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Avian alarm calling: Is there an audience effect?
1570
Animal Behaviour, 34, 5
sequence of resources to the exclusion of another resource which is readily available in the immediate vicinity. Our observations of D. melanogaster foraging in the three patterns suggest that they remain within the pattern in part because of a forward-moving tendency between arrival and departure from a drop, also noted in the foraging behaviour of honeybees (Waddington & Heinrich 1981) and houseflies (Fromm & Bell, unpublished data). However, even the forward-moving tendency seems to be relaxed in the course of dropfinding, since forward-moving orientation would not explain the results in the ZIG pattern. Alternatively, a fly might be able to determine the time it takes to walk from one sucrose drop to another and use this information in ascertaining the spatial configuration of a patch. However, this explanation seems unlikely because the interdrop time intervals for an individual fly are highly variable (Tortorici, unpublished data). The clear benefit gained by this modification of search orientation relative to the spatial configuration of resources is a short-term constraint of search orientation parameters to a resource arrangement that is predictable for a shorter time period. It is possible that D. melanogaster, and perhaps other organisms, exploit resources in a food patch by processing proprioceptive information obtained from spatial locomotory movements, producing short-term retention of the spatial patterning of resources. Supported by the National Science Foundation Psychobiology Program (BNS 85-05679). The authors appreciate the critical comments of R. Jander.
Bond, A. B. 1980.Optimal foraging in a uniform habitat: the search mechanism of the green lacewing. Anim. Behav., 28, 10-19. Dethier, V. G. 1957. Communication by insects: physiology of dancing. Science, N.Y., 125, 331-336. Dixon, A. F. G. 1959. An experimental study of the searching behaviour of the predatory coccinellidbeetle Adelia bipunctata (L.). J. Anim. Ecol., 28, 259-281. Nelson, M. C. 1977. The blowfly's dance: role in the regulation of food intake. J. Insect Physiol., 23, 603612. Smith, J. N. M. i974. The food searching behaviour of two European thrushes. II: The adaptiveness of the search patterns. Behaviour, 49, 1 61. Waage, J. K. 1977. Behavioural aspects of foraging in the parasitoid Nemeritis canescens (Grav.). Ph.D. thesis, University of London, London. Waage, J. K. 1978. Arrestment responses of the parasitoid Nemeritis canescens to a contact chemical produced by its host, Plodia interpunctella. Physiol. Entomol., 3, 135-146. Waddington, K. D. & Heinrich, B. 1981. Patterns of movement and floral choice by foraging bees. In: Foraging Behavior (Ed. by A. C. Kamil & T. D. Sargent), pp. 215-230. New York: Garland Press.
(Received 16 December 1985; revised 1 April 1986; MS. number As-377) Avian Alarm Calling: Is there an Audience Effect?
Alarm calling, an important facet of anti-predator behaviour, is widespread among vertebrates. Various functions of alarm calling have been proposed which are not mutually exclusive, but which differ in their relative emphasis on the effects of alarm signals either on the predator or on other prey (reviewed in Klump & Shalter 1984). In the latter CATHYTORTORICI* case, calls may warn kin, unrelated conspecific ALISON BRODYt companions (Sherman 1985) or even members of WILLIAMJ. BELL~ other species of impending danger (Klump & Shalter 1984). In such situations, one might predict *~ Department of Entomology, University of Kansas, Lawrence, KS 66045, that alarm calling would be elicited in the presence of appropriate signal receivers, but withheld in U.S.A. ? Department of Systematics & Ecology, Univer- their absence. This hypothesis has received support from SuUivan's (1985) study on free-ranging sity of Kansas, Lawrence, KS 66045, U.S.A. downy woodpeckers (Picoides pubescens). Field Department of Physiology & Cell Biology, University of Kansas, Lawrence, KS 66045, studies of the role of the immediate social environment on alarm production are difficult because the U.S.A. relevant variables such as spatial proximity of companions and approaching predators cannot References easily be controlled. Cheney & Seyfarth (1985) Bell, W. J. 1985. Sources of information controlling have shown that in captive vervet monkeys alarm motor patterns in arthropod local search orientation. J. calling is significantlyaffected by the social context Insect Physiol., 31, 837-847. Bell, W. J., Tortorici, C., Roggero, R. J., Kipp, L. R. & in which the animal finds itself at the moment of Tobin, T. R. 1985. Sucrose-stimulatedsearching beha- predator detection. Here we report on laboratory data in which viour of Drosophila melanogasterin a uniform habitat: modulation by period of deprivation. Anita. Behav., 33, domestic chickens (Gallus domesticus) responded 436-448. to systematic variation of social contexts and
