Northeastern Naturalist Vol. 26, No. 1 J.L. Phillips, W.. Haslam, A.E. Ford, and R.A. Rowe 2019 43 2019 NORTHEASTERN NATURALIST 26(1):43–51 Nest-defense Behavior in Response to the Chatter Call in Tachycineta bicolor (Tree Swallow) J. Luke Phillips1,2, Wynn F. Haslam1,3, Alyssa E. Ford1,4, and Richard A. Rowe1,* Abstract - Birds produce a variety of vocalizations ranging in function from attracting a mate to alerting others of danger. One vocalization of Tachycineta bicolor (Tree Swallow), the chatter call, is reported to serve as both a mate-attraction call and a nest-defense call. We used playback techniques to investigate the nest-defense behavior of Tree Swallows to calls of Toxostoma rufum (Brown Thrasher; control) and Sialia sialis (Eastern Bluebird; nest competitor), and Tree Swallow shriek (alarm call) and chatter calls. We measured time spent foraging, remaining in the nest box, and on nest defense. Tree Swallows spent 80% of their time on nest defense in response to the chatter call. The chatter-call and shriek-call nest-defense responses were nearly identical. Our data show that the chatter call functions as an alarm call and elicits nest-defense behavior. Introduction Cohen (1978) first described the vocalizations of Tachycineta bicolor (Vielliot) (Tree Swallow) and presented 9 distinct calls. Others have subsequently supported his descriptions (Leonard et al. 1997, Robertson et al. 1992, Sharman et al. 1994, Winkler 1992). Two of the calls, the shriek and chatter calls, are associated with nest defense, and the chatter call could function to attract mates to the nest (Sharman et al. 1994). Thus, the chatter call appears to carry a message stating either “come here” to potential mates or “go away” to potential competitors; but how the chatter call is modified to relay this message or if it can convey both messages at the same time has not be elucidated. The shriek call is an alarm call directed towards predators near the nest or when attacking a predator, and on occasion, to conspecifics near the nest or during chases (Cohen 1978, Sharman et al. 1994, Winkler et al. 2011). The shriek call, as described by Sharman et al. (1994), is given when perched or in flight and in response to a predator or conspecific flying too close to the nest (Winkler et al. 2011). The shriek call functions to alert a mate or offspring to the presence of a threat and to recruit other Tree Swallows to aid in defense (Curio 1978, Winkler 1994, Winkler et al. 2011). The role of the chatter call in Tree Swallow communication is unclear because it is used in various behavioral contexts. Similar to the shriek call, the chatter call is given near the nest box when a conspecific approaches, and it is most common during the time prior to egg-laying (Cohen 1978, 1989; Robertson et al. 1992; Winkler 1Department of Biology, Virginia Military Institute, Lexington, VA 24450. 2Current address - 2800 Shadow Lake Road, Blacksburg, VA 24060. 3Current address - 361 Bradford Road, Mars Hill, NC, 28754. 4Current address - 3304 8th Street NE, Minot, ND 58703. *Corresponding author - rowera@vmi.edu. Manuscript Editor: Daniel Keppie Northeastern Naturalist 44 J.L. Phillips, W.. Haslam, A.E. Ford, and R.A. Rowe 2019 Vol. 26, No. 1 et al. 2011). The chatter call can also function as a mate-attraction call (Robertson et al. 1992). This call is described as being a nest-site advertising call and does not appear to be associated with defending a mate from potential extra-pair copulations (Sharman et al. 1994). The chatter call is incorporated into the dawn song and day song of Tree Swallows and is used in combination with other calls in these mateattraction and territorial songs (Horn 1996, Taft 2011). Hence, the chatter call may be given in response to conspecifics intruding into nesting space, but there is no direct evidence that it elicits nest-defense behavior from residents in response to a conspecific or predator intruding into a territory. Our goal was to investigate the role of the chatter call in nest defense. We used playback techniques to assess whether: (1) the chatter call elicits nest defense behavior; (2) the response to the chatter call is similar to the shriek call (a wellestablished predator-alarm call: Winkler 1992, Winkler et al. 2011); and (3) the song of a known nest competitor and potential nest usurper, Sialia sialis (L.) (Eastern Bluebird) (Finch 1990; Rowe and Phillips 2016; Tuttle 1987, 1991), near a nest elicits nest-defense behavior. Methods Study site We conducted this study during the 2013–2016 breeding seasons at the Virginia Military Institute Biology Department’s Field Research Site (37°46'45.09"N, 79°23'30.51"W) ~4 km east of Lexington in west-central Virginia. The site is a 9-ha hayfield with 2 small thickets and fence rows and