Shooting and trapping
American Robins were considered a delicacy and worthy of sport from colonial times through the mid-1800s; large numbers were shot and trapped for sale at that time (Sharp 1990).
Pesticides and other contaminants/toxics
Common in both agricultural and suburban areas, American Robins are useful as an indicator of chemical pollution. After DDT was used to control Dutch elm disease on the Michigan State University campus in the mid-1950s (Beaver 1980b), large die-offs of robins were reported in the press and alerted scientists to the long-term effects of chlorinated hydrocarbons in the environment. DDT is known to cause mortality and reduced nest success in American Robins (Mehner and Wallace 1959, Hickey and Hunt 1960, Hickey 1961, Twiest 1965, Wurster et al. 1965, Hunt 1969, Hunt and Sacho 1969, Weller 1971b, Willson 1978; but see Johnson et al. 1976b).
Breeding-season loss of fat reserves in male robins appears to increase susceptibility to DDT poisoning; fatter females show greater tolerance (Hunt 1969). During 1990 and 1991, eggs from orchards in British Columbia were found to contain high levels of both DDE (up to 103 mg/kg) and DDT (up to 26 mg/kg), levels that were significantly higher in orchard habitats than in non-orchard habitats (Elliott et al. 1994). However, there was no difference in reproductive success between robins nesting in conventional orchards and those nesting in organic orchards; no evidence to suggest that reproductive success was affected by organochlorines (Elliott et al. 1994). Another study in British Columbia showed that robins had higher levels of organochlorines in orchard vs. non-orchard habitat (48.6 mg/kg versus 1.10 mg/kg), and similar nest survival rates between habitats (96.7%; Gill et al. 2003). In contrast, during 1990 and 1991 in Pennsylvania, repeated applications of organophosphorus, carbamate, and organochlorine pesticides in conventional orchards reduced reproductive success compared with pairs nesting in organic orchards (Fluetsch and Sparling 1994).
Robin populations appeared to take 10–17 yr to return to pre-DDT levels following spraying (Willson 1978, Beaver 1980b; but see Hunt and Sacho 1969). Robins are poisoned primarily because of their earthworm diet (earthworms are resistant to DDT and concentrate it). Residues in earthworms reach up to 5 times the levels found in associated soils (Edwards et al. 1983). DDT residues persist in soils for long periods (>10 yr) following application and may pass from soils to earthworms to robins for as long as 30 yr after a single application (Dimond et al. 1970).
DDT is transferred up the food chain to robins from earthworms living in contaminated soils of orchards in Canada (Harris et al. 2000c). Robin eggs in the Okanagan Valley, British Columbia in the 1990s contained up to 103 mg/kg DDE and 26 mg/kg DDT, even though these pesticides had not been used since the 1970s (Elliott et al. 1994). Use of methoxychlor instead of DDT was reported to reduce mortality in robins (Hunt and Sacho 1969, Weller 1971b).
Although robins in urban areas were found to contain more lead (from automobile emissions) in feathers, guts, livers, lungs, kidneys, and bones than robins in rural areas, amounts of lead were lower than levels considered to have toxic effects (Getz et al. 1977). Testing in New Jersey found higher concentrations of several metals, including lead and copper, in the feathers of robins at a more polluted ex-urban site compared to a non-polluted forest site (Hofer et al. 2010). Robins carried lower metal levels than House Wrens (Troglodytes aedon), a fact that may be partially attributed to the varied diet and wide-ranging foraging pattern of robins (Hofer et al. 2010). In the mining districts of Oklahoma, Kansas, and Missouri, heavy metal exposure (lead and cadmium) from mining operations was at levels high enough to impair biological functions (Beyer et al. 2004). Despite high selenium levels on mining sites in Idaho, however, no differences were found in clutch size, fledgling success, egg weight and neonate weight between mine and control sites, and in fact overall robin nesting success was higher on mine sites than control sites (Ratti et al. 2006).
