Northeastern Naturalist Vol. 25, No. 2 C.M. Heckscher 2018 291 2018 NORTHEASTERN NATURALIST 25(2):291–307 The Seasonal Pattern of Veery Calls and Song in Relation to Reproductive Contexts Christopher M. Heckscher* Abstract - I investigated the seasonal patterns in the use of Veery vocalizations (song and calls) at a breeding site in northern Delaware. The Veery call repertoire consists of 2 continua (harsh and tonal notes) and 4 discrete call types. Veeries use these calls in long-distance (inter-territory) and short-distance (intra-territory) vocal interaction. To gain insight to Veery communication, I tallied calls during twenty-six 10-min point-counts in seven 11-d periods to determine the pattern of call types used by Veeries from nest initiation through fledging. The pattern of calls used varied depending on the corresponding reproductive context and can be in part explained by their acoustic properties in varying social situations. The use of a variety of calls early in the breeding season coupled with a paucity of song confirms that calls are the primary acoustic signal used by Veeries during the first 2 weeks after spring arrival. Introduction Songbird vocalizations exhibit seasonal and diurnal patterns. Vocalizations are usually separated into complex learned structures (songs) and presumably innate simple structures (calls) (Baptista 1996). Seasonal variation in song may be the product of an individual’s breeding status (paired or unpaired) or current reproductive context (e.g., egg-laying, incubation, rearing nestlings, feeding fledglings). For example, Turdus iliacus L. (Redwing) song is most common in the egg-laying stage and declines during the incubation and nestling stages (Lampe and Espmark 1987), unpaired Saxicola torquata L. (Stonechat) males sing more than paired males (Greig-Smith 1982), and Catharus bicknelli (Ridgway) (Bicknell’s Thrush) sing most during courtship and egg-laying (Ball 2000). Diel patterns in song may depend on functions intrinsic to the signaler’s internal state (i.e., hormonal), social functions (e.g., inter- and intra-sexual), environmental pressures (e.g., optimizing sound propagation dependent on current air turbidity) or a combination of these factors (reviewed in Stacier et al. 1996). For example, Troglodytes brunneicollis Sclater (Brown-throated Wren), like many songbirds, sing with high vocal output in the morning, but, presumably due to a variety of exogenous and endogenous factors, this output decreases as the day progresses (Sosa-López and Mennill 2014). In contrast to songs, far less is known about the seasonal patterns in the use of calls by passerines. During the breeding season, patterns in the calling behavior of songbirds are usually a reflection of social interactions in combination with current breeding status or reproductive context, and are not thought to be influenced directly by hormonal or environmental changes (Beletsky 1991, Beletsky et al. *Department of Agriculture and Natural Resources Delaware State University, 1200 North DuPont Highway, Dover, DE 19901; checkscher@desu.edu. Manuscript Editor: Heather York Northeastern Naturalist 292 C.M. Heckscher 2018 Vol. 25, No. 2 1986, Ficken et al. 1994). For example, calls often promote flock cohesiveness, adult contact, and parent–offspring contact, or convey a threat (Beletsky et al. 1986, Kumar 2004). Ball (2000) examined the seasonal patterns of calls used by Bicknell’s Thrushes in Ontario and found that the greatest peak in calling rate occurred between 30 June and 23 July, but varied in relation to the associated reproductive context (courtship, egg-laying, incubation, rearing nestlings, feeding fledglings). Rimmer et al. (1996) reported that on Mt. Mansfield in Vermont, Bicknell’s Thrush vocalizations (song and call) were most common in mid-June with resurgence in July. An understanding of when birds vocalize and what types of vocalizations they use can have important implications for understanding avian ecology and life history, and can aid in the planning of survey work (Rimmer et al. 1996, Sosa-López and Mennill 2014). The thrushes (Turdidae) are renowned for their complex song structures. However, these species have a remarkably complex call repertoire used by many thrush species (e.g., Andrew 1961, Ball 2000, Grabowski 1979, Kramer 1980, Samuel 1972). In particular, call repertoires used by the world’s forest-dependent thrushes have been poorly documented. Catharus fuscescens (Stephens) (Veery) is a forestinterior thrush that occupies dense boreal and northern temperate North American broadleaf or mixed conifer–deciduous forests. The Veery has a complex repertoire consisting of calls that represent points on 2 frequency continua: a harsh frequencymodulated (i.e., each call shows abrupt shifts in frequency) vibrato continuum, downward sweeping in form, from ~1.3–4.0 kHz, and a less frequency-modulated pure-tone continuum that is also downward sweeping but sometimes consists of multiple harmonics ~1.0–4.5 kHz (Heckscher et al. 2017a). Specific calls used from the 2 continua are continuous in form without discrete boundaries, such that any downward sweeping form of the call can be given within the upper and lower frequency (kHz) boundaries. In addition to the 2 continua, the Veery uses 4 dissimilar and discrete call structures: a high-frequency whistle, a vibrato harsh chatter, a