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American Robin

Turdus migratorius

Order:
Passeriformes
Family:
Turdidae
Sections

Demography and Populations

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Measures of Breeding Activity

Age at first breeding; intervals between breeding

Adults can breed beginning in the first spring following hatching (i.e., their second year of life), and breed annually thereafter. Pairs generally rear 2 broods/yr; approximately 7 d separate fledging of young from the first nest and laying of the first egg of a second clutch (Howell 1942). More information needed.

Clutch size

Clutch size 3–4 eggs, rarely 5; if > 5, likely > 1 female laying in the nest; clutches laid later in the season are likely to have fewer eggs (Howell 1942). Average clutch size in e. Canada declines as maritime influence increases: 3.3 in New Brunswick, 3.2 in Nova Scotia, 3.0 on Prince Edward Island (Howard 1967).

In the following data, number in parentheses is number of nests. In Churchill, Manitoba, 4% of clutches had 2 eggs (1), 39% 3 eggs (9), 52% 4 eggs (12), and 4% 5 eggs (1; J. V. Briskie unpubl. data). In Ithaca, New York, 1% 1 egg (1), 4% 2 eggs (5), 51% 3 eggs (65), 43% 4 eggs (55), and 1% 5 eggs (1; Howell 1942). In Madison, Wisconsin, 1% 1 egg (1), 2% 2 eggs (3), 49% 3 eggs (71), 48% 4 eggs (70), and 1% 5 eggs (1; Young 1955). In the Pacific Northwest (Idaho, Oregon, Washington, Montana), 1% 1 egg (1), 15% 2 eggs (25), 52% 3 eggs (84), 32% 4 eggs (52) and 1% 5 eggs (1; RS). In the Sierra Nevada Mountains, California, average clutch size 3.73; most nests (73%) contained 4 eggs (30; Morton and Pereyra 2004).

Annual and lifetime reproductive success

For one exceptionally productive female followed by Young (Young 1955) for 3 yr, 21 of 30 eggs hatched, and 16 young fledged. Of 257 nests monitored in the Pacific Northwest, a combined total of 248 young fledged; mean 1.0 young/nest ± 0.1 SE (range 0–4; RS).

Analyses of data from studies by Howell (Howell 1942), Farner (Farner 1945), and Young (Young 1949, Young 1955, Young 1956) giving 3.9, 5.0, and 5.6 young produced/female/yr, respectively, have been criticized (Mayfield 1961a, Mayfield 1975a, Niles 1985) for overestimation of percent of successful nests. These authors did not calculate daily failure rates and adjust for nests found after incubation began. According to Mayfield's method, 35% of nests with eggs in a study site in Maine were estimated to produce young, and Graber et al. (Graber et al. 1971) estimated 54% for data from Illinois. For Delaware, Niles (Niles 1985) estimated 40%; 58% of nests with eggs survived to hatching and 70% of nests with young produced at least 1 fledgling. An investigation of nest success across an urban-suburban in Colorado gradient found nest success across all sites was 65%; nests on golf courses had the highest success (73.3%) while those in an urbanized business district had the lowest (42.9%; Reale and Blair 2005).

In the Niles's (Niles 1985) analysis, the percentage of nests with eggs that produced young peaked in May (26% in Apr, 50% in May, and 30% in Jun and later). The higher values in later nests are often, but not always (Yahner 1983c), associated with the tendency for early nests to be low in conifers and later nests to be higher in deciduous trees (Howell 1942, Young 1955, Knupp et al. 1977). This seasonal shift in nest-site location may be related to differences in the phenology of foliage development between coniferous and deciduous trees; early in the season, before deciduous trees are in full leaf, more cover may be provided by conifers. This switch is not seen in all areas, in Nevada and Kentucky birds will raise multiple broods in pines (Warkentin et al. 2003a, ENV).

