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A Mixed Brood Following Usurpation of a Carolina Chickadee Nest by Tree Swallows
Wynn F. Haslam, Richard A. Rowe, and J. Luke Phillips

Southeastern Naturalist, Volume 15, Issue 4 (2016): N44–N49

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2016 Southeastern Naturalist Notes Vol. 15, No. 4 N44 W.F. Haslam, R.A. Rowe, and J.L. Phillips A Mixed Brood Following Usurpation of a Carolina Chickadee Nest by Tree Swallows Wynn F. Haslam1, Richard A. Rowe1,*, and J. Luke Phillips1 Abstract - Naturally occurring mixed species broods are uncommon but can occur due to nest parasitism, and in rare cases, due to usurpation. We report on a mixed brood resulting from a pair of Tree Swallows usurping a Carolina Chickadee nest. The chickadee nest was constructed in a nest box, and 1 egg was laid prior to usurpation. This egg, in addition to the clutch of swallow eggs, was incubated, hatched, and fed by the adult swallows. The chickadee nestling grew and appeared to be healthy at 6–7 days of age, but was approximately 50% smaller than its nest mates at that time. The chickadee died after 8 days, most likely due to starvation. The remaining Tree Swallow nestlings were reared to fledging by the adults. Introduction. The presence of eggs or nestlings of different bird species in the same nest is unusual. Typically, mixed broods are the result of obligate brood parasitism (Payne 1977). Facultative brood parasitism can occur as a result of limited nesting resources (Barrientos et al. 2015, Petrassi et al. 1998) and, in rare cases, with species not associated with nest parasitism (Neal and Rolland 2015, Peer 2010; see Yom-Tov 2001 for a review). Naturally occurring, but accidental, mixed nests due to usurpations have been reported in several species (Austin et al. 2009, Barrientos et al. 2015, Dolenec 2002, Golawski 2007, Petrassi et al. 1998, Samplonius and Both 2013, Suzuki and Tsuchiya 2010). We provide the first report of a naturally occurring mixed brood of a Poecile carolinensis (Audubon) (Carolina Chickadee; hereafter Chickadee) and Tachycineta bicolor (Vieillot) (Tree Swallow; hereafter Swallow) due to usurpation of a Chickadee nest by Swallows. Field Site. Observations were made during April and May of 2016 at the Virginia Military Institute Biology Department’s Field Research Site (37°46'45.09"N, 79°23'30.51"W) located approximately 4 km east of Lexington in west-central VA. The site is a 9-ha hayfield with 30 nest boxes (14 cm x 14 cm x 25 cm, opening 16.5 cm above floor) placed in a grid to attract Swallows. All nest boxes were mounted on poles with antipredator baffles (stove pipe, 20.5 cm x 61 cm). Even though our focus was on Swallow nests, we monitored all nest boxes during the breeding season. Nest status (construction, incubation of eggs, and rearing of young) was checked twice a week, except daily during the egg-laying phase and near expected time of hatching to establish onset of incubation and the nestling phase. The nest-checking protocol used in this study was reviewed by the Animal Subjects Committee at the Virginia Military Institute (VMI ASC permit number 2013-02). None of the birds in this study were marked. Observations. A routine check of nest boxes on 12 April 2016 found no bird activity at nest box 25 (NB 25). On 16 April, we discovered a nearly complete Chickadee nest in NB 25, but a pair of Swallows actively defended the nest box (alarm calls and swooping). We checked the nest box on 19 April, at which time the Chickadee nest contained mammal fur, and again, the Swallows were showing defense behavior. During our nest box check on 21 April, we found the beginnings of a Swallow nest being built on top of the Chickadee nest. At this point, we assumed that the Swallows had usurped the nest and displaced the 1Department of Biology, Virginia Military Institute, Lexington, VA 24450. *Corresponding author - Manuscript Editor: Frank Moore Notes of the Southeastern Naturalist, Issue 15/4, 2016 N45 2016 Southeastern Naturalist Notes Vol. 15, No. 4 W.F. Haslam, R.A. Rowe, and J.L. Phillips Chickadees. Two days later (23 April), we observed a Chickadee exiting the nest box even though the Swallows actively defended it when we approached. The Swallows had added some additional grass to the nest box, creating a small but incomplete nest cup on top of the Chickadee nest. This Swallow nest was not typical; it was not the large, grass nest with a distinct cup that was seen in other nest boxes at our field site. On 26 April, 1 Chickadee egg was found in the nest cup. On 28 April, the Swallows began adding feathers to the nest, an indication that