As recently as 1972, there were major poisoning events of robins due to pesticides; an estimated 10,000 birds were killed by Azodrin in a field near Homestead, Florida (Fisk 1976). Other chemicals known to cause mortality include carbofuran, hydrogen sulphide, furadan, famphur, nemacur, cyanide, chlorpyrifos, dursban, fenthion, chlordane, and carbamate and organophosphate pesticides (National Wildlife Health Center). In Québec, treatment of lawns with insecticides (chlorpyrifos) was found to be negatively correlated with nest productivity, possibly because of reduced earthworm numbers in treated lawns (Décarie et al. 1993), but no cases of adult or juvenile mortality were recorded. Use of diazinon and acephate on ornamental trees was reported to not be harmful to robins despite lower plasma cholinesterase activity in females exposed to these chemicals (Décarie et al. 1993); similar results also found by Brehmer and Anderson (Brehmer and Anderson 1992) for nesting robins in Wisconsin. For two robins in Alleghany County, Virginia, however, the cause of death was diagnosed as poisoning by diazinon (National Wildlife Health Center). Despite no adult mortality as a result of exposure to diazinon in southeastern Québec, robin eggs appeared sensitive to this organophosphorus insecticide, especially when spraying was conducted early in incubation period (Rondeau and Desgranges 1995).
Diazinon residues found in 76% (n = 17) and 89% (n = 18) of American Robin carcasses collected in orchards in Pennsylvania and Washington, respectively, and at both sites earthworms had extremely high concentrations of the pesticide (Cobb et al. 2000). Robin abundance was negatively correlated with granular insecticide use in Canadian prairies (Mineau et al. 2005). In 1998 a major die-off of robins and other birds at a large roost in Scotch Plains, New Jersey was attributed to Chlordane that was transmitted via the soil of suburban areas and golf courses to Japanese beetle (Popillia japonica) and Oriental beetle (Anomala orientalis) and then to birds (Stansley et al. 2001).
The effect of polychlorinated biphenyls (PCBs) on productivity was examined in breeding robin in Massachusetts, and although PCB levels were two times higher than in reference specimens there was no effect on clutch size, number of nestlings fledged, or overall nest success (Henning et al. 2003).
Ingestion of plastics, lead, etc.
Collisions with stationary/moving structures or objects
Robins are often hit by vehicles when flying across roads. Robins are also susceptible to flying into glass windows, especially when flocks feed in vegetation near buildings.
Degradation of habitat: breeding and overwintering
Deforestation, urbanization, and agricultural intensification have generally created, rather than degraded, breeding habitat for the robin. Robins often more abundant at developed sites compared to natural sites. In Michigan, for example, robins were more abundant on one side of a road where housing developments had recently been established compared to the other side that was not developed (Ford and Flaspohler 2010). Although this was only a year-long study, it highlights the fact that differences can be seen at very fine scales and that robins will quickly take advantage of human alterations to the habitat.
New breeding habitat may be leading to an increase in robin populations in some areas. In remnant forests in Ohio nest survival was not impacted by surrounding urban matrix and daily nest survival rates from 2001 to 2011 ranged from 82.6% to 96.9% (Rodewald et al. 2013). However, several studies noted increased nest predation and nest failure with increasing urbanization (See Behavior: Predation).
Disturbance at nest and direct human/research impacts
Research activity (monitoring nests and handling eggs and nestlings) was not found to interfere with nest success (Ortega et al. 1997). Apparently individuals do not try to remove radio transmitters, nor do they have strong enough bills to remove them; cyanoacrylate adhesive used to secure transmitters to birds is believed to be safe and radio-marked birds appear to behave normally (Johnson et al. 1991e). Robins nesting near buildings or areas with high human traffic alarm-call less than those nesting at sites more isolated from buildings and human activity, indicating that individuals may habituate to humans that do not pose any threat (ENV).