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American Robin

Turdus migratorius

Order:
Passeriformes
Family:
Turdidae
Sections

Diet and Foraging

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Feeding

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Once captured, extracting a large earthworm requires strong legs to pull a worm out of its burrow.

© Ryan Schain, Massachusetts, United States, 7 March 2010

Main foods taken

Invertebrates and fruit.

Microhabitat for foraging

Generally a ground feeder on terrestrial invertebrates, a foliage gleaner on foliage invertebrates, and a fruit eater (fruits on plants as well as fruits that have fallen to the ground). Only 1 study, conducted in Wisconsin from Jun to Aug 1976, has compared the time spent on different foraging substrates (Paszkowski 1982). Of 452 min of observation, 78% of foraging occurred in vegetation (prey captures typically involved large insects, such as adult lepidopterans and odonates, taken from the air or foliage), 15% occurred on the ground (prey included items frequently taken from the shore of a nearby lake, such as washed-up insects or insects attracted to lake-side debris, foliage insects fallen from the overhanging canopy, and annelids), and 7% occurred on fruits (birds either picking fruits while perched or on the wing and swallowing them whole). More travel moves and feedings occurred on the ground than predicted by time investments. Fewer travel moves and feedings were made in the vegetation. Fruit foraging proved intermediate, involving fewer travel moves but more feedings than predicted (Paszkowski 1982).

When on the ground tends to forage near cover, such as edge of shrubs or tree lines (Oyugi and Brown 2003). Both adult and juvenile robins are more successful at capturing arthropod prey in well-lit sunny environments and worms in moist soils. Juveniles need very moist soils to be effective at worm capture (see below; Vanderhoff and Eason 2008). In Alberta the distribution of robins is strongly correlated with the presence of invasive earthworms and robins occur most often in locations with high human activity where earthworms were abundant (see Demography and Populations; Cameron and Bayne 2012).

Invasive bush honeysuckle (Lonicera spp.) may provide important habitat for wintering robin communities; in Illinois densities of robins in winter months are higher in areas with honeysuckle than those with native shrubs (McCusker et al. 2010).

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Robin visually searching and listening for earthworm prey in grass.

© Brian Sullivan, South Dakota, United States, 7 July 2009
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Figure 4. American Robins consuming fruit.

The American Robin, shown here with English hawthorn (Crataegus monogyna), consume a large amounts of fruits during the autumn and winter months throughout its range; fruit selection has been well studied in this species. Drawing by Julie Zickefoose.

Food capture and consumption

Employs two foraging modes: a widely foraging one and a sit-and-wait foraging mode (Paszkowski 1982). When foraging for earthworms, uses a combination “Head-Cock” and “Bill-Pounce” behavior (Heppner 1965). In Head-Cocking, one eye points toward a spot on the ground, 3–5 cm directly in front of the bird, along the longitudinal axis of the body. After holding this position for a few seconds, the robin rotates and flexes its head to bring the other eye into a similar relationship with the ground. Bill-Pouncing then occurs, whereby the bill is thrust quickly into the ground, presumably at visually detected prey, at the spot where the eyes had been directed.

Individuals also use their bills to probe the ground and soil, as well as to move leaves, twigs and other objects while foraging (Vanderhoff and Eason 2008). Robins make uncomplicated maneuvers to catch small, slow ground invertebrates and run in brief spurts to chase faster invertebrates, especially those flushed (ENV). They use picks and gleans to harvest fruit; such behavior has been described by Chavez-Ramirez and Slack (Chavez-Ramirez and Slack 1994) as “reaching out” and “gulping fruit.” May also take fruits on the wing (RS).

Capturing large, flying insects in vegetation requires long, complex flights characteristic of sit-and-wait species. Larger prey items are often beaten against the ground or other hard substrate to immobilize prey before consumption (ENV). Choice of foraging mode is related most clearly to food-item attributes, particularly distribution and mobility, rather than to the structure of a foraging site. Ability to shift foraging modes allows the robin to exploit a variety of habitats and food resources (Paszkowski 1982).

Attack rate (the number of times/foraging min that a bird either Bill-Pounced or snapped at prey) was found to increase as grass length increased on a suburban lawn (Eiserer 1980b). Foraging time, on the other hand, decreased with increasing grass length. When offered a choice between short- and long-grassed areas, robins showed a preference for short grass. Finally, the act of mowing itself (independent of grass length) induced a short-term increase in the time robins spent foraging, implying that birds were exploiting temporarily vulnerable prey (Eiserer 1980b).

