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American Robin (Turdus migratorius) Feeds a Nestling Carcass to Nestlings in an Urban Environment
Dustin E. Brewer and Adam M. Fudickar

Urban Naturalist, No. 22 (2019)

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Urban Naturalist 1 D.E. Brewer and A.M. Fudickar 22001199 URBAN NATURALIST No. 2N2o:1. –262 American Robin (Turdus migratorius) Feeds a Nestling Carcass to Nestlings in an Urban Environment Dustin E. Brewer1,* and Adam M. Fudickar1 Abstract - Turdus migratorius (American Robin) is a songbird that commonly eats fruit and invertebrates as an adult and invertebrates as a nestling. This species has only rarely been observed eating vertebrates, or feeding them to nestlings. We observed the first known occurrence of an American Robin feeding a nestling carcass to nestlings. It is unclear if the bird we observed depredated a nest or scavenged a nestling. The observation occurred on the campus of Indiana University in Bloomington, IN, USA. It is currently unknown if urbanization affects the prevalence of this behavior. Studies of the diet and/or foraging ecology of an organism can provide insight into life-history traits and environmental pressures for many taxa (e.g., in birds [Robinson and Holmes 1982], fish [Labropoulou and Eleftheriou 1997], and mammals [Novack et al. 2005]). Turdus migratorius L. (American Robin) is a songbird that occurs in both urban and rural areas throughout much of North America. Here, we report the first known observation of an American Robin feeding a nestling carcass to its nestlings. This event occurred in an urban environment. Gut contents of over 6000 American Robins indicate that adults of this species primarily eat fruit in fall and winter months, a mix of fruit and invertebrates in summer months, and primarily invertebrates in the spring (Wheelwright 1986). Post-fledging American Robins may on rare occasions eat vertebrates, as indicated by several reports, including predatory interactions with fish (Bayer 1980), reptiles (Davis 1969, Netting 1969, Vanderhoff 2007), amphibians (Leighton 2006, Thompson and Waterstrat 2016), and mammals (Penny and Knapton 1977). Several of the above reports occurred during the American Robin breeding season (April to August), and so could be correlated with the presence of nestlings. American Robins generally feed nestlings invertebrates such as Lepidoptera larvae, earthworms, and beetles (Howell 1942). Reports have confirmed that vertebrates, including a Thamnophis elegans vagrans (Baird and Girard) (Wandering Garter Snake; Richmond 1975) and a Sorex sp. (shrew; Powers 1973), have been fed by American Robins to nestlings. However, there are no reports of which we are aware describing an American Robin feeding a bird to nestlings, nor of any species in the family Turdidae attempting to consume another bird. On 30 April 2018, at approximately 2:40 PM EDT, a female American Robin (based on light head, drab breast) was observed feeding 2 feathered nestlings that were no more than than 6 days old (both fledged 10 days later, on 10 May; nestlings 1Environmental Resilience Institute, Indiana University, Bloomington, IN 47405. *Corresponding author - Manuscript Editor: David Krauss Urban Naturalist D.E. Brewer and A.M. Fudickar 2019 No. 22 2 fledge after 9 to 16 days [Howell 1942]). The nest was ~1.1 m above the ground in an ornamental Crataegus sp. (hawthorn) on the campus of Indiana University in Bloomington, IN, USA (Fig. 1). After the presumed female departed, a male Figure 1. American Robin adult with two nestlings that are ingesting the carcass of a nestling bird provided by the adult. Urban Naturalist 3 D.E. Brewer and A.M. Fudickar 2019 No. 22 (based on dark head, solid-orange breast) arrived and began to feed the nestlings the carcass of another nestling bird (Fig. 1). At least 10 seconds elapsed before the adult successfully transferred the carcass. One nestling attempted to ingest the head of the carcass while the other simultaneously attempted to ingest a leg. We retrieved the carcass from the mouths of the nestlings. The carcass (Fig. 2), which we were not able to identify to species, was mostly featherless and appeared to be recently deceased as substantial necrosis had not begun. We recorded the following Figure 2. Four views of the nestling carcass fed to the American Robin chicks. Urban Naturalist D.E. Brewer and A.M. Fudickar 2019 No. 22 4 measurements of the carcass: tarsus length = 9.5 mm, mass = 3.2 g, and culmen length = 6.1 mm. Some abdominal and hindquarter tissues and the left leg were missing so the recorded weight was an underestimate of the bird’s true size. We do not know if the American Robin that brought in the nestling carcass depredated or scavenged it. Nor do we know if our observation was an isolated incident, as we did not observe other feeding events at this nest. The species and age of the carcass are also unknown, though the lack of any visible feathers (Fig. 2) indicates an age of less than 5 days. A coarse, visual estimate that 20% of the carcass is missing (Fig. 2) would suggest that the nestling when alive had a mass of ~4.0 g, which is nearly within the range of masses (4.1–7.7) that Howell (1942) reported for American Robin hatchlings. Desiccation can result in a decrease in mass of more than 5% one hour after death (Clark 1979), meaning that the mass of the carcass could have been within the documented range for day-old American Robins. Melospiza melodia (Wilson) (Song Sparrow) is another species that we observed near the nest which may, as a day-old nestling, possess a mass and tarsus length similar