young left their nests, and females who did not receive male assistance in feeding nestlings gave Chits at higher rates than did assisted females. The rate at ...
Anita. Behav., 1987, 35, 1416-1423
Seasonal change in the vocal behaviour of female red-winged blackbirds, Agelaiusphoeniceus K E N Y A S U K A W A , R E B E C C A A. B O L E Y & SUE E. S I M O N
Beloit College, Department of Biology, Beloit, Wisconsin 53511, U.S.A.
Abstract. Seasonal change in the vocal behaviour of female red-winged blackbirds, Agelaius phoeniceus, was observed in order to test hypotheses for the function of their vocalizations. The 'Chit' vocalization was given at relatively consistent rates throughout the nesting cycle, but females rarely gave Chits once their young left their nests, and females who did not receive male assistance in feeding nestlings gave Chits at higher rates than did assisted females. The rate at which the intrasexually aggressive 'Teer' was given declined through the nesting season and primary (first-to-nest) female redwings gave Teers at lower rates than did later-nesting females. It is tentatively concluded that the Chit functions in intersexual communication and the Teer functions to delay nest initiation by late-arriving females.
A number of potential functions of bird song have been proposed, but identifying the function in a given species has been difficult because sufficient and consistent observational and experimental evidence is often lacking. One exception to this, however, is provided by studies of the male redwinged blackbird, Agelaius phoeniceus. Many observational studies have proposed that male redwing song functions in territory defence (see Nero 1984 for a recent summary), and experimental studies have consistently supported this hypothesis (Peek 1972; Smith 1976, 1979; Yasukawa 1981a; Yasukawa et al. 1982). In this paper, we present the results of our attempt to identify the functions of the vocal behaviour of female redwinged blackbirds. Female Red-winged Blackbird Vocal Behaviour
Female red-winged blackbirds have many distinct vocalizations (Orians & Christman 1968). Most calls are short, simple signals of alarm or agitation, but two are long, loud and complex (Nero 1956a; Orians & Christman 1968; Beletsky 1983a). One of these, the 'Chit' (Hurly & Robertson 1984), is described by Nero (1956a) as a halting, laboured vocalization rendered as 'pee-chee-tachee-ta-chee-chee' (Fig. la), and is thought to function in intersexual communication (Beletsky 1983a, 1985; Beletsky & Orians 1985). The other, the 'Teer' (Hurly & Robertson 1984), is described by Nero (1956a) as a series of shrill, rapid notes rendered as 'spit-a-chew-chew-chew' (Fig. I b), and
appears to function in intrasexual agonistic interactions (Nero 1956a; Orians & Christman 1968; Beletsky 1983a). Despite the recent focus on female red-winged blackbird vocal behaviour, we only partially understand the functions of their vocalizations, and the evidence is far from consistent. For example, although female redwings have at least two distinct songs, female response to broadcasts of the two appears to be identical (Betetsky 1983b, c). Further, although the contexts in which female redwings give the Chit suggest an intersexual communication function (Beletsky 1983a, 1985), experimental studies have failed to support this hypothesis (Beletsky 1983b; Beletsky & Corral 1983). Finally, although the Teer is clearly associated with agonistic interactions among females (Nero 1956a; Orians & Christman 1968; Beletsky 1983a, b, c), the function of these agonistic interactions, and therefore of the Teer, is unclear (Searcy 1986). At present there are two major hypotheses for the function of the Chit of female red-winged blackbirds. The intersexual communication hypothesis proposes that the Chit functions to maintain the pair bond (Beletsky 1983a, 1985; Beletsky & Orians 1985). This vocal pair-bond maintenance would be advantageous to a female because it could ensure unrestricted access to resources in the male's territory, reduce make interference and/or aggression, promote male vigilance against predators, or inform the male of the female's progress through the nesting cycle. In
1416
Yasukawa et al.: Female redwing vocal behaviour
priority of access to resources in the male's territory, to male defence against predators, or to male assistance in feeding young.