In New Jersey, during the fall of 1979, Gochfield and Burger (Gochfield and Burger 1984) compared the behavior of adult and juvenile robins foraging for invertebrates on lawns, revealing significant differences in several measures of feeding efficiency and success. Adults captured larger food items, made more captures per unit time, took fewer steps, and made fewer unsuccessful attempts. Juveniles took 36% longer to capture each food item and required 61% more steps than adults did, obtaining 25% less food/unit time.

In Kentucky, similar results were found. Adult robins consumed almost twice as many worms per minute as did juveniles. Moreover, juvenile robins were only successful at capturing earthworms on the most saturated soils. Juveniles were less successful than adults at capturing other arthropod prey and tended to make more attempts to capture the same amount of prey as adults. However, the success rate of juvenile robins is higher than that of other ground foragers such as starlings and mockingbirds. Juvenile robins also forage more with other juveniles and adults, possibly to increase localization of suitable foraging patches and increase prey consumption (Vanderhoff and Eason 2007, Vanderhoff and Eason 2008). Juvenile robins were also less proficient at foraging for fruit and consumed significantly fewer mulberries and cherries per minute than did adults (Vanderhoff and Eason 2007, Vanderhoff and Eason 2008; see Food Selection).

Few data on daily foraging patterns. Known to forage for earthworms more frequently in early morning and late afternoon (Heppner 1965) and to eat more fruit later in the day (Wheelwright 1986).

Robins foraging on fruits in cold winter conditions increased their rate of fruit consumption before nightfall (Sallabanks 1997). A single bird was observed packing fruits into its esophagus during the final 2 h of foraging; the bird did this by increasing the proportion of picked fruits that it swallowed. Such behavior was presumably in response to the need to survive abnormally low nighttime temperatures (Sallabanks 1997). Indeed, robins are known to die during freezing winter periods, despite the presence of abundant fruit (Courtney and Sallabanks 1992, Sallabanks and Courtney 1992c). The storage of fruits in an extendible esophagus has also been documented in other frugivores (e.g., Levey and Duke 1992) and may be an adaptation to overcome the constraints on food-processing by the gut. Therefore, although robins lack an anatomically distinguishable crop, they nonetheless have an extendable esophagus that may function as a crop.

Diet

Major food items and quantitative analysis

The most comprehensive studies of the stomach contents of robins have used records compiled by the U.S. Biological Survey and U.S. Fish and Wildlife Service (Beal 1915b, Wheelwright 1986, White and Stiles 1990). Wheelwright (Wheelwright 1986) analyzed data on the stomach contents of 1,169 robins collected across their entire range and found that this species ate fruits representing over 50 genera and invertebrates representing over 100 families. Major food classes were soft-bodied invertebrates, hard-bodied invertebrates, and fruits (see Table 1).

The effect of season, habitat, time, and sex on fruit diet was also analyzed by Wheelwright (Wheelwright 1986). Proportion of fruit in robins' diets was influenced by several factors, especially time of year: fruit constituted more of the diet in the fall and winter (median values > 90%) than in the spring (< 10%); summer values were intermediate. Male and female robins did not differ in the proportion of fruit eaten in any month in any region. Stomachs of juvenile robins generally contained a higher proportion of fruit than did stomachs of adults. Robin stomachs contained different proportions of fruit depending on the habitats in which birds were collected (i.e., fruit content was greater for birds collected in orchards, native fruiting trees, and forests than in agricultural fields). Finally, robin stomachs contained a smaller proportion of fruits in the morning than in the afternoon (Wheelwright 1986).

Beal (Beal 1915b) used some of the same early records as Wheelwright (Wheelwright 1986) in his investigation of 1,236 robin stomachs and found them to contain 42% animal matter and 58% vegetable matter. Beal (Beal 1915b) provides a detailed account, including species names, of the different plant and invertebrate taxa found. White and Stiles (White and Stiles 1990) also used data compiled by the U.S. Biological Survey to examine short-term dietary mixing by robins. Stomach content data for 1,085 individuals were combined with field data on the contents of fecal droppings and regurgitations. Plant and animal matter were often mixed in bird stomachs, as were the fruits of different plant taxa. Fecal analyses confirmed these findings. White and Stiles (White and Stiles 1990) concluded that the mixed diet of birds such as robins may be a function both of the need to balance nutrient intake and bird movement and of weak preferences among similar fruits. The robin may be able to increase the nutritional benefit of fruits by selectively consuming those that are insect-infested (Sallabanks and Courtney 1992c).