egg laying was about to commence. On 30 April, the check of NB 25 showed 1 Chickadee egg and 1 Swallow egg present. Subsequently, 1 Swallow egg was added to the nest each day, so that on 5 May the clutch contained 6 Swallow eggs and 1 Chickadee egg. We estimated that the Swallows began incubating on either 4 or 5 May. The average incubation time for Swallows at our field site was 13 days from the laying of the last egg, and we expected that Swallow eggs would begin hatching on 18 May. An inspection of the eggs on 12 May showed that 5 of the 6 Swallow eggs were fertile and that the Chickadee egg was fertile. The Chickadee egg hatched on 17 May, and 2 of the Swallow eggs hatched on 18 May. Our check of the nest on 19 May showed that there were 4 Swallow nestlings, the Chickadee nestling, and 2 remaining eggs (Fig. 1). We checked NB 25 on 22 May, at which time all 5 nestlings were present, were visibly larger, and exhibited an Figure 1. Photograph of the Tree Swallow brood with Carolina Chickadee nestling. The Chickadee is 2 days post-hatching. The Tree Swallow nestlings are 0–1 day post-hatching. (Photograph by R.A. Rowe.) 2016 Southeastern Naturalist Notes Vol. 15, No. 4 N46 W.F. Haslam, R.A. Rowe, and J.L. Phillips active gapping response, indicating that the Chickadee nestling was healthy and being fed by the Swallow adults. On 22 May, there was only 1 Swallow egg (infertile egg) in the nest. The remaining Swallow egg had hatched, and this nestling being several days younger and thus much smaller than its nest mates died. When we checked on 23 May, the Chickadee nestling was noticeably smaller than the Swallow nest mates (Fig. 2), but it appeared to be healthy and was larger than at the time of the previous nest check. We observed NB 25 on 26 May and found only 4 Swallow nestlings. The Chickadee nestling was not visible in the nest box. After the Swallow nestlings fledged on 6 June, we examined the nest box contents and found the Chickadee nestling’s carcass in the debris of the nest. While we do not know the cause of death, we suspect that it died of starvation. Discussion. Naturally occurring mixed broods that are not the result of parasitism are rare. In many cases, the heterospecific egg fails to hatch due differences in egg size, with smaller eggs having less contact with the brood patch and thus developing slower or not at all (Barrientos et al. 2015). Chickadee eggs are smaller (mean size of 14.8 mm x 11.5 mm and 1.04 g; Mostrom et al. 2002) than Swallow eggs (mean size of 18.7 cm x 13.2 cm and 1.9 g; Winkler et al. 2011). Even though the Chickadee egg was noticeably smaller and differed in color from the Swallow eggs, the adult Swallows incubated the egg rather than removing it from the nest. At hatching, the Chickadee nestling was smaller than the Swallows, but the size differential was not substantial. At 6 days old (d.o.) (Fig. 2), the Swallow nestlings (mean mass at 6 d.o. = 11 g, mean mass at 8 d.o. = 16 g; Zach and Mayoh 1982) appeared to be nearly twice the size of the Chickadee nestling (similar data are not available for Carolina Chickadees, but using data from the similar-sized Poecile rufescens (Townsend) [Chestnut-backed Chickadee], the nestling likely weighed about 5 g at 6 d.o.; Gaddis and Corkran 2008). By 6 d.o., Swallow nestlings would be equal to or greater in body mass than an adult Chickadee (9.8–11.0 g; Mostrom et Figure 2. Photograph of the Tree Swallow brood with the Carolina Chickadee nestling. The Chickadee nestling is 7 days post-hatching. The 4 Swallow nestlings are 5–6 days post-hatching. (Photograph by R.A. Rowe.) N47 2016 Southeastern Naturalist Notes Vol. 15, No. 4 W.F. Haslam, R.A. Rowe, and J.L. Phillips al. 2002). Both Zach and Mayoh (1982) and Austin et al. (2009) show that Swallows are in the rapid growth phase at these ages, and approach adult weight (20–21 g; Zach and Mayoh 1982) at 13 days of age. We suspect that between nest checks on 23 May and 26 May, the Chickadee nestling was at such a size disadvantage that either it could not compete for food and died from starvation or it was crushed by the larger Swallow nestlings. In a situation similar to our study, Austin et al. (2009) report on the usurpation of a Troglodytes aedon (Vieillot) (House Wren) nest by Tree Swallows, and the subsequent incubation and rearing of the nestlings. Two House Wren nestlings hatched and survived for 6 and 13 days, respectively, until they died of apparent starvation. Austin et al. (2009) regularly weighed the nestlings and noted that the House Wren growth rates were below normal. Adult House Wrens (10–12 g; Austin et al. 2009) are the same size as Chickadees. The