pure-tone mid-frequency downward-sweeping convex-shaped call, and a frequency-modulated U-shaped call (Heckscher et al. 2017a). These calls—each with links to audio recordings—can be found in more detail in Heckscher et al. (2017a). This classification is inclusive of 9 calls reported by Samuel (1972), 5 of which are represented in the 2 call continua noted above. Although variation in call structures among New World Turdidae have been noted in multiple species (Bent 1949, Grabowski 1979), to the best of my knowledge, the use of call continua has only been reported from Catharus (Veery, Bicknell’s Thrush, C. minimus (Lafresnaye) [Gray-cheeked Thrush]), and Myadestes (M. townsendi (Audubon) [Townsend’s Solitaire]) (Ball 2000, Kramer 1980, Marshall 2000). The use of complex vocalizations for communication often evolves in social species (Morton 1977), particularly in species that inhabit environments where the conveyance of visual displays may be impeded, such as marine systems or, as in the current example, dense forest (e.g., Miller et al. 2004, Tyack 1997). I examined the seasonal pattern of Veery vocalizations (calls and songs) in relation to the temporal shift of reproductive contexts evident within a long-term study Northeastern Naturalist Vol. 25, No. 2 C.M. Heckscher 2018 293 population in part to gain insight into social behavior (i.e., communication) and ecology. The patterns in the use of vocalizations within the population relative to the seasonal changes in reproductive behavior are reported herein. Although the species can exhibit polygynandry (Halley et al. 2016), most male Veeries are territorial despite a social hierarchy that results in overlapping territories among some individuals (Halley 2014; Heckscher 2007, 2017a). The Veery, like its congener Bicknell’s Thrush, uses its call continua for communication across long distances (i.e., across multiple territorial boundaries or the equivalent). Long-distance communication using call notes is unusual in oscine passerines; thus, I also investigated the seasonal pattern of occurrences of calls categorized in the field as long-distance versus those assumed to be intended for short-distance in an attempt to learn more about the use of long-distance communication in this species. The results presented herein will assist in our understanding of vocal communication in this species and can aid in the understanding of how calling behavior can reveal the various reproductive status of individuals. Methods Study area I undertook the study in the mid-Atlantic Appalachian Piedmont physiographic province at White Clay Creek State Park, New Castle County, DE (39.737986°N, 75.760653°W). The study area comprises ~200 ha within a larger forested system, and consists of low hills and creek valleys of mixed mesic hardwood forest dominated by Fagus grandifolia Ehrh. (American Beech), Acer rubrum L. (Red Maple), Fraxinus spp. (ash), Populus deltoides W. Bartram ex Marshall (Cottonwood), Carya spp. (hickory), Quercus spp. (oak), and other species (see Heckscher 2004 for more detail). The forest understory is a mix of native and alien species but is largely comprised of the non-native shrub Rosa multiflora Thunb. (Multiflora Rose) (Heckscher 2004). In 1998, I initiated a study of Veeries, and the population has been continuously studied since then (e.g., Halley et al. 2016, Heckscher 2004, 2017b). Reproductive data For the current study, I monitored nests from 1998 to 2006 and found most of them by following adults as they carried nesting material or food to nest sites. Once located, I checked the nests at 1–3-day intervals until young fledged or the nest failed. I determined nest chronologies from known clutch initiation, clutch completion, hatching, or fledging dates. For dates of events prior to the discovery of the nest I back-dated using the chronology presented in Heckscher et al. (2017a). Vocalizations In 2006, I established 16 unlimited-radius point-count stations separated by a minimum of 200 m along trails and roads (non-vehicular traffic) that traversed the study area. At this study site, peak arrival of Veery adults occurs between 29 April and ~15 May (see Heckscher et al. 2017b). I arbitrarily divided the breeding season Northeastern Naturalist 294 C.M. Heckscher 2018 Vol. 25, No. 2 into seven 11-d periods from 2 May through 23 July (Fig. 1) and separated the 16 points into 2 groups: 1–10 A and 1–6 B. During each period, I visited points 1–10 A twice for 10 min and points 1–6 B once for 10-min. The latter group of 6 was established in more remote regions of the park so they were visited only once in each 11-d period. Thus, I undertook 26 point-counts in each period. At the beginning of each period, I employed a random number generator in Microsoft Excel to pick a number between 1 and 10 to indicate the first point to visit from points 1–10 A. I visited subsequent points in numerical order until all 10 points had been visited once (points after 10 were visited by returning to point 1 A). I chose the starting point for 1–6 B using the same method. Points 1–10 A were then re-visited in the same order picked for that 11-day period. I collected all data during non-inclement weather that lacked precipitation or wind >5 kph. Surveys took place 1700–2000 hrs EDT, when Veeries are most active (Heckscher et al. 2017a). I tallied the number of Veery calls heard