8 a
3
:~ Seasonal Change in Female Vocal Behaviour
,2_ 2 1 0
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1417
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Figure 1. Sound spectrograms of (a) the Chit and (b) the Teer of female red-winged blackbirds.
contrast, the intrasexual communication hypothesis proposes that the Chit functions to coordinate the activities of females nesting in proximity to one another (Beletsky & Orians 1985). This vocal communication could reduce predation by coordinating female vigilance and nest defence, or it could increase foraging efficiency by informing females of locations of profitable foraging areas. There are also two major hypotheses for the function of the Teer of female red-winged blackbirds. The territory defence hypothesis proposes that the Teer functions to defend the female's territory (Beletsky 1983a). Vocal advertisement would be an inexpensive way to reduce female density, to increase nest dispersion, or to defend territorial resources such as food, nest sites, or cover from predators. In contrast, the status defence hypothesis proposes that the Teer functions to defend a female's status within a harem (Yasukawa & Searcy 1982). If, as a result of her status, a high-ranking female has higher reproductive success than a female of lower rank, then it would be advantageous for her to defend her status vocally. Advantages of high status could include
Functional studies of bird song can be based on the pattern of seasonal change in vocal behaviour. For example, one of the early attempts to test the hypothesis that male red-winged blackbird song functions in territory defence was that of Peek (1971), who examined seasonal change in song rate and other aspects of male redwing behaviour. Peek (1971) demonstrated that male song rates were highest during the early portion of the breeding season when males were establishing their territories, and when competition for territories was most intense. This seasonal pattern is consistent with the territory defence hypothesis (Peek 1971). Seasonal change can also be used to test hypotheses for the function of female vocal behaviour. For example, Ritchison (1986) used this method to study song in the female northern cardinal, Card# nalis cardinalis. The seasonal timing and contexts in which female northern cardinals sang suggested a pair-bonding function, and playback experiments supported this hypothesis (Ritchison 1986). We studied seasonal change in the vocal behaviour of female red-winged blackbirds to test the four hypotheses for the functions of their Chit and Teer vocalizations. Each of the four hypotheses makes specific predictions about the pattern of seasonal change in female red-winged blackbird vocal behaviour. The intersexual communication hypothesis predicts that the Chit should be given at relatively high rates throughout the breeding season because the pair bond is maintained throughout this period. This prediction assumes that the net benefit of pairbond maintenance, and therefore of intersexual communication, remains constant throughout the breeding season. In contrast, the intrasexual communication hypothesis predicts that the Chit should be given at low rates prior to the onset of nesting, be given at relatively high rates through the nesting cycle, and then decline in frequency once the young leave the nest because coordinatioriOwith other females is necessary only while nests are active and vulnerable to predation. Thus the benefit of intrasexual communication is high while
Animal Behaviour, 35, 5
1418
the nest contains eggs or nestlings because coordinated nest defence and/or vigilance is assumed to reduce the risk of predation. Both the territory and status defence hypotheses predict that the Teer should be given at relatively high rates early in the breeding season when aggressive behaviour and the rate of interaction among females are highest, and both predict that Teer rates should decline thereafter. Because territory defence would be advantageous for all females residing on a male's territory, however, the territory defence hypothesis also predicts that the pattern of seasonal change in vocal behaviour should not vary with a female's status within a harem. Although females are likely to vary in their vocalization rates, this variance should be independent of female status because the assumed benefits of territory defence should accrue to all females within a harem. In contrast, the status defence hypothesis predicts status-related differences in the pattern of seasonal change in vocal behaviour. Primary (first-to-nest) females and/or females who receive male assistance in feeding young are most likely to benefit from vocal status defence, therefore these females should sing the Teer at higher rates than do non-primary and/or unassisted females. We tested these predictions by observing the seasonal change in the vocal behaviour of female red-winged blackbirds.
