N31 2015 Southeastern Naturalist Notes Vol. 14, No. 2 J.A. Fowler Neal and V. Rolland A Potential Case of Brood Parasitism by Eastern Bluebirds on House Sparrows Jessica A. Fowler Neal1 and Virginie Rolland1,* Abstract - Sialia sialis (Eastern Bluebird) and Passer domesticus (House Sparrow) are common cavity-nesting birds that can compete with each other for nest sites in habitats where they overlap. Both species occasionally lay eggs in the nests of conspecifics, but evidence of interspecific brood parasitism (IBP) is lacking. Here we report the first detailed case of an Eastern Bluebird laying an egg in a House Sparrow nest and discuss potential explanations, including intentional IBP, IBP by default, and failed nest usurpation. Introduction. Some female birds lay 1 or more of their eggs in the nest of another female (Payne 1977). This phenomenon, known as brood parasitism, is a reproductive strategy that females adopt to increase their reproductive output by laying more eggs than would constitute a typical clutch size, thus producing more offspring than they would otherwise be able to raise (Robert and Sorci 2001). Brood parasitism has 2 forms: interspecific brood parasitism (IBP) in which eggs are laid in a nest of another species, and conspecific brood parasitism (CBP) where eggs are laid in a nest of the same species. Although a few species are obligate interspecific brood parasites, e.g., Molothrus ater (Bodaert) (Brown-headed Cowbird) and Cuculus canorus L. (Eurasian Cuckoo), CBP has been reported in at least 236 avian species (Yom-Tov 2001), including in Sialia sialis L. (Eastern Bluebird, hereafter Bluebird) (Gowaty and Bridges 1991, Meek et al. 1994) and Passer domesticus L. (House Sparrow; hereafter Sparrow) (Kendra et al. 1988). Neither Bluebirds nor Sparrows have been known to practice IBP. An anecdotal report suggested IBP by Bluebirds on Sparrows (Gowaty and Plissner 1998) but observations and circumstances were not documented. We report here the details of a case of functional brood parasitism by Bluebirds on Sparrows and discuss the potential causes of this occurrence. Field-site description and methods. We detected this case of IBP in 2013 while conducting a demographic study of Bluebirds and other cavity-nesting species in a nest-box trail located along CR 766, about 10 km north of Jonesboro, AR. The 178 nest boxes on our trail were established in 2003. Most boxes were placed at ~100-m intervals along roadsides, fences, tree lines, or forest edges, and faced yards, hay fields, or pastures for an approximate density of 20–21 nest boxes per km2. Throughout our study, we routinely inspected all nest boxes every 2–5 days from mid- March through late August (nesting period of both Bluebirds and Sparrows) and recorded nest contents. In April 2012 and 2013, Bluebirds occupied 62–64% of the monitored nest boxes; 3–5% of nest boxes were used by Poecile carolinensis (Audubon) (Carolina Chickadee), Baeolophus bicolor L. (Tufted Titmouse), Thryothorus ludovicianus (Latham) (Carolina Wren), and Glaucomys volans L. (Flying Squirrel). Sparrows attempted to nest in 8% of the nest boxes (usually closer to houses) throughout each season. Active competition between Sparrows and Bluebirds sometimes resulted in Bluebird fatalities, including in the focal nest box in February 2012. The focal nest box (35°56'4''N, 90°41'7''W), was located 1 Department of Biological Sciences, Arkansas State University, PO Box 599, State University, AR 72467. *Corresponding author - Jessica.A.Fowler.Neal@gmail.com. Manuscript Editor: Karl E. Miller Notes of the Southeastern Naturalist, Issue 14/2, 2015 2015 Southeastern Naturalist Notes Vol. 14, No. 2 N32 J.A. Fowler Neal and V. Rolland in a mowed yard ~50 m from a house, 100 m from a forested area, and 2 m from a hedgerow of mixed shrubs. Sparrows consistently occupied this box and the nearest 2 boxes with no observed successful Bluebird attempt since 2009. We typically controlled the spread of Sparrows into the site by removing their nests, eggs, and occasionally adult Sparrows from nest boxes. Observations. From 15 March to 11 April, Sparrow nesting attempts were removed every 2–3 d from the focal and nearest 2 nest boxes. The next 4 nest boxes on the road (650–900 m from the focal box) were occupied by Bluebirds, Tufted Titmice, and Carolina Chickadees starting 11 April. On 13 April, we found a new Sparrow nest in the focal nest box, but allowed the nesting attempt to continue so that we could trap the adults at a later date. On 15 April, we found 2 Sparrow eggs that we left in place (Fig. 1A), but we were unsuccessful in trapping the adults inside the nest box. When we returned