during the 10-min survey as long-distance or short-distance tonal continuum, T3, long-distance or short-distance harsh continuum, H3, chatter, whistle, downward call, or U-call (Figs. 2, 3). The T3 and H3 calls represent vibrato vocalizations that reach the lowest frequency points on the tonal and harsh continua, respectively. Although the T3 and H3 calls occur on the tonal and harsh continua, I noted them separately because they are easily distinguishable and therefore may have unique effects. Calls from the 2 continua sound similar but remain identifiable in the field. Therefore, all categories used in analyses reflect calls that are easily discernible in the field. Of interest, the Veery uses its call continua for communication with congeners over long-distances. I define long-distance signals as the use of acoustic signals at high amplitude apparently intended to travel across multiple territorial boundaries Figure 1. Breeding-season chronology of Catharus fuscescens (Veery) in northern Delaware as determined from 57 nests monitored from 1998 to 2006. The breeding season is separated into seven 11-day intervals. Arrival of Veeries from South America peaks during the first 2 weeks of May. Northeastern Naturalist Vol. 25, No. 2 C.M. Heckscher 2018 295 Figure 2. Spectograms showing various call forms representing Catharus fuscescens (Veery): (A) harsh-call continuum, and (B) tonal-call continuum. The H3 and T3 calls denoted are easy to differentiate in the field and reach the lowest frequency points on the harsh and tonal continua, respectively. See Heckscher et al. (2017a) for audio recordings of various harsh and tonal continua calls. (or the equivalent distance). In contrast, some signals are intended to be perceived solely within a territorial boundary (very low amplitude) or with a receiver (hereafter, assessor) in an adjacent territory (low to mid-amplitude). The categorization of long-distance versus short-distance calls in the field was somewhat subjective in nature, but I categorized calls I felt were loud and emphatic as long-distance because I felt there was a high probability that the call was intentionally projected over long-distances (e.g., multiple territorial boundaries). I categorized calls as short-distance if they were comparatively low in amplitude and seemed intended for within- or adjacent-territory assessment. My objective was to obtain an index reflecting the relative frequency of call use—independent of all other call types—thereby generating a pattern of its use throughout the breeding season; therefore, the numbers of individuals emitting calls is immaterial considering the number of Veeries available to call remains approximately constant throughout the breeding season. I made a direct comparison of call type only between harsh- and tonal-continuum calls, which was justified because both are of similar form and are projected with similar amplitude dependent on social context (Heckscher 2007). I made 2 assumptions: (1) all audible calls emitted by Veeries during the 10-min point count were tallied, and (2) detectability for each call type remained constant throughout the season. The implications of these Northeastern Naturalist 296 C.M. Heckscher 2018 Vol. 25, No. 2 assumptions, and the methods used for this survey, differ from traditional avian point-count studies in which an avian species assemblage is being surveyed for comparative purposes and the probability of detection varies greatly for each species being compared. In contrast to calls, I did not count songs; rather, I recorded the maximum number of individuals detected simultaneously singing during each 10-min period. Statistical analyses I used a one-way ANOVA to test whether the number of calls tallied differed among the seven 11-d periods. For each 11-d period, I then calculated the mean number of calls and standard error for each call type detected. I then examined the results in relation to the temporal shift of reproductive contexts evident within the population to identify patterns in the use of different calls throughout the 7 periods. For tonal and harsh continua, I used t-tests to compare the difference in the number of long-distance calls and short-distance calls. After examining the data and observing a progressive decline in chatter calls as the season progressed, I used a simple linear-regression model to determine if the decline was significant. I employed boxplots to examine all data used in statistical analyses to ensure that the assumptions of normality and equal variances were met prior to analysis (Logan Figure 3. Four discrete calls that supplement Catharus fuscescens (Veery) call continua: (A) whistle, (B) chatter, (C) U-call, and (D) downward. See Heckscher et al. (2017a) for audio recordings. Northeastern Naturalist Vol. 25, No. 2 C.M. Heckscher 2018 297 2010). Some data violated the assumptions of parametric tests; thus, I assessed each call type for significance in the change across the 7 periods using a Kruskal–Wallis non-parametric test. Regardless of statistical significance, I chose to report patterns evident in the data if they were thought to be of potential interest. For tests where P ≤ 0.15 but > 0.05, I report results as trends (e.g., Heckscher et al. 2017b). All statistical tests were accomplished in R 3.2.1 (R Development Core Team 2015) statistical software. Results Reproductive