to that of the carcass (Sogge et al. 1991). Species similar to the size of Song Sparrows that were also seen within 500 m of the nest include Haemorhous mexicanus (Statius Müller) (House Finch) and Passer domesticus (L.) (House Sparrow). Regardless of the species of the carcass, it is still of interest to speculate on how the provisioning American Robin may have collected it. We doubt that the carcass was from the same nest as the one to which it was brought, because the nestlings being fed were feathered and the carcass was not (Fig. 1). The American Robin may have found the nestling, alive or dead, on the ground under/near a nest from which it fell, or in a nest. Since American Robins forage both on the ground and in vegetation (Paszkowski 1982), all of these possibilities are plausible. Previous observations (see above) demonstrate that American Robins are capable of killing small prey. Infanticide is a phenomenon that occurs in many taxonomic groups (Hrdy 1979). Our observation could be consistent with any or all of the following hypotheses reviewed by Hrdy (1979): the resource competition hypothesis (e.g., infanticide decreases competition for food), the sexual selection hypothesis (e.g., infanticide induces fertility in a potential mate), and the exploitation hypothesis (infanticide provides a direct food source). There are reports of infanticide in many bird species, including Troglodytes aedon musculus Naumann (Southern House Wren), which depredate conspecific and heterospecific nests (Kattan 2016). Given that infanticidal behavior has not yet been reported in the family Turdidae, the most plausible explanation for our observation is that the provisioning bird found an already dead nestling and foraged opportunistically. Wildlife, including mammals, birds, and herpetofauna, encounter unique selection pressures in urban environments, and sometimes consequently exhibit abnormal behavior (Ditchkoff et al. 2006). At the very least, our observation provides further evidence that American Robins forage opportunistically, especially when feeding nestlings. It is currently unknown if American Robins in urban environments Urban Naturalist 5 D.E. Brewer and A.M. Fudickar 2019 No. 22 are more likely to engage in the provisioning behavior that we observed than are conspecifics in non-urban environments. Future studies could determine if our observation is a rare occurrence, or just a rarely observed occurrence, in urban and/or non-urban environments. Acknowledgments This work was supported by the Environmental Resilience Institute, funded by Indiana University’s Prepared for Environmental Change Grand Challenge initiative. We would like to thank the editor of this manuscript, as well as 2 anonymous reviewers, who provided useful comments. Literature Cited Bayer, R.D. 1980. Novel use of an unusual food: American Robins eating parts of fish. Journal of Field Ornithology 51:74–75. Clark, G.A. 1979. Body weights of birds: A review. Condor 81:193–202. Davis, W.F. 1969. Robin kills snake. The Wilson Bulletin 81:470–471. Ditchkoff, S.S., S.T. Saalfeld, and C.J. Gibson 2006. Animal behavior in urban ecosystems: Modifications due to human-induced stress. Urban Ecosystems 9:5– 12. Howell, J.C. 1942. Notes on the nesting habits of the American Robin (Turdus migratorius L.). American Midland Naturalist 28:529–603. Hrdy, S.B. 1979. Infanticide among animals: A review, classification, and examination of the implications for the reproductive strategies of females. Ethology and Sociobiology 1:13–40. Kattan, G.H. 2016. Heterospecific infanticidal behavior by Southern House Wrens (Troglodytes aedon musculus) suggests nest-site competition. The Wilson Journal of Ornithology 128:899–903. Labropoulou, M., and A. Eleftheriou 1997. The foraging ecology of two pairs of congeneric demersal fish species: Importance of morphological characteristics in prey selection. Journal of Fish Biology 50:324–340. Leighton, D. 2006. Predation of a Columbia Spotted Frog by an American Robin. Wildlife Afield 3:134–135. Netting, M.G. 1969. Does the Robin eat DeKay’s Snake? The Wilson Bulletin 81:471. Novack, A.J., M.B. Main, M.E. Sunquist, and R.F. Labisky. 2005. Foraging ecology of Jaguar (Panthera onca) and Puma (Puma concolor) in hunted and non-hunted sites within the Maya Biosphere Reserve, Guatemala. Journal of Zoology 267:1 67–178. Paszkowski, C.A. 1982. Vegetation, ground, and frugivorous foraging of the American Robin. The Auk 99:701–709. Penny, C., and R.W. Knapton. Record of an American Robin killing a shrew. Canadian Field-Naturalist 91:393. Powers, L.R. 1973. Record of a Robin feeding shrews to its nestlings. The Condor 75:248–248. Richmond, M. 1975. American Robin feeds Garter Snake to its nestlings. The Wilson Bulletin 87:552. Robinson, S.K., and R.T. Holmes. 1982. Foraging behavior of forest birds: The relationships among search tactics, diet, and habitat structure. Ecolog y 63:1918–1931. Sogge, M.K., M.D. Kern, R. Kern, and C. van Riper III. 1991. Growth and development of thermoregulation in nestling San Miguel Island Song Sparrows. The Condor 93:773–776. Urban Naturalist D.E. Brewer and A.M. Fudickar 2019 No. 22 6 Thompson, C.E., and F.T. Waterstrat. 2016. American Robin (Turdus migratorius) predation of a Plethodontid salamander on the Olympic Peninsula, Washington. Northwestern Naturalist 97:257–259. Vanderhoff, E.N. 2007. Predator–prey interaction between an American Robin, Turdus migratorius, and a Five-lined Skink, Eumeces fasciatus. The Canadian Field-Naturalist 121:216. Wheelwright, N.T. 1986. The diet of American Robins: An analysis of US Biological Survey records. The Auk 103:710–725.