Short Communications predator approach. The predator's approach was controlled by the use of small aerial models of raptors, similar to those employed in the classic experiments of Lorenz, Tinbergen and others (Schleidt 1961). We focused on the calling behaviour of cockerels, which are known to give specific alarm calls to different classes of natural predators (Konishi 1963; Guyomarc'h 1974). Six mated Golden Sebright cockerels were tested in a chamber 3.5 m long and 1.2 m wide, with walls 1.8 m high. The subject was confined to a small cage (1.0 x 0.6 x 0.6 m) in the centre of the chamber. When required, a second bird was introduced into another cage immediately adjacent. Two tracks of double nylon monofilaments were set at 2.3 m above ground, located 40 cm on either side of the middle of the long axis of the chamber. The models, pieces of black cardboard shaped like a raptor, measured 5.0 and 7-5 cm across. As presented, they mimicked a hawk with a wingspread of 120 cm flying either at 48 or 32 m above the ground. An electric motor pulled the models by means of a third filament at 0.5 m/s and 0.7 m/s, respectively, giving an apparent speed of approximately 45 kin/ h, the cruising speed of several raptors reported by Meinerthagen (1955). We tested each male in two experiments separated by 4 weeks. During the week preceding each experiment, the birds were placed alone in the chamber for two half-hour intervals for acclimation without model presentation. Each experiment was divided into three sessions, the order of which was varied systematically. In one session the male was tested alone; in another, with its mate. In the remaining session the social context differed between the two experiments. In experiment 1, the males were tested with a hen mated to another male and housed in the same room as the subjects for several weeks prior to the study. A different female was used for each male. In experiment 2, each of the subjects was tested with a single male which was unmated and housed in another building prior to the experiment. During a 30-min period immediately preceding testing, the subjects and the audience birds were familiarized with the chamber and each other. Then large and small models of flying raptors were each presented once with a 20-min interval between them to each of the six cockerels, giving a total of 12 presentations for each social context. We avoided habituation by varying the size, direction of transit and track of the model and by never repeating the same configuration to a given bird in the same experiment. Separate sound recordings of the subject and audience bird were made by means of a biotelemetric system developed by C. Clark in conjunction with the electronics laboratory of the Rockefeller University.
1571
Vocal responses to the models were highly predator-specific. In response to a total of 72 presentations, we recorded 99 aerial predator alarm calls and no ground predator alarm calls at all. There was a significant overall effect of the presence of an audience on the number of alarm calls given by the males in each experiment (Table I, Friedman two-way ANOVA, experiment 1: Z2= 9.75, df= 2, P < 0.0055; experiment 2: Z~= 9.0, df=2, P = 0.0081). In experiment 1 (Table I), paired comparisons of solitary and social situations showed significant differences. With their mate or another female as an audience, males increased their alarm calling rates significantly. In experiment 2 (Table I), the same relationship was found. Males called significantly more frequently when accompanied by either their mate or another male than when alone. Males called more frequently to an unfamiliar female than to their mate, but this difference was not significant (experiment 1), nor was there any significant difference in the number of calls when the audience was another male as compared to their mate (experiment 2). The audience birds could also see the hawk model and were free to call. There was a clear sexual dimorphism in alarm calling. Females seldom gave alarm calls. We recorded only five instances of alarm calling by a hen in a total of 36 presentations and their vocalizations always fol-
Table I. Effect of social context on male alarm call production Male Condition Experiment 1 Alone Mate Other female Experiment 2 Alone Mate Unfamiliar male
1 2 3 4 5 6 Median Range 2* 0 0 0 0 0 3 3 2 3 3 4 5 6 6 5 2 4
0 3t 5t$
1 3 8
0 0-1 3t 18 3.5~'w 1-8
0 0 0 0 0 8 8 3 1 3 1 7 3 1 4
0~ 2~4 2-6
* Each number represents the sum of alarm calls for two model presentations. t Comparison with the alone condition, Wilcoxon matched-pairs signed-ranked two-tailed test, N=6, T=0, P<0.05. $ Comparison between mate and other female condition, two-tailed sign test, N= 5, x= 1, P= 0.376. wComparison between mate and other male condition, two-tailed sign test, N=4, x=2, P= 1.0.