METHODS
The study site was Newark Prairie, approximately 13 km west of the Beloit College campus in southern Rock County, Wisconsin, U.S.A. Newark Prairie is a 13-ha short-grass and sedgemeadow habitat, which is maintained by approximately biannual burning, but is otherwise undisturbed. Approximately 30 male redwings defended territories on the prairie. We employed 10-rain focal-animal sampling throughout the breeding season of 1985, from 22 April to 12 July, and between sunrise and 1000 hours. We observed 25 female red-winged blackbirds on the territories of 19 males. Females were identified by unique colour combinations of plastic bands ( N = 8) or by a combination of plumage, vocalization and nest location ( N = 17). Females were captured under licence in mist nets or Potter
traps baited with cracked corn, and were marked with United States Fish and Wildlife Service numbered aluminium bands and coloured plastic bands. We classified their vocalizations by ear in the field following Nero (1956a), Beletsky (1983a) and Hurly & Robertson (1984). The Chit (Fig. la) corresponds with the halting, laboured vocalization described by Nero (1956a), and with the Type 1 of Beletsky (1983a). The Teer (Fig. lb) corresponds with the shrill vocalization described by Nero (1956a), and with the Type 2 of Beletsky (1983a). The Chit and Teer were sometimes combined (see also Beletsky 1983a, 1985). These combinations were classified for analysis as Teers because they too were given in agonistic contexts. We calculated rates of vocalization and compiled them by date and by stage of the nesting cycle. Following Peek (1971), we divided the breeding season into 10-day blocks, from 21 April to 29 June. In addition, we divided the nesting cycle into six periods: Pre-nesting, Nest Building, Laying, Incubating, Brooding and Feeding Fledglings. Because female red-winged blackbirds begin incubating with the penultimate egg, our Incubating Period underestimates the actual period of incubation by 1 day. We were able to observe only the first 5 days of the fledgling period. Rates of vocalization were also compiled separately for females of known status. Females classified as primary were first to initiate nesting within a male's territory, while non-primary females nested later. In our population, each male had one primary and up to four non-primary females. In addition, We classified each female whose nestlings survived to day 6 of the Brooding Period as assisted or unassisted by observing her nest for 30 rain each day and noting whether the male fed the nestlings (see Patterson 1979; Yasukawa 1981b; Yasukawa & Searcy 1982; Muldal et al. 1986). All nest observations were conducted from blinds or parked vehicles. All observations were made between 0600 and 1200 hours. We did not observe during periods of heavy rain or high wind. The effects of the stage of the nesting cycle and the status of the female on vocal behaviour were analysed statistically using a ModeM, two-way ANOVA. When the ANOVA indicated a significant status effect, we performed paired comparisons using the planned comparisons t-test (Sokal & Rohlf 1981). Statistical significance was accepted at the 0'05 level.
Yasukawa et al.: Female redwing vocal behaviour RESULTS
1419
z~
We observed the vocal behaviour of 25 females: 13 primary (first-to-nest) and 12 non-primary (laternesting). Only 21 females had nestlings that survived to day 6 of the Brooding Period, and 13 of these females received assistance from their mates in feeding nestlings.
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When plotted by date (Fig. 2), Chit rate was high early in the breeding season, was somewhat lower through the middle of the season, and was high again late in the season. One complication in the use of date to examine seasonal change in vocalization rate is that females did not nest synchronously on our study area. M a n y females initiated nesting late in the breeding season, either because their earlier attempts failed, or because they arrived late on our study area. One way to control for this asynchrony is to compile vocalization rates by stage of the nesting cycle, rather than by date. A somewhat different seasonal pattern emerged when vocalization rates were thus plotted (Fig. 3). Chits were given at relatively consistent rates throughout the nesting cycle, but their rates dropped precipitously once the young left the nest. When Chit rates were plotted separately for primary and non-primary females (Fig. 4), and for assisted and unassisted females (Fig. 5), some status-related differences emerged. Chit rates of primary and non-primary females did not differ
(22)
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LA IN BR Stage of the Nesting Cycle
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Figure 3. Seasonal change in vocalization rates of female
red-winged blackbirds. Total rate (solid line), Chit rate (dot-dash line), and Teer rate (dashed line) are plotted by stage of the nesting cycle. PN=Pre-nesting, NB =Nest Building, LA = Laying, IN = Incubating, BR = Brooding and FL=Feeding Fledglings. Number of females observed and number of 10-min focal-animal observations (in parentheses) are shown.