data The breeding season showed a strong degree of synchrony; each reproductive stage occurred for most pairs within 22-d periods (Fig. 1). Eighty-seven percent (n = 57) of the nests were initiated before 26 May; hatch-day occurred between 26 May and 17 June in 90% (n = 57) of nests; and 64% (n = 43) of nests that fledged young did so between 6 June and 29 June (Fig. 1). During the study period, nest initiation was not detected after 17 June, which resulted in a unimodal pattern of breeding behavior (Fig. 1). Vocalizations There was no significant difference in the total number of calls among the seven 11-d periods (F = 1.1, df = 6, P = 0.37); however, of the 7 periods, I detected the lowest number of calls between 2 and 13 May prior to the season’s first nestlings (Fig. 4). The pattern of tonal- and harsh-continua calls varied seasonally (excluding T3 and H3 calls, respectively). Harsh long-distance calls were most common when clutch completion peaked and were less common during and after fledging (Fig. 5). Although tonal long-distance calls peaked during fledging, the change among the 7 periods was not significant (Fig. 5). Both calls were least common Figure 4. Mean number of Catharus fuscescens (Veery) calls detected during twenty-six 10-min point-counts visited during each of seven 11-d periods throughout the breeding season at a Delaware breeding site in 2006. Bars represent ± SE. Northeastern Naturalist 298 C.M. Heckscher 2018 Vol. 25, No. 2 during territory establishment and early nest initiation (Fig. 5). The mean number of harsh long-distance calls from the 7 periods was significantly greater than the mean number of tonal long-distance calls (t = -3.01, df = 317, P = 0.002; Fig. 5). Tonal short-distance calls were most common early in the season, whereas harsh shortdistance calls peaked at the end of the season (Fig. 5). From the 7 periods, the mean number of tonal short-distance calls was greater than the mean number of harsh short-distance calls (t = 1.6, df = 304, P = 0.05). I detected neither T3 nor H3 calls while Veeries were establishing territories (2 May–13 May; Fig. 5). However, once territories became established, these calls became more abundant, although the variation across the 7 periods was not significant and showed only a weak statistical trend (P ≤ 0.15) for T3 calls (Fig. 5). After nest initiation, T3 calls were most abundant but became less common once most young had fledged (Fig. 5). H3 calls were rare throughout the season but peaked from 6 to 17 June when most nests contained young or were fledging young; however, differences among the 7 periods were not significant (Fig. 5). The chatter call was most abundant early, while the majority of birds were establishing territories, initiating nests, and completing clutches. The use of this call then declined through the season. The seasonal decline in the use of chatter was significant (r2 = 0.63, F5 = 8.61, P = 0.03; Fig. 6). Figure 5. Seasonal patterns of Catharus fuscescens (Veery) harsh- and tonal-continua calls throughout the breeding season as detected during twenty-six 10-min point-counts visited during each of seven 11-day periods at a Delaware breeding site in 2006. Bars represent ± SE. All P-values are from Kruskal–Wallis tests of significant differences among the 7 reproductive periods. Northeastern Naturalist Vol. 25, No. 2 C.M. Heckscher 2018 299 Whistle calls occurred during territory establishment and nest initiation but were not recorded during peak incubation periods. They became more common again during and after fledging (Fig. 7). U-calls peaked during and just after most young fledged (Fig. 7). Downward calls, the least common Veery call, were most abundant Figure 6. Relationship between chatter calls and the progression of the Catharus fuscescens (Veery) nesting season. Data are from twenty-six 10-min point-counts visited during each of seven 11-day periods at a Delaware breeding site in 2006. Figure 7. Seasonal patterns of whistle, U-call, and downward Catharus fuscescens (Veery) calls as detected from twenty-six 10-min point-counts visited during each of seven 11-d periods at a Delaware breeding site in 2006. P-values from Kruskal–Wallis tests of significant differences among the seven periods: whistle P = 0.006, U-call P = 0.05, and downward P = 0.83. Northeastern Naturalist 300 C.M. Heckscher 2018 Vol. 25, No. 2 following territory establishment and nest initiation, but the change across the 7 periods was not significant (Fig. 7). Song was rare from first arrival until approximately 14 May when most pairs were initiating nests and completing clutches. The number of birds recorded singing during point counts remained steady thereafter until cessation of the breeding season ≥ 23 July (Fig. 8). The change in song production across the 7 periods was highly significant (Fig. 8). Discussion The reproductive stages of this Veery population showed a unimodal distribution pattern with rapid cessation—a typical pattern for a single-brooded species. In contrast, Hylocichla mustelina (Gmelin) (Wood Thrush) is double-brooded and has 2 seasonal periods for reproductive activity (Brown and Roth 2002). The pattern of vocalizations varied in accordance with the temporal shifts in reproduction. Overall, calls were more frequent when females were incubating and brooding and when adults were tending mobile