1572
Animal Behaviour, 34, 5
lowed the alarm calls of the cockerels. We demonstrated that the cockerels were not being cued by their female companions by presenting the models in a subsequent experiment in such a way that the female could see them and the males could not. In no case did a male alarm call despite the fact that the females responded to the models with evasive behaviours such as crouching, freezing, monitoring and, rarely, with an alarm call of their own. This is the first demonstration in a controlled environment that alarm calling in an avian species varies according to the presence or absence of another bird. This suggests that an audience could be an important factor eliciting alarm calls. However, an alternative hypothesis does not require the presence of a conspecific receiver to account for an audience effect. Alarm calls could inform the predator that it has been detected by an alert prey. Call frequency might be higher when the cockerel is accompanied by another potential prey, conspecific or otherwise, than when he is alone because of the sharing with other prey of the residual risk that the predator may attack nonetheless. In either case the cockerels might be expected to act so as to minimize the risk to their mates as opposed to other conspecifics, and in this respect the two hypotheses make different predictions. If calling functions to warn, cockerels should call more frequently in the presence of their mates than with other adult males. Alternatively, if calling signals to the predator that it has been detected, and entails some shared risk, cockerels should call more frequently when with other males than when with their mates. Since in our experiments no significant difference was found in the frequency of calling with mates as compared to other adult males, the question of calling frequency as a function of audience identity bears further investigation. Cockerels also modulate their food calling according to the social context (Marler et al. 1986). In contrast to alarm calling, the gender of the audience is important for food calling. Cockerels inhibit food calling in the presence of another male. Accordingly, the alarm call data cannot be explained by general arousal effects that are due to the presence of a conspecific companion, or by assuming that cockerels make no functional distinction between males and females as a communicative audience. Chickens thus have the capacity to modulate their vocal output with reference to their social environment. As evidenced by the comparison between food and alarm calling, the nature of this modulation varies with the type of communication involved. The fact that vocal systems are subject to the influence of an audience in animals as diverse as the domestic chicken and the vervet monkey (Cheney
& Seyfarth 1985) provides significant opportunities for a comparative analysis of phenomena lying along the continuum ranging from reflexive signal production to communicative intentionality. We thank E. Balaban, G. Ball, W. Buskirk, S. Clark, D. Griffin, R. Hegner, G. Klump, J. Mitani, D. Nelson, S. Nowicki, C. Ristau and R. Suter for helpful comments on an earlier draft. Financial support was provided to M. Gyger by the Swiss National Science Foundation (No. 83.115.0.83) and by the Winston Foundation, to P. Marler by the National Science Foundation (BNS No. 8416451) and by BRSG S07 RR07065. MARCEL GYGER STEPHEN J. KARAKASHIAN PETER MARLER
Rockefeller University, Field Research Center for Ethology and Ecology, Millbrook, N Y 12545, U.S.A.
References Cheney, D. L. & Seyfarth, R. M. 1985. Vervet monkey alarm calls:manipulation through shared information? Behaviour, 94, 150-166. Guyomarc'h, J.-C. 1974. Les vocalisationsdes gallinac6s. Structure des sons et des r6pertoires. Ontogen~se motrice et acquisition de leur s~mantique. Th6se d'Etat, Universit~ de Rennes. Klump, G. M. & Shalter, M. D. 1984.Acoustic behaviour of birds and mammals in the predator context. Z. Tierpsychol., 66, 189526. Konishi, M. 1963. The role of auditory feed-back in the vocal behaviour of the domestic fowl. Z. Tierpsychol., 20, 349-367. Marler, P., Dufty, A. & Pickert, R. 1986.Vocal communication in the domestic chicken: II. Is a sender sensitive to the presence and nature of a receiver?Anim. Behav., 34, 194-198. Meinerthagen, R. 1955. The speed and altitude of bird flight. Ibis, 97, 81 117. Schleidt, W. M. 1961. Reaktionen von Truthfihnern auf fliegende Raubv6gel und Versuche zur Analyse ihrer AAM's. Z. Tierpsychol., 18, 534-560. Sherman, P. 1985. Alarm calls of Belding's ground squirrels to aerial predators: nepotism or self-preservation? Behav. Ecol. Soeiobiol., 17, 313-324. Sullivan, K. 1985. Selective alarm calling by downy woodpeckers in mixed-species flocks. Auk, 102, 184187. (Received 2 January 1986; revised 17 March 1986: MS. number As-379)
Song Development by Castrated Marsh Wrens Gonadal steroids play a role not only in controlling adult song but also in the ontogeny of the song
Animal Behaviour, 34, 5
sequence of resources to the exclusion of another resource which is readily available in the immediate vicinity. Our observations of D. melanogaster foraging in the three patterns suggest that they remain within the pattern in part because of a forward-moving tendency between arrival and departure from a drop, also noted in the foraging behaviour of honeybees (Waddington & Heinrich 1981) and houseflies (Fromm & Bell, unpublished data). However, even the forward-moving tendency seems to be relaxed in the course of dropfinding, since forward-moving orientation would not explain the results in the ZIG pattern. Alternatively, a fly might be able to determine the time it takes to walk from one sucrose drop to another and use this information in ascertaining the spatial configuration of a patch. However, this explanation seems unlikely because the interdrop time intervals for an individual fly are highly variable (Tortorici, unpublished data). The clear benefit gained by this modification of search orientation relative to the spatial configuration of resources is a short-term constraint of search orientation parameters to a resource arrangement that is predictable for a shorter time period. It is possible that D. melanogaster, and perhaps other organisms, exploit resources in a food patch by processing proprioceptive information obtained from spatial locomotory movements, producing short-term retention of the spatial patterning of resources. Supported by the National Science Foundation Psychobiology Program (BNS 85-05679). The authors appreciate the critical comments of R. Jander.