mature trees separating the field site from adjacent farmland. In 2002, we placed 30 nest boxes (14 cm x 14 cm x 25 cm, with an opening 16.5 cm above floor) 1.5–1.8 m above the ground in a grid on the hayfield to attract Tree Swallows. We mounted all nest boxes on posts with anti-predator baffles (stove-pipe, 20.5 cm x 61 cm). The mean distance between nest boxes was 46.2 m. Field methods In west-central Virginia, most Tree Swallows arrive in mid-March (R.A. Rowe, pers. observ.) and begin nest construction in mid-April; first-egg date varies from 27 April to 13 May (R.A. Rowe, unpubl. data). We checked nest-building status weekly in March and twice weekly in the first 2 weeks of April until complete nest cups were present. We monitored nests every other day until feathers were present in the nest cup (indicating that egg laying was imminent). We examined individual nests daily during egg laying and at the expected time of hatching to establish onset of incubation (we defined day 0 of incubation as the first day when no new egg was laid) and the nestling phase. We defined nest day (ND) 0 of the nestling phase as the first day a nestling was found in the nest. When clutches were complete or the nestlings had hatched, we checked nest boxes every other day to ensure the eggs or nestlings were present. We emptied all nest boxes in late February, prior to the onset of the nesting season. The study was approved by the Animal Subjects Committee at the Virginia Military Institute. None of the birds in this study were marked. Northeastern Naturalist Vol. 26, No. 1 J.L. Phillips, W.. Haslam, A.E. Ford, and R.A. Rowe 2019 45 Playback protocol We used playback tapes of Tree Swallow vocalizations to elicit nest-defense behaviors from resident pairs of Tree Swallows. Each playback tape consisted of 5 min of silence followed by 5 min of playback scenario. We used 4 different playback scenarios to investigate behavioral responses to the chatter call. We used the typical singing of Toxostoma rufum (L.) (Brown Thrasher) as a control for the presence of the tape recorder, tripod, and the presentation of sounds at the nest box. Brown Thrashers were present at our field site but do not present a threat to Tree Swallows because they are not cavity nesters and are too large to enter a nest box. During the 2016 nesting season, we used playback of an Eastern Bluebird singing to investigate whether the presence of a potential nest competitor that was not a predator would elicit nest-defense behavior. We employed both the Brown Thrasher and Eastern Bluebird calls with permission from the Borror Laboratory of Bioacoustics, The Ohio State University, Columbus, OH (Brown Thrasher BLB 4508; Eastern Bluebird BLB 17538). We also played chatter and shriek calls to provide a comparison of the chatter call to a known alarm call. I recorded both the chatter and shriek calls during 2013 at the research site using a Telinga Pro parabolic dish (Tekinga, Tobo, Sweden), Sennhieser microphone (MKH 20; Wedemark, Germany), and a Fostex FR-2 digital field recorder (Fostex, Wiltshire, UK). I created playback loops for all tapes using Raven Pro 1.4 (Bioacoustics Research Program, The Cornell Laboratory of Ornithology, Ithaca, NY). We presented playback tapes at each nest box during each nesting phase, incubation (day 7 or 8 after the last egg was laid), young nestlings (ND 4 or 5), and older nestlings (ND 12 or 13). Over the years of this study, the mean length for incubation was 13.5 d at our field site (n = 62), and fledging occurred at ~ND 19 (n = 38). The order of presentation of playback tapes was varied so that each playback was played at a nest during the nesting period (incubation, young nestlings, or older nestling) but no nest was tested with the same playback twice. During the 2013–2015 nesting seasons, the 3 playback tapes were the Brown Thrasher singing, shriek call, and chatter-call tapes. During the 2014–2015 nesting seasons, we documented suspected nest-usurpation by Eastern Bluebirds (Rowe and Phillips 2016). As a result, during the 2016 nesting season, we substituted playback of a singing Eastern Bluebird for the shriek-call playback in order to investigate whether Tree Swallows would respond to the presence of a potential nest competitor or usurper near the nest box. We conducted all playback trials between 0700 and 0930. During playback trials, we placed the tape recorder on a tripod ~1 m in front of a nest box and 1.5 m above ground level; 2 observers were positioned ~30 m from the focal nest box and 30 m from each other. An initial 5-min silent period allowed the focal pair to adjust to the presence of the tape player and tripod and to allow the observers to locate the focal pair for observations during the playback trial. During a playback