fledglings, although differences among the stages were not significant. During brooding, while females are on nests, male Veeries regularly interact with neighboring males via calls and song as they defend territories and occasionally move beyond their territorial boundaries, likely seeking extra-pair copulations (cf. Halley et al. 2016). In a study population in Ontario, Bicknell’s Thrush also showed a peak in calling behavior during the female incubation period (Ball 2000). When fledglings are mobile, adults continually remain in contact with their young via calls (Heckscher 2017a). Calls were least common early in the breeding season during nest initiation, when females were building nests and males Figure 8. Pattern of Catharus fuscescens (Veery) song throughout the breeding season as measured by the number of individuals detected singing during twenty-six 10-min point-counts in each of seven 11-d periods at a Delaware breeding site in 2006. Bars represent ± SE. P-value from Kruskal–Wallis test of significant difference among the 7 periods: P < 0.0001. Northeastern Naturalist Vol. 25, No. 2 C.M. Heckscher 2018 301 were mate-guarding (see Halley 2014, Heckscher 2007). The number of individuals singing per point was lowest early in the season, rose and remained steady for several weeks, then terminated abruptly late in the season. There were some seasonal differences in the use of long- and short-distance continua calls. Long-distance calls were less common early in territory establishment and nest initiation. However, harsh long-distance calls became common after 14 May, showing that after males became settled and females began initiating clutches, males became engaged in long-distance interaction. Harsh long-distance calls may facilitate the assessor’s ability to judge the signaler’s precise location because they are frequency modulated, begin and terminate abruptly, and are of a vibrato nature. These properties make the harsh calls less susceptible to acoustic degradation (attenuation and reverberation) than tonal calls (Konishi 1973; Marler 1955, 1967; Wiley and Richards 1982), which allows them to be perceived as propagating from a finite direction (Richards and Wiley 1980, Wiley and Richards 1982). Consequently, they may be adaptive to both signalers and assessors because they provide spatial cues among widespread neighbors in dense forest. In contrast, tonal calls are less frequency-modulated, are of a pure tone nature, and begin and terminate gradually. Therefore, tonal calls may be more common in short-distance communication because they effectively conceal the precise location of the signaler from unintended assessors (i.e., they are more difficult from which to detect point-of-origin than harsh calls due to their acoustic properties). Concealing precise location from unintended assessors can be adaptive if unintended assessors might otherwise take advantage of that information (e.g., trespassing males might more readily seek extra-pair copulations from neighboring females if they know the location of the territory-holding male). Figure 9 depicts how harsh and tonal calls convey precise and imprecise location to assessors. Indeed, playback trials have shown that harsh continua calls are easier than tonal continua calls for Veeries to locate (Heckscher 2007). Harsh short-distance calls are often used by adults during fledgling management (Heckscher 2007); in the present study, they became more common later in the nesting season coinciding with that context. During fledgling management, adults often trespass on neighboring territories as they move or follow young through the forest (C. Heckscher, pers. observ.). Maintaining contact with fledglings via precise location-cues may be important in this context (Benedict 2007). Tonal short-distance calls were most common early in the breeding season, perhaps because they are used for within-territory communication among mates during courtship, mateguarding, and while females are incubating and brooding nestlings (Heckscher 2007). It is difficult for unintended assessors (e.g., neighboring males) to locate the point-of-origin of tonal short-distance calls, which may be an advantage to the signaler if there are unintended receivers nearby. T3 calls became common once eggs hatched, and they remained common through the fledging stage. This call is often used when nests or fledglings are threatened. T3 calls are also emitted during territory establishment and may represent a non-aggressive precise-location cue in that context, considering that the Northeastern Naturalist 302 C.M. Heckscher 2018 Vol. 25, No. 2 T3 call occurs on the tonal continuum and tonal calls are expected to be of a less aggressive nature compared to harsh calls (Morton 1977). T3 calls disappeared as fledglings became more independent. H3 calls were most common for a short period during fledging, suggesting a role in fledgling management. Adults often emit H3 calls when contacting fledglings prior to delivery of food, and older juveniles have been observed emitting the H3 call when begging for food (Heckscher 2007). The structure of H3 calls indicates they are extremely easy for assessors to locate because of their low frequency, broad-band, and harsh vibrato nature (Konishi 1973, Marler 1967, Wiley and Richards 1982), which facilitates the ability of an assessor (parent or fledgling) to