Fecal analyses, rather than stomach contents, were used by Hamilton (Hamilton 1940) in his quantification of diet for robins in Ithaca, New York. Over 700 droppings, collected during the summer (Jun–Aug) of 1939, were analyzed and found to contain a similar mix of animal and plant matter. Insects (especially Coleoptera) dominated the animal component of the diet, and fruits of the chokecherry (Prunus virginiana) dominated the plant component. Extensive studies in the western U.S. have shown that fruits of the English hawthorn (Crataegus monogyna) are a staple food item in winter (Sallabanks 1992b). Robins also consume large quantities of fruits of juniper (Juniperus spp.) and are consequently an important and efficient disperser of this genus (Livingston 1972, Chavez-Ramirez and Slack 1994). Also known to become intoxicated from feeding exclusively on fruits of the honeysuckle (Lonicera tatarica; Bergtold 1930).

Isotope analysis of blood and claws indicate that despite the importance of fruit in their diet, robins retain high 15N levels year-round and there was no noticeable shift in 15N levels during migration periods (Gagnon and Hobson 2009).

Other unusual food items consumed include fish (Phillips 1927, Michael 1934, Kimball 1944, Bayer 1980c), small snakes (Davis 1969c, Netting 1969, Richmond 1975, Erickson 1978, Foreman 2017), shrews (Powers 1973, Penny and Knapton 1977), damselfly nymphs (Hall and Northcote 1986), frogs (Preston 2005, Leighton 2006) and skinks (Vanderhoff 2007).

Food Selection and Storage

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Fruiting plants provide an important source of nutrition for juveniles, which are less experienced at foraging.

© Noëlla Aubry, Quebec, Canada, 7 September 2015
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American Robins are important seed dispersers of many woody plant species.

© Chris Wood, Minnesota, United States, 24 October 2004
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Adult male carrying earthworm prey to feed young.

© Michael J Good, Maine, United States, 6 May 2016

Well studied, especially fruit selection (Figure 3). Facing a hierarchy of foraging decisions (i.e., which plants to feed in, which fruits to pick, and which fruits to swallow once picked), wintering robins may use a variety of cues (Sallabanks 1993c). In descending order of importance, fruit crop size, fruit size, and fruit pulpiness appear to be especially important decision cues. Once plants are selected, individuals make visual choices among fruits; large fruits are preferred to small ones. Once fruits are picked, additional discrimination does not seem to occur. The use of a repertoire of cues to make foraging decisions may have evolved in response to the environmental uncertainty typical of fruiting systems. Such flexibility in foraging behavior may, in itself, promote variability within and among fruiting plants (Sallabanks 1993c, Sallabanks and Courtney 1993d).

In old fields and mixed-age and mature woods in central New Jersey, wintering robins fed on larger fruits more efficiently than smaller fruits, harvesting more biomass while swallowing fewer fruits in shorter visits (White and Stiles 1991).

In Kentucky, adult foraging rates (number of fruits consumed per minute) are similar for mulberry and cherry fruit, but juvenile robins consume significantly less fruit than adults and rates are lower for mulberry than cherry fruits (Vanderhoff and Eason 2007, Vanderhoff and Eason 2008). Mulberry fruits are available sooner in the season when young robins have had less experience with fruit; overall lack of fruit foraging proficiency may be due to juvenile's inability to distinguish between unripe fruit and ripe fruit. Both adults and juveniles were observed testing fruit with their beak, possibly to gauge fruit quality and ripeness. Adults were more likely to drop fruit than juveniles, possibly indicating that they are more skilled at determining fruit ripeness (Vanderhoff and Eason 2007).

Among captives, individuals often favor blue, and sometimes red, artificial fruits over green or yellow ones (Willson 1994). High-lipid and high-glucose fruits are often selected when dark in color, but not when the higher nutrient level is a lighter shade of the same color. Slight decreases in accessibility often alter preference rankings, and robins reject fruits with high seed loads (Willson 1994). Other studies have shown captives favor red over blue artificial fruits in choice trials (Lepczyk 1993, RS). Choice trials also indicate that robins prefer sugar-rich and red fruit to lipid-rich and purple fruit in the summer (Lepczyk et al. 2000). Individuals differ in choice among mulberry, honeysuckle, and dogwood (Cornus stolonifera) fruits (Jung 1992). As a group, prefer mulberry (Morus alba) and avoid honeysuckle fruits; heavier robins tend to prefer larger fruits. Large seeds are voided rapidly by regurgitation, resulting in higher pulp consumption rates for large-seeded fruits than for small-seeded ones, whose seeds pass through the gut (Murray et al. 1993). Most captives prefer large-seeded fruits, presumably using seed packaging as a measure of fruit profitability. Rotten fruits consumed at lower rates than control fruits (Cipollini and Stiles 1993).

