Northeastern Naturalist 482 R.A. Rowe and J.L. Phillips 22001166 NORTHEASTERN NATURALIST 2V3(o4l). :2438,2 N–4o8. 94 Suspected Usurpation of Tree Swallow Nests by Eastern Bluebirds in West-central Virginia Richard A. Rowe1,* and J. Luke Phillips1 Abstract - Researchers have reported that both Tachycineta bicolor (Tree Swallow) and Tachycineta thalassina (Violet-green Swallow) are more aggressive than Sialia sialis (Eastern Bluebird) and often outcompete bluebirds for nest sites. Therefore, we would not have expected that Tree Swallows would be victims of usurpation by Eastern Bluebirds. However, as part of a study on Tree Swallows, we found that although Tree Swallows occupied 68% of the nest boxes during May, there was also evidence of nest loss in Tree Swallows due to usurpation by Eastern Bluebirds. Suspected usurpation by Eastern Bluebirds resulted in the loss of 18% of Tree Swallow nests over 2 years. Almost all usurpation events coincided with the conclusion of the first nesting cycle in Eastern Bluebirds. Our data suggest that usurpation by Eastern Bluebirds is more common than reported in the literature. Introduction The swallows Tachycineta bicolor (Vielliot) (Tree Swallow) and Tachycineta thalassina (Swainson) (Violet-green Swallow) are more aggressive than Sialia spp. (bluebirds) and these swallows outcompete bluebirds and exclude them from empty nest-boxes (Prescott 1982, Prigge 1981). Later in the nesting season, these aggressive interactions can result in nest usurpation (Boone 1982, Lindell 1996, Prescott 1982). Most data on nest usurpation point to Tree and Violet-green Swallows usurping Sialia sialis (L.) (Eastern Bluebird) and Sialia currucoides (Bechstein) (Mountain Bluebird) nests (Brawn 1990; Harris and Siefferman 2014; Hersey 1933; Tuttle 1987, 1991). However, Wiebe (2016) showed that the nest owner generally prevailed during usurpation attempts between Mountain Bluebirds and Tree Swallows. Usurpation is not an uncommon phenomenon with other species that are attracted to nest boxes. Troglodytes aedon (Vieillot) (House Wrens) (Belles-Isles and Picman 1986, 1987; Finch 1990) and Passer domesticus (L.) (House Sparrows) (Gowaty 1984, Tuttle 1987) can destroy eggs and kill occupants. Tree Swallows can kill conspecific nest holders and use their nests (Lombardo 1986), and Mountain Bluebirds have killed Tree Swallows in the nest (Frye and Rogers 2004). Usurpation of nests by bluebirds is uncommon. Harris and Siefferman (2014) did not observe Eastern Bluebirds usurping nests at their field site in North Carolina, and there are only a few reported examples of bluebirds usurping nests of other species (Frye and Rogers 2004; Gowaty and Plissner 2015; Tuttle 1987, 1991). In this paper, we report evidence for Eastern Bluebirds usurping Tree Swallow nests and show that the timing of these usurpation events falls between the first and second nesting efforts of bluebirds. 1Department of Biology, Virginia Military Institute, Lexington, VA, 24450. *Corresponding author - rowera@vmi.edu. Manuscript Editor: Jean-Pierre Savard Northeastern Naturalist Vol. 23, No. 4 R.A. Rowe and J.L. Phillips 2016 483 Methods Study site We made the observations presented here during a study on Tree Swallows in 2014 and 2015 at the Virginia Military Institute Biology Department’s Field Research Site (37°46'45.09"N, 79°23'30.51"W) located ~4 km east of Lexington, in west-central VA. The site is a 9-ha hayfield with 2 small thickets and fencerows with mature trees that separate the field site from adjacent farmland. We placed 30 nest boxes (14 x 14 x 25 cm, opening 16.5 cm above floor) in a grid on the hayfield to attract Tree Swallows. The mean distance between nest boxes was 46.2 m, which exceeds the minimal distance for natural nest cavities of 27-m (Robertson and Rendell 1990) and the 36-m minimal distance for Tree Swallow nest-box grid distribution (Muldal et al. 1985). All nest boxes were mounted on posts with antipredator baffles (stove pipe, 20.5 x 61 cm) and had nest-box openings (1.5–1.8 m above the ground) facing into the open hayfield. Field methods We monitored nesting activity at the 30 nest boxes during the 2014 and 2015 nesting seasons (March through June). Bluebirds, which are year-round residents in west-central Virginia, began nesting in late March, with the first eggs generally laid between 6 and 21 April. The majority of Tree Swallows arrived in mid-March, began nest construction in late April, and the first eggs appeared in the nests between 27 April and 13 May. Although the focus of our research was on Tree Swallows, we monitored the status of all nest boxes throughout the breeding season, regardless of the bird species occupying