In Maine, the mean number of young produced/successful nest was 2.5 ± 0.15 SE (n = 38 nests; Knupp et al. 1977). In southwestern British Columbia, 58.5% of 53 nests produced young with an egg-to-fledging success rate of 86.6% and a mortality of 1.2% between nestling and fledgling stages (Kemper and Taylor 1981). For 257 nests monitored in the Pacific Northwest, 138 failed, 103 were successful, and 16 had unknown fates; these data yield Mayfield estimates of a daily mortality rate of 0.05 and a probability of nest success (probability that a nest would successfully raise at least one fledgling) of 0.26 (RS). In Oregon, nest survival higher in logged versus control plots (Kroll et al. 2010). No data on lifetime reproductive success (number of female offspring surviving to breed).

Number of broods normally reared per season

Robins usually rear 2 broods per breeding season, but sometimes 3, especially in southern portions of their range. In Ithaca, New York, 15% of n = 27 pairs were estimated to raise 3 broods in a season (Howell 1942). In a hypothetical robin population of 1,000 pairs, modeled using data from Madison, Wisconsin, 147 pairs (15%) would successfully raise 3 broods, 621 (62%) 2 broods, 191 (19%) 1 brood, and 41 (4%) would fail to breed successfully (Young 1955).

Proportion of total females that rear at least one brood to nest-leaving or independence

More information needed.

Life Span and Survivorship

One banded wild bird lived 13 yr and 11 mo (Klimkiewicz et al. 1983). Using field data to model a hypothetical robin population, Young (Young 1955) estimated the rate of survival of young from fledging to 1 Nov to be only 25%. Even so, there were estimated to be fewer adults in the population than immatures by 1 Nov; ratio of adults to immatures estimated to be 93:100 by Young (Young 1955), 88:100 by Farner (Farner 1945). On the basis of 855 records of banding returns and assuming that population was stable, Farner (Farner 1945, Farner 1949) estimated that the average young T. m. migratorius surviving until 1 Nov or 1 Jan will live another 1.7 yr and that there is 52%/yr (6%/mo) mortality in all age cohorts after age of 6 mo. Thus, within 6 yr there is a nearly complete turnover in the population. Survival estimates for areas west of Michigan were somewhat lower. In a study of post-fledgling survival, Whittaker and Marzluff (Whittaker and Marzluff 2009) estimated daily survival rates of 97%; however the survival of radio-tagged birds whose fate was known was lower (28%) and predation accounted for a significant amount of mortality.

MAPS survivorship data for the American Robin can be explored here: http://www.birdpop.org/nbii/surv/survresults.asp?strRegion=aa&strSpec=amro. Data show survival probability, based on estimates of annual adult apparent survival probability from modified Cormack-Jolly-Seber mark-recapture analyses, to range between 0.46-0.54 (regions with adequate sample sizes).

Disease and Body Parasites

Diseases

Documented records of four infectious diseases causing mortality: Yersinia tuberculosis, avian pox, an unidentified protozoan, and an unknown (National Wildlife Health Center [NWHC]). One report of a malignant bone tumor in an adult robin constitutes first report of an osseous neoplasm in a wild passerine (Hartup and Steinberg 1996). In Minnesota Mycoplasma sturni (leads to conjunctivitis in starlings) was found in asymptomatic robins suggesting the species can contract the bacteria from other birds (Wellehan et al. 2001b).

The robin has been the focus of work centered on the spread of West Nile virus, introduced in the US in 1999. West Nile is transmitted by the mosquito vector Culex pipiens that preferentially feeds on robins, compared to other birds such as House Sparrows (Passer domesticus) and European Starlings (Sturnus vulgaris; Simpson et al. 2009). Other studies have shown no such relationship (Suom et al. 2010), but the majority of studies on host selection and tests of mosquito blood meals reveal that robins are a major player and a key reservoir for the disease (Molaei and Andreadis 2006b, Savage et al. 2007, Hamer et al. 2008, Molaei et al. 2007, Molaei et al. 2008, Hamer et al. 2009, Kent et al. 2009) and it is likely that the robin acts as a “super spreader” of West Nile in the summer months (Kilpatrick et al. 2006, McKenzie and Goulet 2010).