Fruit selection preferences of captives also change with season (Wheelwright 1988). Preference for native versus non-native fruits has been investigated with varying results. Captive studies in Connecticut indicate that robins prefer fruits of invasive plants to native fruits and are more likely to try novel fruits than are starlings (LaFleur et al. 2007). However, tests with red and dark blue fruits yielded contrasting results. For example, given a choice between two red fruits, native winterberry (Ilex verticillata) and the invasive autumn olive (Elaeagnus umbellate) robins always chose the latter. These differences might be due to the nutritional content of the fruit; invasive species contained more protein than the native fruit (LaFleur et al. 2007). However given a choice between two blue fruits, the native highbush blueberry (Vaccinium corticillata) and invasive glossy buckthorn (Frangula alnus), robins preferred the native blueberry. In another study conducted in California, robins preferred native toyon fruits (Heteromeles arbutifolia) to non-native olive fruit (Olea europaea). When non-native fruit are similar in color to native fruit, frugivores like the robin may be more apt to choose non-native when natives are unavailable or when non-natives are more abundant and/or produce larger fruit crops (Aslan and Rejmánek 2012).

When robin diets were switched from fruits to insects, digestive efficiency was initially compromised, perhaps due to a lag in digestive responses to the new diet (Levey and Karasov 1989). Available energy from fruit pulp was surprisingly low, probably a result of rapid transit times in the gut for fruits (Levey and Karasov 1989). Lacking the digestive enzyme sucrase (Karasov and Levey 1990) and therefore unable to digest sucrose, robins may develop an aversion to high-sucrose fruits (Brugger 1992). When tested, captives did avoid 15% sucrose solutions, but only during the first hour of testing. Fruit was consumed at higher rates than insects, yet robins did not eat enough fruits to meet energy and nitrogen requirements.

Invertebrate selection was examined in only 2 studies. Because the sounds of earthworms burrowing in the soil are of such a low intensity that they are masked by background noise at locations where robins normally forage, individuals probably look for visible signs of worms in their burrows, rather than relying on auditory cues (Heppner 1965; see Feeding). Adults generally consume small prey and feed larger items to their young (Swihart and Johnson 1986). The length of foraging bouts is primarily a function of the cumulative size of prey eaten prior to loading rather than the size of the load. Foraging models indicate that prey size, quantity of prey in the bill, and time away from the nest are important in determining when a bout is terminated (Swihart and Johnson 1986). Rate of foraging in territories increases with age of nestlings.

Mated pairs partition territories roughly in half, presumably as a mechanism for increasing foraging efficiency in a fairly homogeneous environment. Central-place foraging behavior was modified in response to predation risk; foraging adult robins nesting near an American Kestrel (Falco sparverius) nest had increased mean foraging bout duration and mean load of prey per bout compared with pairs without apparent predation risk (Johnson and Swihart 1989).

Not known to store food in the traditional sense, but see comments on the storage of fruits in an extendible esophagus in Feeding.

Nutrition and Energetics

Kendeigh's (Kendeigh 1969b) equation for existence metabolism predicts an 18.9 kcal/d existence energy requirement for a 55-g robin (Hazelton et al. 1984). If captives required no more than 130% of existence metabolism and could metabolize 80% of gross energy in fruits, Hazelton et al. (Hazelton et al. 1984) calculated that robins required 30.7 kcal of energy intake/d. Average daily assimilation (total energy of consumed food – total energy of excreta) of captives was calculated by Levey and Karasov (Levey and Karasov 1989) to be 80 kJ/d.

Robins have short gut retention times and have relatively low digestive efficiency for sugars. In vivo digestive efficiency of radio-labeled glucose was 73% in fruit-eating robins (Karasov and Levey 1990); sucrose could not be digested. Mean assimilation efficiency for sugars and lipids was 97.8% and 77.2%, respectively (Lepczyk 1993). At the typical rate of consumption of 3 fruits/h, robins derive 2.62 kcal/h when feeding on sugar-rich fruits but only 2.25 kcal/h when feeding on lipid-rich fruits; for this reason, robins preferred sugar-rich fruits over lipid-rich fruits in choice tests (Lepczyk 1993).

Robins show seasonal differences in assimilation efficiencies. Summer assimilation efficiencies for sugars are similar to the above, but decrease in the autumn to a mean of 96.6%. In preparation for migratory fattening, lipid assimilation efficiency increases from 76.6% in the summer to 90.7% in the autumn (Lepczyk et al. 2000).