them. We checked nest status (construction, laying and incubation of eggs, hatching, and presence of young) weekly in March and twice weekly in the first 2 weeks of April until the onset of egg laying. We monitored individual nests daily during egg laying and at the expected time of hatching to establish onset of incubation and the nestling phase. During the incubation and nestling phases, we checked nest boxes every other day to ensure that the eggs or nestlings were present and to identify Tree Swallow eggs, which we marked with a pencil to monitor for egg dumping. We emptied all nest boxes in late February, prior to the onset of the nesting season. The Animal Subjects Committee at the Virginia Military Institute approved the nest-checking protocol used in this study. Results Ninety percent of the nest boxes were occupied during the April–May nesting period. Eastern Bluebirds began building nests and laying eggs before Tree Swallows, and occupied 18% of the nest boxes during their first breeding attempt (6 of 30 in 2014 and 5 of 30 in 2015). In general, Eastern Bluebirds began laying eggs 3 weeks before Tree Swallows. Tree Swallows occupied 65% of the nest boxes (19 in 2014 and 20 in 2015), and once egg-laying began, most clutches were complete within a 2-week period. During both years, Poecile carolinensis (Audubon) (Carolina Chickadee) occupied 2 nest boxes and 3 nest boxes were not used. Northeastern Naturalist 484 R.A. Rowe and J.L. Phillips 2016 Vol. 23, No. 4 We recorded suspected usurpation events at 7 Tree Swallow nests; 1 in 2014, representing 5% of the nests and 6 in 2015, representing 30% of the nests (Table 1). During 2014, 35% (7 of 20 nests) of the Tree Swallow nests were lost due to predation by Elaphe obsoleta (Say) (Black Rat Snake) and 1 nest was abandoned. Nests lost in 2014 to Black Rat Snake predation occurred early in the nestling phase, and by the beginning of June, ~40% of the nest boxes were unoccupied. During 2015, only 1 Tree Swallow nest was lost due to predation before the middle of June, and as a result, most of the Tree Swallows still occupied their nest boxes when the Bluebirds began their second nesting cycle. All attempted usurpation events occurred after the end of the first Eastern Bluebird nesting period (nest termination due to abandonment, predation, or fledging) and before the second nesting period (Table 1). At all of the attacked Tree Swallow nest boxes, we found broken eggs in the nest box or eggs and nestlings on the ground next to the nest box (Table 1). Nests were attacked either during the egglaying phase (29% within 5 days of the first egg), during incubation (29% between the laying of the last egg and hatching), or at the beginning of the nestling phase (42% had 1 to 5 day old nestlings). In one nest box (NB 21; Table 1), 5-day-old nestlings were missing from the nest box and the female Tree Swallow was found dead in the nest box with head and neck wounds. Although we did not observe the attack on this nest box, which was located in the middle of the hay field, we observed Eastern Bluebirds perching on the nest box for 3–4 days following the event. At NB 27 (Table 1), we found 3 broken eggs and 2 intact eggs in the nest. We continued to monitor this nest, and 2 days after finding the broken eggs, Eastern Bluebirds began construction of a nest on top of the Tree Swallow nest. A complete Bluebird nest was constructed but no eggs were laid in the nest. Our nest records show that most usurpation events occurred within a 24-h period, but one unsuccessful event (NB 10; Table 1) occurred over a 3-day period. We observed Eastern Bluebirds near and on this nest box during these 3 days. On 11 May, there were 3 Tree Swallow eggs (I, II, and III) in the nest box. On 12 May there were 2 eggs present, but eggs I and III were missing, egg II remained, and a new egg, IV, was present. On 13 May, egg II was pierced and on the ground and eggs IV, and V (a new egg) were in the nest box. Ultimately, the Tree Swallow pair defended their nest and reared 1 nestling to fledging (the other egg was infertile). We only directly observed 1 attempted usurpation of an Eastern Bluebird nest by Tree Swallows during the 2 y of this study. In this instance, 3 Tree Swallows were harassing an Eastern Bluebird pair who had occupied a nest box and laid eggs. The Tree Swallows repeatedly dove at the bluebirds when they were perched on the nest box and chased the birds when they flew. The Eastern Bluebirds retained control of the nest box and successfully reared a brood. Discussion The suspected usurpation events occurred during a period when the Eastern Bluebird nests had either fledged, failed, or were abandoned, and the Tree