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Figure 4. Comparison of seasonal change in Chit rates of primary (solid line) and non-primary (dashed line) female red-winged blackbirds. Stages of the nesting season as in Fig. 3. Sample sizes are shown in Fig. 6.
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Figure 2. Seasonal change in vocalization rates of female red-winged blackbirds. Total rate (solid line), Chit rate (dot-dash line), and Teer rate (dashed line) are plotted by date. Number of females observed and number of 10-min focal-animal observations (in parentheses) are shown.
significantly (F=0.008, P>0"9), but unassisted females gave Chits at significantly higher rates than did assisted females ( F = 6.34, P < 0-05). Unassisted females gave Chits at significantly higher rates (1.82/rain) than did assisted females (0-62/rain) during the Nest Building Period (t=2.54, P0-05), although the rates for unassisted"and assisted females during the Incubating Period (l'05/min and 0.64/rain, respectively) were marginally non-significant (P < 0" 1).
1420
Animal Behaviour, 35, 5 1.0[
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Figure 5. Comparison of seasonal change in Chit rates of assisted (solid line) and unassisted (dashed line) female red-winged blackbirds. Stages of the nesting cycle as in Fig. 3. Number of females observed and number of 10min focal-animal observations (in parentheses) are shown.
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Figure 7. Comparison of seasonal change in Teer rates of assisted (solid line) and unassisted (dashed line) female red-winged blackbirds. Stages of the nesting cycle as in Fig. 3. Sample sizes are shown in Fig. 5. rates than did primary females i F = 9.80, P < 0.01), but Teer rates of assisted and unassisted females i F = 0.002) did not differ significantly (P > 0'9). Non-primary females gave Teers at significantly higher rates (0.51/mini than did primary females (0.07/mini during the Laying Period (t=3.31, P < 0-01). Comparisons of Teer rates for primary and non-primary females during all other stages of the nesting cycle were not statistically significant (P > 0.05).
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Figure 6. Comparison of seasonal change in Teer rates of primary (solid line)and non-primary (dashed line) female red-winged blackbirds. Stages of the nesting cycle as in Fig. 3. Number of females observed and number of 10min focal-animal observations (in parentheses) are shown.