locate a signaler in dense vegetation. Although uncommon in inter-territory communication among males, H3 calls also occur in that context (C. Heckscher, pers. observ.). There were also differences in the use of the 4 discrete calls. Chatter calls were common early in the season when males were becoming settled, often prior to the arrival of females. Chatter calls are of a harsh vibrato structure and should be easy for assessors to locate. They are frequently used in relation to aggression (Samuel 1972). Their function as an aggressive and easily locatable acoustic cue may assist in spatially coordinating and reinforcing territorial boundaries early in the season. Figure 9. Schematic diagram depicting the theoretical difference in how tonal and harsh calls are perceived by assessors (A) in multiple locations through 100 m of dense forest when the signaler (S) emits a call from one location. Due to differences in sound structure, as sound travels through forest, tonal calls are more difficult to locate and can be perceived by assessors as originating from an imprecise location, whereas harsh-frequency–modulated or vibrato-type calls are easy to locate and are perceived as originating from a precise location (cf. Richards and Wiley 1980, Wiley and Richards 1982). Northeastern Naturalist Vol. 25, No. 2 C.M. Heckscher 2018 303 The seasonal decline of chatter can be attributed to adults becoming increasingly settled on territories followed by the progressively greater number of nests that are completed, and consequently, those nest-owners becoming non-territorial in preparation for molt. Marshall (2000) described a similar call of Bicknell’s Thrush and also associated that call with aggressive behavior. Whistle calls were common during territory establishment and early nest initiation, disappeared in mid-season, and were common again after most nests had fledged. Indeed, whistle calls are used during territorial confrontation and courtship, possibly as an appeasement vocalization given their very high frequency and tonal nature (see Morton 1977). Therefore, their use early in the season is expected. Late-season use of whistles may be the result of adults trespassing on neighboring territories as they care for mobile fledglings. Trespassing adults caring for young may benefit from emitting an appeasement call to potentially hostile territory owners. U-calls were most common during fledgling management. Adults frequently use the U-call when attempting to contact concealed young, and they are also used among adults for the same purpose (Heckscher et al. 2017a). Downward calls were uncommon throughout the season but were most abundant once males became settled on territories. Considering their pure tone nature, they likely convey nonthreatening imprecise location cues among males. Veery song was absent or rare during territory establishment and did not become common until after nest initiation during incubation. Conversely, Ball (2000) reported that in Ontario, peak song-periods of Bicknell’s Thrush coincided with territory establishment and nest initiation. Of note, Ball’s reproductive data were limited, and most data used were obtained from outside the study area, which may be problematic if reproductive stages are not synchronized across the species’ range. The scarcity of Veery song early in the season has been emphasized by previous observers (Dilger 1956, Tyler 1949). Delay of song initiation may be due to a period of intense male mate-guarding after females arrive (Heckscher et al. 2017a). The primary functions of song are thought to be territory establishment and maintenance, attraction of mates, and individual identity (e.g., Falls 1982, Kroodsma and Miller 1996, Weary et al. 1990). Song was not emphasized in territory establishment, and if male quality can be adequately assessed by other means (e.g., resource-holding potential), then song may not be necessary for females to assess male quality. Indeed, in the rare instances when Veery courtship has been observed, song has been absent (Dilger 1956, Heckscher et al. 2017a, Tyler 1949). Weary et al. (1987) presented strong evidence that Veery song conveys individual identity, and Stein (1956) and Marshall (2000) proposed the same for Gray-cheeked Thrush. I hypothesize that Veery song conveys individual ownership of local resources after resources and mates have been secured, rather than act in the process of territory establishment, perhaps acting as a “fingerprint” on the landscape (see also Marshall 2000). Male Veeries have age-dependent social hierarchies: as males become older they absorb the territories of missing neighbors and surrounding younger Northeastern Naturalist 304 C.M. Heckscher 2018 Vol. 25, No. 2 males, trespassing aggressively and freely, supposedly because that increases the chances of successful extra-pair copulations with neighboring females (see Halley 2014, Halley and Heckscher 2012, Halley et al. 2016, Heckscher 2007). Regardless, a proximate effect of Veery song clearly is territory maintenance; however, the ultimate mechanism may be a cue to individual identity especially considering that individual recognition often evolves in tandem with social hierarchies (e.g., Barnard and Burke 1979). The acoustic complexity of Veery song is not expected to be retained through the thick vegetation that characterizes Veery breeding habitat