In Connecticut, robin roosting sites have higher rates of infected mosquitos; this increased amplification of the virus may increase chances of transmission to humans (Diuk-Wasser et al. 2010); however, no such relationship was found among roosts in Illinois (Benson et al. 2012). West Nile may also be harming robin populations; since its introduction, there has been a regional decline in robin populations, most notably in Massachusetts, Maryland and Minnesota (Foppa et al. 2011). A vaccine for robins lowers viral load and thus may reduce the spread of the virus to mosquitos, and thence to other robins and humans (Kilpatrick et al. 2010). The robin has also been studied as a reservoir for eastern equine encephalitis (Molaei and Andreadis 2006b, Estep et al. 2011).

Body parasites

Sixty percent of robins sampled in Chicago, IL, were infested with chewing lice; those infested tended to have lower immune response to bacteria such as E. coli (Girard et al. 2011). In addition to a regular complement of external parasites such as lice, flies, ticks and mites (Peters 1936, NWHC), most robins harbor worms. Fifty-eight of 62 robins examined in Colorado had at least 1 of 11 species of helminths (Slater 1967). Some of the common helminth parasites of robins have invertebrate intermediate hosts such as land snails, earthworms, and insects (Ching 1993). Sporozoan (e.g., coccidiosis) and protozoan (e.g., trichomoniasis) parasites have also been reported (NWHC).

Robins that forage on the ground may be more likely than non-ground-foraging passerines to pick up and spread ticks, and thus may act as a reservoir for tick-borne diseases (Giardina et al. 2000, Richter et al. 2000, Ginsberg et al. 2005, Morshed et al. 2005). In s. NY State, some individuals were heavily infested with larvae of the tick Ixodes dammini and found to carry the Lyme disease spirochete, Borrelia burgdorferi (Battaly and Fish 1993). In British Columbia, I. aurutulus were found on robins that were also positive for Lyme disease spirochetes (Scott et al. 2010b). In nw. California, of 11 captured robins none were infested with ticks harboring spirochetes, suggesting regional differences exist; more data needed (Slowik and Lane 2001).

Scaly leg disease is caused by the mite (Knemidokoptes jamaicensis); infection leads to thickening of scales on the feet, leading in some cases to loss of digits or the entire foot (Pence et al. 1999); cases have been reported in Oklahoma, Georgia, Florida and Kentucky (ENV, Pence et al. 1999). Infected robins in Kentucky were still able to move and forage; the fitness effect of the mite is unknown (ENV).

Causes of Mortality

Most reproductive failure is at the egg rather than the nestling stage (Kendeigh 1942) and is probably due to predation (see Behavior: Predation) or desertion due to disturbance or bad weather. Of 138 failed nests in the Pacific Northwest, 95 (69%) were depredated, most likely by small mammalian nest predators such as the yellow pine chipmunk (Eutamias amoenus), red squirrel, northern flying squirrel (Glaucomys sabrinus), and deer mouse (Peromyscus maniculatus; RS). In Ithaca, NY, 7% of eggs were either sterile or failed to hatch for some other reason (Howell 1942). Female Brown-headed Cowbirds remove a robin egg before laying one of their own, so even though female robins remove cowbird eggs, potential productivity is reduced.

Adult mortality is known to be caused by predation (see Behavior: Predation), poisoning (see Conservation and Management: Effects of Human Activity), and infectious diseases (see Demography and Populations: Diseases and Parasites), as well as parasites (coccidiosis, trichomoniasis, Knemidocoptes mites), trauma (e.g., attack by predator, gunshot wound), car strikes, emaciation, bill malformation, electrocution, apergillosis, and salmonellosis (NWHC). In a state-wide survey of car mortalities in Indiana, robins were the most common road-side bird carcasses (Glista and DeVault 2008).

Range

Initial dispersal from natal site

Banded young remain on breeding grounds for up to 4 mo (Young 1955). Fledged young roost with males during the remaining breeding season and eventually flock and roost with adults before molt and migration (Young 1955). Radio-tagged juveniles recently fledged in Seattle, WA, travelled on average within an area of 258 ha and preferentially used residential areas over urban and forested areas, seeking out grassy lawns with ample food (Whittaker and Marzluff 2009).

From banding returns for T. m. migratorius, Farner (Farner 1945) estimated that 70% of individuals that survived to 1 yr bred within 40 km of natal sites.