Seasonal Change in Teer Rate When plotted by date (Fig. 2), Teer rate was high early in the breeding season and was low once nesting began on 30 April When plotted by stage of the nesting cycle (Fig. 3), Teer rate was high prior to the onset of nesting, and was low once incubation began. When Teer rates were plotted separately for primary and non-primary females (Fig. 6), and for assisted and unassisted females (Fig. 7), some status-related differences again emerged. Nonprimary females gave Teers at significantly higher
Although females of most species of song birds have limited vocal repertoires, in some species female vocalizations are prominent. Female vocal behaviour has recently begun to receive attention, and several functional studies have been attempted. Proposed functions of female vocalization include territory defence and pair-bond formation (Nottebohm 1975; Ritchison 1986), pair-bond maintenance (Beletsky 1982, 1983a), family-group maintenance and reproductive synchronization (Ritchison 1983, 1986) and status defence (Yasukawa & Searcy 1982). Our study of the female redwinged blackbird demonstrated seasonal change in vocal behaviour, which we used to test hypotheses for the function of female redwing vocalizations. Intrasexual Versus Intersexual Communication Functions of the Chit Our results support the presumed intersexual function of the Chit. This vocalization was given at high rates early in the breeding season and prior to
Yasukawa et aL: Female redwing vocal behaviour
the onset of nesting, and it was given relatively consistently throughout the nesting cycle. Once nesting began, Chits were often given in response to the male's song (see also Beletsky 1983a, 1985), and when the female left and returned to her nest (see also Beletsky & Orians 1985). If the Chit functions to maintain the pair bond, then we might expect rates to be high initially, as the pair bond is being formed, and consistent thereafter, as the pair bond is maintained. We might not expect Chit rates to fall sharply when the young leave the nest, however, because the pair bond continues through the period of dependence. Females continue to have access to the male's territory, and males continue to feed fledglings (Nero 1956a, b; personal observation). Perhaps, however, the Chit serves pair-bond maintenance functions throughout the nesting cycle, but the costs and benefits of this vocal maintenance change once the young leave the nest, producing a sharp drop in the net benefit of the Chit during the Fledgling Period. Our results do not support the intrasexual communication hypothesis. If the Chit functions to coordinate nest defence among females nesting in proximity to one another, then we would expect Chit rates to be high once nest construction or egg laying begins, but we would not expect high rates prior to the onset of nesting because coordination would be unnecessary prior to that time. Our results do not conform to these expectations. In addition, we would expect a vocally mediated coordination of female nest attendance, but there is no evidence of such coordination (Beletsky & Orians 1985). A study ofintersexual song answering in the redwinged blackbird (Beletsky 1985) described results similar to those of the present study. Females sang Chits in response to the songs of males, and this song answering was most likely when the males sang in sexual contexts. In addition, females were most likely to answer male song during the Preincubating and Incubating Periods, but the proportion of male songs answered declined during the Brooding Period, and was very low during the Fledgling Period (Beletsky 1985). Neither the intrasexual nor the intersexual communication hypothesis predicted a status-related difference in Chit rates because all females were presumed to benefit equally from vocal coordination and pair-bond maintenance. Our results, however, demonstrate that unassisted females gave
1421
Chits at higher rates during the Nest Building Period than did assisted females. Apparently there are status-related differences in the costs and benefits of the Chit. One potential explanation for this difference is that nests of unassisted females are at greater risk of predation than are nests of assisted females. A male may be more likely to defend the nest of the female he assists, or he may be better informed about the status of the nesting attempt. An unassisted female may therefore benefit by informing her mate vocally that her nest also requires defence against predators. Another possibility is that an unassisted female risks a greater reduction in nesting success as a result of male interference or attention than does an assisted female. An unassisted female may therefore benefit by vocalizing to reduce direct interaction with her mate (Beletsky & Orians 1985). A third possibility is that the Chit Junctions as a request for assistance from the male in feeding young. Unassisted females may sing Chits at higher rates than do assisted females because they are requesting assistance. In any case, more information on status-related differences in intersexual interactions of red-winged blackbirds is needed.