trial, observers used a hand-held voice recorder (Olympus WS-822 digital voice recorder) to verbally document the behavior of the birds. We employed Raven Pro 1.4 to analyze these recordings for time spent performing each behavior. We calculated the time spent for each behavior as the average from both observers. Northeastern Naturalist 46 J.L. Phillips, W.. Haslam, A.E. Ford, and R.A. Rowe 2019 Vol. 26, No. 1 Behaviors We noted the following behaviors during playback sessions: foraging, perched, in the nest box, and circling. We combined active feeding, flying in a direct line above the grass, or flying out of sight (away from the nest box) as foraging. We defined perched as being perched on or within 10 m of the nest box. The “in the nest box” category occurred when either or both birds (a rare event) were known to be in the nest box and, thus, we could not observe their actual behavior. Our initial observations during playback trials in 2013 revealed that Tree Swallows left the nest almost immediately (within the first 30 sec) after either the chatter- or shriek-call playbacks began. As a result, we decided not to include “in the nest box” as a nestdefense behavior. We defined circling as flying that was centered around the nest box. For the analyses in the present study, nest defense was the sum of the amount of time spent circling and perched on/near the nest box. During the 2016 nesting season, we recorded vocalizations from male Tree Swallows perched on their nest box during the egg-laying and incubation phases. These vocalizations were recorded using a Telinga Pro parabolic dish, a Sennhieser microphone (MKH 20), and a Fostex FR-2 digital field recorder. From these recordings, we were able to identify 11 cases in of a male singing from his nest box when the location of his mate was known and an intruding conspecific Tree Swallow flew into/through the territory. We analyzed these 11 recordings using Raven Pro 1.4 (Bioacoustics Research Program, The Cornell Laboratory of Ornithology) to investigate if the rate or the percentage of chatter calls changed when a conspecific was present in the territory. Statistical analysis We converted data collected on time spent performing each behavior to percentage of time spent during a playback session. We transformed data to the arcsine square-root of the percent for statistical analysis (Sokal and Rolf 1995). As a means to validate our methods, we used the general linear model ANOVA (threshold for significance P ≤ 0.05) and a Tukey post-hoc test (in Minitab 17.3.1) to determine if there was an effect (increase in behavioral responses) on subsequent Brown Thrasher playback trials by playing a shriek or chatter call first. We found no effect of playing a shriek or chatter call first on the behavioral responses in subsequent playback trials across the nesting phases (n = 48 for all ANOVAs; foraging F2 = 1.62, P = 0.21; in the nest box F2 = 2.03, P = 0.138; circling F2 = 0.48, P = 0.62; nest defense F2 = 1.68, P = 0.19). Thus, we are confident that our experimental procedure did not sensitize the resident pair and elicit an enhanced response during future trials. We conducted a repeated-measures ANOVA and Tukey post-hoc test to determine if mean differences in time spent performing behaviors in response to the playbacks at each nest phases (R Core Team 2018) were meaningful. We used a paired t-test to evaluate changes in the number and percentage of chatter calls given prior to and during the intrusion of a conspecific into a territory (Minitab 17.3.1). Northeastern Naturalist Vol. 26, No. 1 J.L. Phillips, W.. Haslam, A.E. Ford, and R.A. Rowe 2019 47 Results Brown Thrasher playback trials In response to the Brown Thrasher, the mean percentage of time Tree Swallows spent performing the different behaviors (Table 1) during the 3 nesting phases (incubation, young nestlings = ND 4–5, old nestlings = ND 12–13) did not differ. As expected, the amount of time spent in the nest box (Table 1) decreased at ND 12–13, when both adults were foraging and nestlings had attained adult weight (R.A. Rowe, unpubl. data). It should be noted that only female Tree Swallows incubate the eggs and thus, the mean values are indicative of only the females’ behavior. During Brown Thrasher playback trials at all nesting phases, Tree Swalllow males spent less than 3% of their time in the nest box. The time spent on nest defense showed no significant changes as the nest progressed in age, thus, the time spent on this behavior was consistent (Table 1). Chatter and shriek call playback trials Our comparison of the mean time spent performing different behaviors in response to both the chatter and shriek calls (Table 2) revealed that there were no Table 2. Mean percent time (mean ± SD) spent by nesting Tree Swallow pairs