Blood metabolite profiles at spring refueling stops in Ontario indicate stopover site quality, with higher levels of triglycerides at high quality sites than at low quality sites (Guglielmo et al. 2005). Glucose metabolism occurs passively in the gut and the lack of cell-mediated metabolism provides robins and other frugivores with an advantage when it comes to plant toxins, such as flavonoids, that are capable of blocking cell-mediated metabolic pathways; thus robins can maximize energy intake (McWhorter et al. 2010, Skopec et al. 2010, Karasov 2011). There are few studies on metabolic differences of ingested native and non-native fruit. Captive studies suggest that non-native fruits such as Chinese tallow (Triadica sebifera Syn. Sapium sebifera) provide high apparent metabolizable energy for robins and thus could serve as another food source for wintering and migrating birds (Baldwin et al. 2008).

Intestines for 2 salvaged robins were 230 and 257 mm long (White and Stiles 1990).

Metabolism and Temperature Regulation

See Food Selection and Storage. No information on temperature regulation; like other birds, however, robins will fluff feathers in winter to trap heat. In summer they pant and raise wings to cool themselves, especially when forging on sun-exposed lawns and fields (ENV).

Drinking, Pellet-Casting, and Defecation

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Like other songbirds, American Robin lowers its bill to the water and tilts its head back to drink.

© John Atkinson, Indiana, United States, 20 January 2016

Frequently regurgitate seeds when eating fruits, especially if seeds are large (Murray et al. 1993, RS), ridding themselves of indigestible seeds that would otherwise occupy gut space and reduce fruit handling times (Courtney and Sallabanks 1992). During fruit-eating foraging bouts, robins regurgitate and defecate frequently and are known to regurgitate fruits and reswallow them repeatedly (e.g., White and Stiles 1990, Sallabanks 1992b). Each fecal dropping of robins foraging on English hawthorn fruits contained a mean of 2 seeds ± 0.10 SE (n = 246 droppings). The ratio of eaten to regurgitated/defecated fruits was 2.52:1. Robins dropped 20% of the fruits they picked and regurgitated/defecated 40% of fruits swallowed before leaving fruiting hawthorn bushes; over 50% of fruits removed from bushes by robins were actually dispersed (Sallabanks 1992b). A typical full-sized fecal dropping from a robin measures 20–25 mm long by 4–5 mm in diameter; the end emerging from the cloaca tends to be the widest and is the locus of urates when they occur (White and Stiles 1990). Coprophagy observed in Kentucky—robins observed eating fecal droppings on the ground during or shortly after bouts of fruit eating (ENV).

The ingestion of fruits and subsequent defecation or regurgitation of seeds makes the robin an effective seed-dispersal agent. As well as dispersing native fruits (western chokecherry Prunus virginiana var, demissa (Beck and Vander Wall 2011), robins are important dispersers of several invasive plants including English hawthorn Crataegus laevigata (Sallabanks 1993b), Chinese tallow (Triadica sebifera syn Sapium sebiferum; Renne et al. 2000), bush honeysuckle (Lonicera maackii; Bartuszevige and Gorchov 2006), and English holly (Ilex aquifolium; Zika 2010). Robins may be important transporters of ornamental plants, such as holly, from suburban yards to adjacent woodlots; in Seattle robins account for 99% of holly frugivory: a single robin can remove up to 20 seeds, an entire flock an estimated 3,187 seeds (Zika 2010). Robin foraging behavior also aids in dispersal; robins will remove fruit and process the fruit in “relay” trees 10–50 m away from fruiting trees, as well as forage in nearby clearings on the ground for invertebrates (Zika 2010).

Ingestion of seeds has been shown to reduce germination success in one study (Crossland and Kloet 1996), but in Ohio 86% of honeysuckle seeds defecated by robins were viable and germination rates were higher for defecated seeds than for seeds from intact fruit. Robins also tend to disperse honeysuckle seeds in edge environments which are often more suitable for invasive plants such as the honeysuckle (Bartuszevige and Gorchov 2006). In Louisiana and South Carolina, robins are important dispersers of Chinese tallow, but their overall importance varies between sites and robins are not the primary dispersal agent as with holly above (Renne et al. 2002).

Drinks water from ponds, slow moving water, puddles and man-made structures; lowers beak into water, raises head, and swallows in short rapid bursts (ENV).

Recommended Citation

Vanderhoff, N., P. Pyle, M. A. Patten, R. Sallabanks, and F. C. James (2016). American Robin (Turdus migratorius), version 2.0. In The Birds of North America (P. G. Rodewald, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bna.462