because the subtle details of Veery song are easily obscured by attenuation and reverberation (Morton 1982). Thus, the finite elements of Veery song should degrade rapidly with increasing distance; consequently, it should be difficult to discern the precise location of a distant songster. This effect may be adaptive in the sense that the point-of-origin may be farther than it seems, thereby allowing a singing individual to deceive assessors and defend a greater area than it would otherwise. Conversely, simple call notes, which are relatively well preserved over great distances due to their simple sound structure, function effectively as spatial cues during territory establishment when location is important to convey. This reasoning provides an explanation for why calls instead of song are used early in the season during settlement, and the data presented herein confirm singing is delayed until after adults have acquired territories. Summary The unimodal distribution of the 4 major reproductive stages indicates this population is single-brooded. Veeries show seasonal patterns of calling behavior that vary with regard to their discrete reproductive contexts depending on their effectiveness for differing communicative means. Some calls were more common during territory establishment and nest initiation while others were more frequent during the incubation, nestling, or fledgling stages. In long-distance communication, harsh-continuum calls are more common than tonal-continuum calls, while in short-distance communication tonal-continuum calls are more common than harsh continuum calls. As documented in previous studies, harsh and tonal calls are assessed differently, depending on attenuation and reverberation thereby revealing or concealing the precise location of the signaler. This location-based framework, likely a product of the environment where there is impaired visibility (i.e., dense forest understory) typical of this species, may explain the patterns of calls used in the different stages of the breeding season as revealed herein. The scarcity of song coupled with the use of a variety of calls early in the season, especially the use of the chatter call, implies that calls were the primary vocal cues necessary to accomplish the spatial distribution of territory-holding males across the landscape. As reported by previous authors, singing is delayed in the Veery and apparently does not play a role in initial territory establishment or pair formation. The ultimate function of song may be as an individual recognition cue among neighbors after they have secured seasonal resources. Northeastern Naturalist Vol. 25, No. 2 C.M. Heckscher 2018 305 Acknowledgments I thank R.R. Roth, E.S. Morton, C.K. Williams, J.H. McDonald, D.W. Tallamy, and J.L. Woods for assistance in study design and helpful review of the research. Delaware Division of Parks and Recreation approved research permits and permitted study at White Clay Creek State Park. I am particularly grateful to C. Bennett, R. Line, and N. McFadden for their support. This manuscript benefitted from the efforts of 2 anonymous reviewers. Literature Cited Andrew, R.J. 1961. The motivational organization controlling the mobbing calls of the Blackbird (Turdus merula). IV. A general discussion of the calls of the Blackbird and certain other passerines. Behaviour 18:161–175. Ball, M. 2000. Vocal behavior of Bicknell’s Thrush. M.Sc. Thesis. Dalhousie University, Halifax, NS, Canada. Baptista, L.F. 1996. Nature and its nurturing in avian vocal development. Pp. 39–60, In D.E. Kroodsma and E.H. Miller (Eds.). Ecology and Evolution of Acoustic Communication in Birds. Cornell University Press, Ithaca, NY. 587 pp. Barnard, C.J., and T. Burk. 1979. Dominance hierarchies and the evolution of “individual recognition”. Journal of Theoretical Biology 81:65–73. Beletsky, L.D. 1991. Alert calls of male Red-winged Blackbirds: Call rate and function. Canadian Journal of Zoology. 69:2116–2120. Beletsky, L.D., B.J. Higgins, and G.H. Orians. 1986. Communication by changing signals: Call switching in Red-winged Blackbirds. Behavioural Ecology and Sociobiology 18:221–229. Benedict, L. 2007. Offspring discrimination without recognition: California Towhee responses to chick-distress calls. Condor 109:79–87. Bent, A. C. 1949. Life histories of North American thrushes, kinglets, and their allies. Dover Publications, Inc., New York, NY. 452 pp. Brown, W.P., and R.R. Roth. 2002. Temporal patterns of fitness and survival in the Wood Thrush. Ecology 83:958–969. Dilger, W.C. 1956. Hostile behavior and reproductive isolating mechanisms in the avian genera Catharus and Hylocichla. Auk 73:313–353. Falls, J.B. 1982. Individual recognition by sound in birds. Pp. 237–273, In D.E. Kroodsma and E.H. Miller (Eds.). Acoustic Communication in Birds, Volume 2. Academic Press, New York, NY. 392 pp. Ficken, M.S., E.D. Hailman, and J.P. Hailman. 1994. The chick-a-dee call system of the Mexican Chickadee. Condor 96:70–82. Grabowski, G.L. 1979. Vocalizations of the Rufous-backed Thrush (Turdus rufopalliatus) in Guerrero, Mexico. Condor 81:409–416. Greig-Smith, P.W. 1982. Seasonal patterns of song production by male Stonechats, Saxicola torquata. Ornis Scandinavica 13:225–231. Halley, M.R. 2014. Kin structure and mating system of the Veery (Catharus fuscescens) in the Mid-Atlantic Piedmont. M.Sc. Thesis. Delaware State University, Dover, DE. Halley, M.R., and C.M. Heckscher. 