in response to playback of the chatter-call and shriek-call tapes. Nest defense is the sum of time spent circling and perched during a trial. We conducted statistical analyses using a repeated-measures ANOVA (Fnumdf,dendf) with P-values from a Tukey post-hoc comparison of means. Means with different letters in each column are different at ≤ 0.05. % time spent on behavioral responses Playback n Foraging In nest box Nest defense Brown Thrasher 134 58.0 ± 40.8A 21.6 ± 8.1 A 20.4 ± 31.0 A Bluebird 52 45.6 ± 39.9 A 11.3 ± 27.9 AB 43.2 ± 38.2A Chatter call 134 18.2 ± 25.6B 2.9 ± 14.0B 79.2 ± 27.4B Shriek call 40 16.4 ± 16.6B 0.08 ± 4.3B 82.9 ± 17.5B Statistics F3,74 = 27.29 F3,74 = 13.55 F3,74 = 67.65 P = 0.01 P = 0.01 P = 0.01 Table 1. Mean percent time (mean ± SD) spent foraging, in the nest box, and on nest defense by nesting pairs of Tree Swallows during Brown Thrasher playback trials. The Brown Thrasher tape served as a control in our study; the data represent behaviors of Tree Swallows in response to the playback protocol. Nest defense is the sum of time spent circling and perched during a trial. We conducted statistical analyses using a repeated-measures ANOVA (Fnumdf,dendf) with P-values from a Tukey post-hoc comparison of means. Means with different letters in each column are dif ferent at ≤0.05. % time spent on behavioral responses Playback n Foraging In nest box Nest defense Incubation 56 49.4 ± 44.5A 24.6 ± 42.3A 26.1 ± 36.5A ND 4–5 50 59.4 ± 38.9A 25.4 ± 38.4A 15.2 ± 26.2A ND 12–13 28 73.0 ± 32.1A 8.6 ± 24.4B 18.3 ± 25.5A Statistics F2,74 = 3.68 F2,74 = 4.59 F2,74 =2.15 P < 0.03 P < 0.01 P = 0.12 Northeastern Naturalist 48 J.L. Phillips, W.. Haslam, A.E. Ford, and R.A. Rowe 2019 Vol. 26, No. 1 significant differences in time spent foraging, in the nest box, or on nest defense (P = 0.07–1.0, Tukey comparisons). Both the chatter and shriek calls elicited similar responses when compared to the Brown Thrasher playback. The time spent foraging, in the nest box, or on nest defense in response to either the chatter call or shriek call was different (P < 0.01 for all comparisons) from the Brown Thrasher playback. Eastern Bluebird playback trials During the 2016 nesting season, we investigated if the presence of an Eastern Bluebird singing near the nest box would elicit any changes in behaviors by Tree Swallows in response to the Brown Thrasher, Eastern Bluebird, and chatter call playbacks. A comparison of the Eastern Bluebird and Thrasher playbacks showed no differences in Tree Swallow behavior as nests progressed from incubation through ND 12–13 (F 2,74, for phase-effect, P values from Tukey: F = 0.12–2.16, P = 0.13–0.89). In addition, there were no meaningful differences in the amount of time spent foraging, in the nest box, or on nest defense when the Eastern Bluebird and Brown Thrasher tapes were compared (F 3,74 [see Table 2 for repeated-measures F values for each behavior], P = 0.25–0.99). We compared the Tree Swallow responses to Eastern Bluebird and chatter-call playbacks to examine if a singing Eastern Bluebird elicited behavioral changes in the Tree Swallows. There were differences in the responses for nest defense and foraging (P values from Tukey comparison of means: foraging P = 0.01, circling P = 0.01, nest defense P = 0.01) but no differences in the time spent in the nest box (perched P =1.00, in the nest box P = 0.23). Chatter calls in response to intruding Tree Swallow During 2016, we recorded 11 situations in which a male Tree Swallow was vocalizing while perched on a nest box and, subsequently, another Tree Swallow flew into the territory. We compared the number and percentage of chatter calls given prior to the intrusion and during the time the conspecific was within the territory. The mean number of chatter calls increased significantly (paired t-test: t = 4.06, P = 0.01, n = 11) from 6.2 ± 0.8 (mean ± SD) chatters per second prior to the intrusion to 9.3 ± 0.5 chatters per second during the time the conspecific was present. During the time prior to the intrusion, chatter calls constituted 78.3% ± 6.7 of the vocalizations, and 95.9% ± 2.2 during the intrusion (paired t-test: t = 2.46, P = 0.03, n = 11). Discussion Our data demonstrates that the chatter call primarily elicits nest-defense responses and increases nest defense by 4-fold, that the percentage of time spent on nest defense is similar to the response elicited by the shriek call (constituting ~80% of the swallows’ time, Table 2), and that the responses to the chatter call differ from those to Brown Thrasher or Eastern Bluebird playback trials. Both the chatter and shriek calls elicit similar nest-defense behavior; thus, we speculate that Tree Swallows have 2 alarm calls to provide additional information about the threat. We note that the data