts contained similarly small, available insect prey to four other overwintering migrant warblers, and prey were small (many < 2 mm length) and often patchily distributed among the gut samples within a foraging species, indicative of foraging regularly on small patches of aggregated prey. Thus, redstarts are evolutionarily generalized—i.e., behaviorally opportunistic—in terms of foraging behavior. This opportunism is consistent with the seasonally changing demands and diverse ecological conditions encountered annually (Sherry 1990).
Convergent in terms of morphology and associated foraging behavior not only with Myioborus warblers, but also with New World and Old World flycatchers (Holmes et al. 1979b, Sherry 1979, Bennett 1980, Keast et al. 1995). Has flattened flycatcher-like beak—depth 66% of width (Keast et al. 1995; see also Table 1), proportionately large wing area (i.e., low wing-loading) and large tail area for aerobatic attacks and pursuits of evasive insect prey, and rictal bristles typical of tyrannid flycatchers that pursue prey aerially (Bennett 1980, Sherry 1983, Keast et al. 1995). Distinctive wing and tail flash patterns (contrasting colors exposed by fanning wings and tail during foraging) make this species unique among North American passerines (Keast et al. 1995). Morphological, foraging behavioral, and dietary similarities of American Redstart to Least Flycatcher appear to underlie their competitive ecological interactions on breeding grounds (Sherry 1979, Sherry and Holmes 1988, Martin et al. 1996a, TWS).