Swallows were in the early stages of their nesting phase (Table 2). In all suspected usurpation Northeastern Naturalist Vol. 23, No. 4 R.A. Rowe and J.L. Phillips 2016 485 Table 1. Evidence for usurpation of Tree Swallow nests during 2015 and 2014. Date of usurpation event, nest status 24 h prior to the event, evidence of usurpation, and status of each nest following each usurpation events are provided. NB = nest box, TRES = Tree Swallow, EABL = Eastern Bluebird, do = day old). Date of suspected Nest-box number and status usurpation 24 hr prior to usurpation event Evidence of usurpation Post-usurpation status 9 May 2015 NB 24; egg-laying phase, 2 eggs in nest, Eggs missing from nest and found TRES pair retained nest, 5 eggs laid over (1st egg: 8 May) on ground near nest an 8-day period, and then abandoned the nest. No EABL activity after 9 May 12–14 May 2015 NB 10; egg-laying phase, 2 eggs in nest, 5 eggs laid during usurpation TRES pair retained nest with 2 eggs, 1 egg 2 eggs removed from nest during usurpation event, period, 3 eggs found on ground, infertile, other reared to fledging EABL seen at nest box (1st egg: 9 May) 1 pierced 14 May 2015 NB 27; incubation phase, 5 eggs in nest (1 st egg: 3 broken eggs in nest, 2 unbroken EABL nest construction began on 16 May, 9 May) complete nest on 18 May, no EABL eggs laid 22 May 2015 NB 25; 4, 1-do nestlings and 1 egg, (1 st egg: 3 May); Nestlings and egg found on ground No EABL activity after 22 May EABL seen on and near the nest box 2 days prior near nest box 27 May 2015 NB 21; 5, 5-do nestlings (1 st egg: 3 May) Nestlings missing, female dead in No EABL nest constructed. EABL seen on nest box nest box for several days after 27 May 27 May 2015 NB 30; 4, 2-do nestlings, 1 egg (1 st egg: 7 May) Egg found in nest, nestling on No EABL activity after 27 May ground next to nest box 28 May 2014 NB 29; End of incubation phase, 5 eggs, (1 st egg: 1 broken egg in nest box, 5 eggs EABL nest construction 9 days later, 1st 8 May) missing, partial egg found outside EABL egg 14 days after 28 May of nest box Northeastern Naturalist 486 R.A. Rowe and J.L. Phillips 2016 Vol. 23, No. 4 cases, Eastern Bluebirds appear to have been the attacking species. Although Gowaty and Plissner (1997) report that Eastern Bluebirds with unsuccessful nests tend to move away from the failed nest box and successful breeders tend to remain at the nest box and often reuse it, none of the Eastern Bluebirds we studied reused their nest box. Instead, all pairs selected a new nest box for their 2nd brood during both years. During 2015, our casual observations suggest that Tree Swallows opportunistically occupied nest boxes while the Eastern Bluebirds were off the nests with their fledglings. These Tree Swallow pairs were either re-nesting due to early nest failure or were pairs with second-year females (based on brownish coloration; Hussel 1983). Because of the earlier onset of breeding by Eastern Bluebirds at our site, their young were fledging and vacating nest boxes when Tree Swallow nesting was at a peak. Tree Swallows who arrived later at our site, or who could not compete with more experienced birds, represented a floating population of potential breeders looking for a nest box. As a result of the Tree Swallow activity, the Eastern Bluebirds were forced to look for new nest boxes for their 2nd broods. Davis et al. (1994) and Pinkowski (1979) found that Eastern Bluebirds preferred nest boxes with old nests to empty nest boxes, which is consistent with our observations that the Bluebirds selected nest boxes that were previously used by Tree Swallows. We suspect that the nest usurpation events we observed in this study are associated with re-nesting due to a failed nest or a 2nd brood. Tuttle (1987, 1991) reported 11 usurpation events by Eastern Bluebirds (11 of 333 Tree Swallow nests over a 5-y period, accounting for 3.3% of the nests) at his research site near Delaware, OH. He noted that Eastern Bluebirds usurped Tree Swallow nests between the 1st and 2nd breeding attempts, and that 6 nests were disrupted during the egg-laying phase, 4 during incubation, and 1 during the nestling phase, with the 9 day-old nestlings buried under a new nest. Our suspected usurpation events occurred within the timeframe reported by Tuttle (1987, 1991), and the Eastern Bluebirds’ behaviors were consistent with second-nesting behaviors. Waring (1912) and Gowaty and Plissner (2015) reported that Eastern Bluebirds destroyed nests and removed Carolina Chickadee nestlings from nest boxes and dropped them away from the nest box. This observation is consistent with our findings (Table 1); we found eggs and nestlings outside of the nest boxes in 63% of Table 2. Eastern Bluebird nesting records in 2015. First-egg date, nest-box number, and fate of nest are presented. 