Fidelity to breeding site and winter home range

A high proportion of breeding adults return to the same site in subsequent years (e.g., 10 of 14 banded birds on Vancouver I.; Martin 1973e). Travel widely in large flocks in winter, tracking sources of food, so are unlikely to remain long at any particular site. Some populations qualify both as temperate migrant and as resident. For example, birds banded on breeding grounds from Nebraska to Manitoba to Pennsylvania have been recovered in Arkansas in winter (James and Neal 1986), but whether individuals return to the same wintering sites is doubtful.

Dispersal from breeding site

Need data on distances dispersed between breeding attempts (between years and within seasons).

Home range

Few data; needs study. Defends nest sites but may use undefended feeding grounds up to 300 m away (Knupp et al. 1977).

Population Status

Enlarge
Figure 7. Regional trends in American Robin breeding populations.

Based on data from the North American Breeding Bird Survey, 1966-2013 (Sauer et al. 2014). Data show estimates of annual population change over the range of the survey; areas of increase are shown in blue and declines are shown in red. See Sauer et al. (2014) for details.

Enlarge
Figure 8. Relative abundance of American Robin during the breeding season.

Based on data from the North American Breeding Bird Survey, 1966-2013. See Sauer et al. (2014) for details.

Enlarge
Figure 9. Early winter density of the American Robin.

Based on data from the Christams Bird Count, 2003. Numbers show the number of individuals counted per 100 party hours in each region with CBC count circles.

Numbers

See Figure 7. Recorded on Breeding Bird Surveys (BBS) between 1966 and 2011 in every state of the U.S. and every province of Canada (Sauer et al. 2012). Highest counts (robin encountered on > 200 survey routes) occurred in the Appalachian Mountains followed by the Prairie Potholes, Atlantic Northern Forest, Boreal Hardwood, Eastern Tallgrass Prairie, Northern Rockies and Great Basin. Lowest counts were in the Sierra Nevada mountains and deserts. In the U.S., California had the highest counts and Rhode Island had the lowest. In Canada Manitoba had the highest counts and Prince Edward Island the lowest (Sauer et al. 2012).

After October, may form enormous roosts; e.g., in Arkansas, several hundred thousand individuals have roosted at night in close concentration (James and Neal 1986). Winter abundances are highest in the southeast and in central California.

Trends

Population trends have been estimated from BBS data, 2001-2011 (http://www.mbr-pwrc.usgs.gov/cgi-bin/atlasa10.pl?07610&1&10). Survey-wide, robins are increasing throughout most of their range, with a slight increase in the US and a slight decrease in Canada (Figure 8). Largest increase was in the Shortgrass Prairie and Prairie Pothole regions, with the largest decrease in the Chihuahuan Desert and Sierra Nevada Mountains. Texas and Florida saw the largest increase in the US and Saskatchewan had the largest increase in Canada. Largest decreases occurred in Oklahoma and British Colombia (Sauer et al. 2012).

Population Regulation

Few data; poorly understood. Densities and perimeter of geographic range seem to be determined by resources on breeding grounds. Productivity is so high that variation in reproduction, predation on eggs and nestlings, and fledgling survival are unlikely to be density-dependent limiting factors. Mortality rates of adults appear no higher during migration and wintering than in the breeding season (Farner 1945), but this needs confirmation.

Thus, population regulation is most probably a balance between variation in the specific resources required (wet grassy foraging sites and suitable nest sites protected from inclement weather in breeding season, roosting and foraging sites with abundant fruit during migration and winter). MacArthur (Macarthur 1972) proposed that the southern limit of the breeding range is determined by competition with other members of genus Turdus, but this is unlikely as there are no such members in central Florida or Texas -- the southern limit of breeding in the USA (see Distribution and Habitat).

Recommended Citation

Vanderhoff, Natasha, Peter Pyle, Michael A. Patten, Rex Sallabanks and Frances C. James.(2016).American Robin (Turdus migratorius), The Birds of North America (P. G. Rodewald, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America: https://birdsna.org/Species-Account/bna/species/amerob

DOI: 10.2173/bna.462