2012. Multiple male feeders at nests of the Veery. The Wilson Journal of Ornithology 124:396–399. Halley, M.R., C.M. Heckscher and V. Kalavacharla. 2016. Multi-generational kinship, multiple mating, and flexible modes of parental care in a breeding population of the Veery (Catharus fuscescens), a trans-hemispheric migratory songbird. PLoS ONE 11(6):e0157051. Northeastern Naturalist 306 C.M. Heckscher 2018 Vol. 25, No. 2 Heckscher, C.M. 2004. Veery nest sites in a Mid-Atlantic Piedmont forest: Vegetative physiognomy and use of alien shrubs. American Midland Naturalist 151:326–337. Heckscher, C.M. 2007. Use of the Veery (Catharus fuscescens) call repertoire in vocal communication. Ph.D. Dissertation. University of Delaware, Newark, DE. Heckscher, C.M., L.R. Bevier, A.F. Poole, W. Moskoff, P. Pyle, and M.A. Patten. 2017a. Veery (Catharus fuscescens), version 3.0. In P.G. Rodewald (Ed.). The Birds of North America. Cornell Lab of Ornithology, Ithaca, NY. Available online at https://doi. org/10.2173/bna.veery.03. Accessed 14 November 2017 Heckscher, C.M., M. Gutierrez Ramirez, A.H. Kneidel. 2017b. Reproductive outcomes determine the timing of arrival and settlement of a single-brooded Neartic–Neotropical migrant songbird (Catharus fuscescens) in South America. The Auk: Ornithological Advances 134:842–856. Konishi, M. 1973. Locatable and nonlocatable acoustic signals for Barn Owls. The American Naturalist 107:775–785. Kramer, S.P. 1980. The vocal behavior of Townsend’s Solitaires on winter territories. M.Sc. Thesis. Northern Arizona University, Flagstaff, AZ. Kroodsma, D.E., and E.H. Miller. 1996. Acoustic Communication in Birds, Volume 2. Academic Press, New York, NY. 587 pp. Kumar, A. 2004. Acoustic communication in the Red-vented Bulbul, Pycnonotus cafer. Annals of the Brazilian Academy of Sciences 76:350–358. Lampe, H.M., and Y.O. Espmark. 1987. Singing activity and song pattern of the Redwing Turdus iliacus during the breeding season. Ornis Scandinavica 18:179–185. Logan, M. 2010. Biostatistical Design and Analysis Using R: A Practical Guide. Wiley- Blackwell Publishing, West Sussex, UK. 574 pp. Marler, P. 1955. Characteristics of some alarm calls. Nature 176:6–8. Marler, P. 1967. Animal communication signals. Science 157:769–774. Marshall, J.T. 2000. The Gray-cheeked Thrush, Catharus minimus, and its New England subspecies, Bicknell’s Thrush, Catharus minimus bicknelli. Publication of the Nuttall Ornithological Club, No. 28. Miller, P.J.O., A.D. Shapiro, P.L. Tyack, and A.R. Solows. 2004. Call-type matching in vocal exchanges of free-ranging resident killer whales, Orcinus orca. Animal Behavior 67:1099–1107. Morton, E.S. 1977. On the occurrence and significance of motivation–structural rules in some bird and mammal sounds. The American Naturalist 109:17–34. Morton, E.S. 1982. Grading, discreteness, redundancy, and motivation–structural rules. Pp. 183–212, In D.E. Kroodsma and E.H. Miller (Eds.). Acoustic Communication in Birds, Volume 1. Academic Press, Inc., New York, NY. 360 pp. R Development Core Team. 2015. R: A language and environment for statistical computing. R Foundation for Statistical Computing, Vienna, Austria. Richards, D.G., and R.H. Wiley. 1980. Reverberations and amplitude fluctuations in the propagation of sound in a forest: Implications for animal communication. The American Naturalist 115:381–399. Rimmer, C.C., J.L. Atwood, K.P. McFarland, and L.R. Nagy. 1996. Population density, vocal behavior, and recommended survey methods for Bicknell’s Thrush. Wilson Bulletin 108:639–649. Samuel, D.E. 1972. Song variation and other vocalizations of Veeries. Bird Banding 43:118–127. Sosa-López, J.R., and D.J. Mennill. 2014. The vocal behavior of the Brown-throated Wren (Troglodytes brunneicollis): Song structure, repertoires, sharing, syntax, and diel variation. Journal of Ornithology 155:435–446. Northeastern Naturalist Vol. 25, No. 2 C.M. Heckscher 2018 307 Stacier, C.A., D.A Spector, and A.G. Horn. 1996. The dawn chorus and other diel patterns in acoustic signaling. Pp. 426–454, In, D.E. Kroodsma and E.H. Miller (Eds.). Ecology and Evolution of Acoustic Communication in Birds. Cornell University Press, Ithaca, NY. 587 pp. Stein, R.C. 1956. A comparative study of “advertising song” in the Hylocichla thrushes. The Auk 73:503–512. Tyack, P.L. 1997. Studying how cetaceans use sound to explore their environment. Pp. 251–297, In D.H. Owings, M.D. Beecher, and N.S. Thompson (Eds.). Perspectives in Ethology: Communication, Volume 12. Plenum Publishing Corporation, New York, NY. 481 pp. Tyler, W.M. 1949. Hylocichla fuscescens fuscescens (Stephens) Veery. Pp. 217–231, In A.C. Bent (Ed.). Life Histories of North American Thrushes, Kinglets, and their Allies. US National Museum Bull., No 196. 452 pp. Weary, D.M., R.E. Lemon, and E.M. Date. 1987. Neighbor–stranger discrimination by song in the Veery, a species with song repertoires. Canadian Journal of Zoology 65:1206–1209. Weary, D.M., K.J. Norris, and J.B. Falls. 1990. Song features birds use to identify individuals. The Auk 107:623–625. Wiley, R.H., and D.G. Richards. 1982. Adaptations for acoustic communication in birds: Sound transmission and signal detection. Pp. 131–170, In D.E. Kroodsma and E.H. Miller (Eds.). Acoustic Communication in Birds. Academic Press, New York, NY. 362 pp.