House Wrens showed initial growth and development, but as the House Wrens and Swallows entered the rapid growth phase, the differences in body size led to a decrease in ability to compete for food and ultimately death due to starvation. Some mixed broods do survive to fledging. Suzuki and Tsuchiya (2010) report the rearing and fledging of a mixed brood (1 Parus major (L.) [Great Tit] and 5 Poecile varius (Temminck & Schlegel) [Varied Tit]) that occurred as a result of usurpation by the Varied Tits. A pair of Sitta europaea (L.) (Nuthatch) usurped a Great Tit nest, incubated the single egg in the nest, and reared the Great Tit to fledging (Dolenec 2002). Uniquely, Samplonius and Both (2013) report on a mixed brood that contained 3 species (Ficedula hypoleuca (Pallas) [Pied Flycatcher], Cyanistes caeruleus (L.) [Blue Tit], and Great Tits) that resulted from 2 usurpation events. The Great Tits successfully reared the nest to fledging. Petrassi et al. (1998) report on several instances of Great Tits usurping Blue Tit nests and rearing the mixed broods to fledging. Robinson et al. (2005) report on nest sharing by Sitta canadensis (L.) (Red-breasted Nuthatch) and Poecile gambeli (Ridgway) (Mountain Chickadee) that resulted in fledging of nestlings from both species. This mixed nest did not appear to be the result of usurpation because both species were observed feeding the mixed brood. In these cases, the mixed broods were from similar species or similarly sized birds and as a result competition between nestlings did not result in starvation of the smaller nestling. Also, it is likely that food choice and provisioning by the adults met the dietary needs of the nestlings. Secondary cavity nesters such as Tree Swallows, Carolina Chickadees, and Sialia sialis (L.) (Eastern Bluebird) can be limited by the availability of suitable nesting sites (Gowaty and Plissner 2015, Holroyd 1975, Mostrom et al. 2002). The presence of artificial cavities (nest boxes) in areas where natural cavities are limited creates a situation in which competition for nest boxes can lead to usurpation (Barientos et al. 2015, Petrassi et al. 1998). We are confident that the presence of the Chickadee egg in the nest was due to usurpation of the Chickadee nest by the Swallows and not the result of nest parasitism by a female Chickadee because the Chickadee nest was constructed prior to construction of the Swallow nest. An adjacent nest box (NB 26) that was 62 m away contained a Chickadee nest that had completed a clutch of 5 eggs on 21 April, 5 days prior to the egg appearing in NB 25. It is unlikely that the female from NB 26 produced an egg 5 days into incubation and parasitized another Chickadee nest. At the time of the usurpation event (21 April), 67% of the nest boxes in the grid were occupied (11 by Swallows, 6 by Eastern Bluebirds, 2 by Chickadees and 1 by a Baeolophus bicolor (L.) [Tufted Titmouse]) and 33% of the nest boxes were open. By 23 April, 74% of the nest boxes were claimed. The usurping pair of Swallows consisted of a male (after-hatch-year plumage) and, a second-year female (ascertained by plumage characteristics; Hussell 1983). It is unclear to us why this pair chose to usurp the Chickadee nest rather than select an unoccupied nest box. This pair may have been less aggressive or experienced and could not compete with other Swallows for nest boxes. 2016 Southeastern Naturalist Notes Vol. 15, No. 4 N48 W.F. Haslam, R.A. Rowe, and J.L. Phillips The mixed brood described in this report appears to have been the result of nest usurpation and subsequent incubation and rearing of the heterospecific nestling. While the Chickadee nestling was fed by the host Swallows, it is likely that a combination of differences in body size between the Chickadee and Swallow nestlings, prey types presented by the adults, and feeding rates resulted in death of the Chickadee nestling. Success of a mixed brood is dependent on similarity between the species in development rates, body sizes, and food requirements. Our study and that of Austin et al. (2009) are the only 2 reports of Tree Swallows rearing heterospecific nestlings. In both of these cases, the heterospecific nestlings were from smaller species and died prior to fledging. Acknowledgments. We would like to thank Drs. Anne Alerding and Paul Moosman and 2 anonymous reviewers for their comments on this manuscript. This study was funded in part through the VMI Biology Department’s Carroll Fund, VMI’s Jackson-Hope Fund, VMI SURI funding, and a grant from the North American Bluebird Society. Literature Cited Austin, S.H., T.R. Robinson, W.D. 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