1st-egg date Nest-box # Fate of nest 3 April NB 9 Hatched by 22 April; fledged on 11 May 17 April NB 8 Abandoned by 7 May with 2 eggs 18 April NB 19 Hatched 5 May; fledged 22 May 21 April NB 31 Abandoned by 11 May with 2 eggs 29 April NB 11 Hatched 15 May; nest lost to predation 26 May with 4 nestlings 3 June NB 16 Hatched 20 June; fledged 10 July 4 June NB 18 Hatched 21 June; fledged 10 July 9 June NB 28 Hatched 28 June; fledged 16 July 16 June NB 3 Hatched 28 June; fledged 15 July Northeastern Naturalist Vol. 23, No. 4 R.A. Rowe and J.L. Phillips 2016 487 usurpation events. Tuttle (1987) reported that out of 506 nests he monitored during a 5-y period, 68 usurpation events were directed at Tree Swallows or Eastern Bluebirds, with 72% of these events committed by House Wrens. Of the 11 usurpations of Eastern Bluebird nests by Tree Swallows, Tuttle (1987) observed only 1 case of Tree Swallows removing eggs from a nest. Leffelaar and Robertson (1985) reported on usurpation of Tree Swallows by conspecifics. In both of these cases, the birds built new nests on top of old nests or added new eggs to the nest cup; no eggs/nests/ nestlings were destroyed during usurpation. We documented 1 usurpation event in which eggs were buried under a new nest, but Eastern Bluebirds constructed the new nest. In all other events, eggs and/or nestlings were removed from the nest box. Although House Wrens are well documented as nest usurpers (Belles-Isles and Picman, 1986, 1987; Doherty and Grubb 2002; Finch 1990; Quinn and Holroyd 1989; Tuttle 1987, 1991), we do not believe House Wrens were involved at our site because we heard only 1 House Wren singing there during the 2015 nesting season and we saw none at nest boxes. Our data suggest that Tree Swallows were not responsible for usurpation because (1) no new Tree Swallow nests were constructed in the nest boxes and (2) Tree Swallows are only reported to bury usurped eggs or nestlings and very rarely to remove eggs from the nest. During 2014 and 2015, only 1 out of 39 Tree Swallow nests was abandoned (NB 24; Table 1). While we did not observe any usurpation activity at this nest, the presence of eggs on the ground rules out predation as an explanation. In the case of NB 24, we assume that Eastern Bluebirds had attacked the nest because of the close proximity and timing of events at 2 other nest boxes (NB 27 usurped 5 days later and NB 25 attacked 13 days later; Table 1). Thus, it seems likely that Eastern Bluebirds were responsible for all of the suspected nest usurpations. Our data suggest that usurpation by Eastern Bluebirds may be more common than suspected. Researchers monitoring bluebird nest boxes may not recognize usurpation events because these individuals are focused on successful nesting of the Eastern Bluebirds and overlook usurpation events in which an Eastern Bluebird evicts a Tree Swallow. Our data suggest that destruction of eggs in the nest and removal of eggs and nestlings can occur as a result of usurpation by Eastern Bluebirds. The removal of eggs and nestlings from nest boxes is not uncommon during usurpation by House Wrens (Belles-Isles and Picman 1986), but the frequency of this behavior by Eastern Bluebirds is unknown. Information on the frequency of nest usurpation and egg removal by Bluebirds could be noted and reported through nest-monitoring programs such as NestWatch (nestwatch.org). In west-central Virginia, the timing of Tree Swallow migration coupled with the occurrence of Eastern Bluebirds as year-round residents gives an occupancy advantage to bluebirds. As established occupants, the bluebirds’ successful defense of a site during competition for available nest boxes (Wiebe 2016) may reduce Tree Swallow aggression towards Eastern Bluebirds during March and April. The timing of the completion of the 1st nesting cycle for Eastern Bluebirds creates a situation in which Eastern Bluebirds have the opportunity to usurp Tree Swallow nests during the early stages of egg laying and incubation by the latter. Northeastern Naturalist 488 R.A. Rowe and J.L. Phillips 2016 Vol. 23, No. 4 Acknowledgments The Virginia Military Institute’s Jackson Hope Fund, SURI Program, Grants-in-Aid of Research, and Biology Department provided funding for this project. We thank Drs. Anne Alerding and Paul Moosman for comments on earlier versions of this manuscript. Critical comments from 3 anonymous reviewers greatly helped focus and improve this manuscript. Literature Cited Belles-Isles, J-C., and J. Picman. 1986. House Wren nest-destroying behavior. 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