presented in the present study do not rule out the possibility that the chatter call also serves to attract mates as suggested by Sharman et al. (1994). Northeastern Naturalist Vol. 26, No. 1 J.L. Phillips, W.. Haslam, A.E. Ford, and R.A. Rowe 2019 49 Eastern Bluebirds pose a threat because they are known to usurp the nests of other species, including Tree Swallows (Frye and Rogers 2004; Gowaty and Plissner 2015; Rowe and Phillips 2016; Tuttle 1987, 1991). Eastern Bluebirds have been documented to usurp nests during the building, egg-laying, incubation, and the early nestling phases. Loss of a nest due to usurpation or nest parasitism results in a loss of reproductive effort, which is especially critical in Tree Swallows, which only produce 1 clutch per year (Leffelaar and Robertson 1985). Thus, the reproductive value of each clutch is high in Tree Swallows, and possessing a specific call, such as the chatter call, that can alert a mate of the presence of a nest competitor can be important. The responses of Tree Swallows to chatter-call playbacks showed changes in the behaviors that we monitored when compared to the Brown Thrasher (control). The increase in nest defense (the sum of circling and perched behaviors in this study) in response to the chatter-call playback provides the opportunity to be seen by the threatening bird and to monitor the activity of the threat. Curio (1978) notes that alarm calls serve to notify the predator that it is seen and thus, the element of surprise is lost. This visibility during nest defense performs some of the functions of an alarm. Tree Swallows commonly fly in response to a threat (Winkler et al. 2011), which provides an immediate avenue for escape, allows the bird to visually locate the threat, and allows the swallow to monitor the threat’s activity. We describe the Tree Swallows’ nest-defense behavior as being vigilant or patrolling the territory in response to the potential presence of a threat to the nest. Some researchers have suggested that nest defense should increase with the increasing age of a nest and thus, the inherent value of the nestlings (Forbes et al. 1994, Montgomerie and Weatherhead 1988). However, Tree Swallows do not appear to increase their nest-defense behaviors as nests progress from eggs through older nestling stages (Table 1; Hainstock et al. 2010, Lombardo 1991, Winkler 1991, present study). Although both the chatter and shriek calls have an alarm-call function, they may serve additional functions in the vocal repertoire. Our data on nest defense in response to the shriek call are consistent with other studies; however, the possibility that the chatter call serves multiple functions is likely (Sharman et al. 1994, Winkler et al. 2011). Referential-based alarm-call systems have been described in only a few species of birds. Templeton et al. (2005) first described encoding of information into alarm calls by Parus atricapillus (L.) (Black-capped Chickadee) and the Baeolophus bicolor (L.) (Tufted Titmouse). These species varied the notes within their alarm calls to encode information about the type of threat present. Our data suggest the possibility that one use of the chatter call of Tree Swallows is to broadcast the presence of a threat from a nest competitor or usurper, of the same or different species, that has entered the territory, and to alert an intruding Tree Swallow that it has been seen. The increase in chatter calls during intrusion events that we documented could serve both of these functions. Nest usurpation is found in a number of species (Lindell 1996) and is common in Tree Swallows (Finch 1990; Frye and Rogers 2004; Haslam et al. 2016; Hersey 1933; Rowe and Phillips 2016; Tuttle 1987, 1991). Northeastern Naturalist 50 J.L. Phillips, W.. Haslam, A.E. Ford, and R.A. Rowe 2019 Vol. 26, No. 1 Our study shows that the chatter call elicits responses that are similar to the shriek call, which is an alarm call; however, it would be necessary to present threats (predators or competitors) to the nest and record the vocalizations of Tree Swallows in response to these threats in order to confirm the chatter call’s role as an alarm call. Tree Swallows are a nest-cavity–limited species that will readily occupy nest boxes. As a result, large aggregations of Tree Swallows are possible along Eastern Bluebird trails and nest-box grids. The increase in availability of nest boxes associated with Eastern Bluebird conservation has resulted in an expansion of their breeding range (Winkler et al 2011) and has created large, semi-colonial breeding groups. In these large breeding assemblages, it is common for shriek calls to recruit individuals from neighboring nests to assist in harassing a predator (Winkler 1994). 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