on 19 April to attempt to catch the adults again, the nest contained 5 Sparrow eggs and a single Bluebird egg (Fig. 1B). A pair of unbanded Bluebirds was perched on a power line about 20 m away from the nest box, but they did not attempt to drive away the Sparrows when they entered the nest box. On 21 April, we found that all of the eggs were warm, including the Bluebird egg (Fig. 1C). We checked the nest every 2 d, and the first Sparrow egg hatched on 29 April (Fig. 1D). On 10 May, we collected the unhatched but intact Bluebird egg and euthanized 5 well-developed Sparrow nestlings. We opened the Bluebird egg and found no signs of development. We did not observe adult Bluebirds near this particular nest box again until an unbanded pair initiated a nest there in late May. Discussion. This report is the first to document a case of functional IBP by Eastern Bluebirds, providing more insight into this rare phenomenon reported previously only anecdotally. We believe this apparent IBP was likely a misdirected case of CBP, i.e., IBP by default. CBP occurs in ~10% of Bluebird nests (Meek et al. 1994), most commonly in response to limited nest availability or nest depredation during the laying period (Lyon and Eadie 2008). However, the nearest (810 m) known Bluebird nest was already incubating a clutch of 5 eggs when the focal nest box was parasitized. This distance and the occupancy of the 3 nearest nest boxes by other bird species likely did not provide an opportunity for CBP and may have led the Bluebird female to parasitize the focal Sparrow nest. Although our Figure 1. Contents of a House Sparrow nest parasitized by an Eastern Bluebird in 2013. (A) 15 April: two House Sparrow eggs; (B) 19 April and (C) 21 April: five House Sparrow eggs and one Eastern Bluebird egg; (D) 29 April: first House Sparrow egg to hatch. N33 2015 Southeastern Naturalist Notes Vol. 14, No. 2 J.A. Fowler Neal and V. Rolland observation could have been a case of intentional IBP, it seems unlikely given the recognizable difference in nest shape and material between Sparrow and Bluebird nests. Finally, we cannot rule out the possibility of a failed attempt at nest usurpation by Bluebirds, but that also seems unlikely given the lack of Bluebird nest material in the focal box and the Sparrow’s persistence in rebuilding its nest after we repeatedly removed nesting material over the course of a month. Also of note from this episode is that the Sparrows did not appear to reject the Bluebird egg. It remained intact throughout the entire nesting period with no puncture marks or obvious scratches, nor was it pushed to the side or buried in the nest material. Although Sparrows reject roughly 30% of conspecific eggs, mainly based on egg-spot patterns rather than color (Lopez de Hierro and Moreno-Rueda 2010), they may not have evolved strategies to recognize and reject eggs of other species (Moksnes et al. 1991), such as the unpatterned Bluebird egg. In conclusion, although Bluebirds are well studied, functional IBP remains a poorly understood aspect of their biology. Compared to previous anecdotal observations, our report provides more details about the circumstances of this probable case of IBP by Bluebirds but the data are still insufficient to conclude whether what we observed was true IBP, IBP by default, or failed nest usurpation. Contributions from citizen scientists represent a valuable opportunity to increase detection and understanding of rare phenomena (Cooper et al. 2012) such as IBP by Bluebirds. We encourage individuals who monitor Bluebird nest boxes to report their observations to programs such as NestWatch (nestwatch.org). Acknowledgments. We thank M. Bobowski, B. Cansler, and E. Mizell for their precious assistance in the field as well as T. Boves and anonymous reviewers for helpful comments. Our research was funded by the College of Science and Mathematics of Arkansas State University (A-State). Our bird-handling protocol was approved by the A-State Institutional Animal Care and Use Committee. Literature Cited Cooper, C.B., W.M. Hochachka, and A.A. Dhondt. 2012. The opportunities and challenges of citizen science as a tool for ecological research. Pp. 99–113, In J.L. Dickinson and R. Bonney (Eds.). Citizen Science: Public Participation in Environmental Research. Cornell University Press, Ithaca, NY. Gowaty, P.A., and W.C. Bridges. 1991. Nestbox availability affects extra-pair fertilizations and conspecific nest parasitism in Eastern Bluebirds, Sialia sialis. Animal Behavior 41:661–675. Gowaty, P.A., and J.H. Plissner. 1998. Eastern Bluebird (Sialia sialis). Number 381, In A. Poole (Ed.). 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