Territory Versus Status Defence Functions of the Teer Several studies have shown that the Teer is given by female red-winged blackbirds in intrasexual agonistic interactions (Nero 1956a; Orians & Christman 1968; Beletsky 1983a, b, c; Hurly & Robertson 1984). In addition, the Teer is clumped in time early in the breeding season, suggesting that females interact vocally,by responding to the Teers of others with aggressive vocalizations of their own (Hurly & Robertson 1984). Our results are consistent with the presumed agonistic function of the Teer. Female red-winged blackbirds gave the Teer at high rates both early in the breeding season and prior to nest building. During these times, female redwings interact aggressively and residents respond with vocal and visual displays to the presence of other females (Nero 1956a, b; Orians & Christman 1968; Hurly & Robertson 1984). While it seems clear that the Teer is an agonistic vocal signal, the function of female-female aggression in the red-winged blackbird has not been clearly established. There is evidence that female redwings occupy exclusive areas within the territories of males (Hurty & Robertson 1984), but resident
1422
Animal Behaviour, 35, 5
females often tolerate intruders and other residents once nesting has begun (Nero 1956a, b; Orians 1980), and Orians (1980) describes female redwings as only weakly territorial. Further, display sites of females overlap considerably, and the restriction of activity to areas near nests may be sufficient to account for apparently exclusive occupation of putative territories by females (Searcy 1986). Removal experiments have indicated that the presence of nesting female red-winged blackbirds within a male's territory may inhibit further recruitment into the male's harem (Hurly & Robertson 1985), but female redwing aggressive behaviour is not necessarily effective in preventing additional females from settling in an area (Nero 1956b; Orians & Christman 1968; Orians 1980), and there is no evidence of over-dispersion of nest sites (Yasukawa & Searcy 1981) or of territory defence by all females in a harem (Yasukawa & Searcy 1982). Our results do not support the hypothesis that the Teer functions to defend a female's territory (Nero 1956a, b; Beletsky 1983a; Hurly & Robertson 1984, 1985). The territory defence hypothesis predicts that vocal advertisement should not vary with female status because all females would benefit from vocal territory defence. Our results, however, demonstrate that female red-winged blackbird vocal behaviour varies with female status. Both primary and non-primary females gave Teers at high rates during the Pre-nesting Period, but non-primary females gave Teers at significantly higher rates during the Laying Period than did primary females. Our results are also not consistent with the status defence hypothesis, which proposes that the Teer functions to defend a female's status within a harem by delaying or preventing nesting by females of lower status. According to the status defence hypothesis, primary females should give the Teer at higher rates than non-primary females to defend their primary status, and this difference is most likely to occur early in the nesting cycle. Although Teer rates of primary and non-primary female redwings did differ significantly early in the nesting cycle, non-primary females gave Teers at higher rates than did primary females. The seasonal pattern of change in Teer rate that we observed supported neither of the hypotheses we tested. Our results, however, do suggest that there are status-related differences in the costs and benefits of singing the Teer in female red-winged
blackbirds. One explanation for the differences we observed is that primary females sing the Teer at high rates during the Pre-nesting Period to delay nesting by other females and thus to preserve their primary status. Once nest construction has begun, however, such status defence is unnecessary, and rates of the Teer drop in primary females. In contrast, non-primary females give Teers at high rates until the onset of incubation. One potential advantage of this relatively prolonged use of the Teer is that non-primary females may suffer a greater reduction in reproductive success than do primary females as a result of further recruitment to the harem. Such reduction could occur as a result of the increased density of nesting females (Weatherhead & Robertson 1977) and/or as a result of the reduced probability of male assistance in feeding nestlings (Patterson 1979; Muldal et al. 1986). The greater risk to non-primary than primary females could result from age/experience differences (Crawford 1977) and/or from differences in the strength of the pair bond (Patterson 1979). Another explanation is that there are statusrelated differences in the degree or expression of territorial behaviour in female red-winged blackbirds. For example, a primary female could defend all or portions of her mate's territory, while a nonprimary female could be non-territorial. In contrast to vocal territory defence by primary females, non-primary females could use vocal advertisement to delay nesting by later-arriving or nonresident females, as described above. In any case, further tests of the territory defence hypothesis should focus on the advertisement behaviour of primary females, while tests of the status defence hypothesis will require detailed examination of the differences between primary and non-primary females. ACKNOWLEDGMENTS We are pleased to thank W. A. Searcy, L. D. Beletsky, J. Zanocco, (3. Ritchison and an anonymous reviewer for their comments on earlier drafts of this paper. This material is based on work supported by the National Science Foundation under Grant No. BNS 84-05221. REFERENCES
Beletsky, L. D. 1982. Vocalizations of female northern orioles. Condor, 84, 445~447.
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(Received 6 June 1986; revised 8 October 1986; MS. number: A4798)