Bond, A. B. 1980.Optimal foraging in a uniform habitat: the search mechanism of the green lacewing. Anim. Behav., 28, 10-19. Dethier, V. G. 1957. Communication by insects: physiology of dancing. Science, N.Y., 125, 331-336. Dixon, A. F. G. 1959. An experimental study of the searching behaviour of the predatory coccinellidbeetle Adelia bipunctata (L.). J. Anim. Ecol., 28, 259-281. Nelson, M. C. 1977. The blowfly's dance: role in the regulation of food intake. J. Insect Physiol., 23, 603612. Smith, J. N. M. i974. The food searching behaviour of two European thrushes. II: The adaptiveness of the search patterns. Behaviour, 49, 1 61. Waage, J. K. 1977. Behavioural aspects of foraging in the parasitoid Nemeritis canescens (Grav.). Ph.D. thesis, University of London, London. Waage, J. K. 1978. Arrestment responses of the parasitoid Nemeritis canescens to a contact chemical produced by its host, Plodia interpunctella. Physiol. Entomol., 3, 135-146. Waddington, K. D. & Heinrich, B. 1981. Patterns of movement and floral choice by foraging bees. In: Foraging Behavior (Ed. by A. C. Kamil & T. D. Sargent), pp. 215-230. New York: Garland Press.
(Received 16 December 1985; revised 1 April 1986; MS. number As-377) Avian Alarm Calling: Is there an Audience Effect?
Alarm calling, an important facet of anti-predator behaviour, is widespread among vertebrates. Various functions of alarm calling have been proposed which are not mutually exclusive, but which differ in their relative emphasis on the effects of alarm signals either on the predator or on other prey (reviewed in Klump & Shalter 1984). In the latter CATHYTORTORICI* case, calls may warn kin, unrelated conspecific ALISON BRODYt companions (Sherman 1985) or even members of WILLIAMJ. BELL~ other species of impending danger (Klump & Shalter 1984). In such situations, one might predict *~ Department of Entomology, University of Kansas, Lawrence, KS 66045, that alarm calling would be elicited in the presence of appropriate signal receivers, but withheld in U.S.A. ? Department of Systematics & Ecology, Univer- their absence. This hypothesis has received support from SuUivan's (1985) study on free-ranging sity of Kansas, Lawrence, KS 66045, U.S.A. downy woodpeckers (Picoides pubescens). Field Department of Physiology & Cell Biology, University of Kansas, Lawrence, KS 66045, studies of the role of the immediate social environment on alarm production are difficult because the U.S.A. relevant variables such as spatial proximity of companions and approaching predators cannot References easily be controlled. Cheney & Seyfarth (1985) Bell, W. J. 1985. Sources of information controlling have shown that in captive vervet monkeys alarm motor patterns in arthropod local search orientation. J. calling is significantlyaffected by the social context Insect Physiol., 31, 837-847. Bell, W. J., Tortorici, C., Roggero, R. J., Kipp, L. R. & in which the animal finds itself at the moment of Tobin, T. R. 1985. Sucrose-stimulatedsearching beha- predator detection. Here we report on laboratory data in which viour of Drosophila melanogasterin a uniform habitat: modulation by period of deprivation. Anita. Behav., 33, domestic chickens (Gallus domesticus) responded 436-448. to systematic variation of social contexts and
Short Communications predator approach. The predator's approach was controlled by the use of small aerial models of raptors, similar to those employed in the classic experiments of Lorenz, Tinbergen and others (Schleidt 1961). We focused on the calling behaviour of cockerels, which are known to give specific alarm calls to different classes of natural predators (Konishi 1963; Guyomarc'h 1974). Six mated Golden Sebright cockerels were tested in a chamber 3.5 m long and 1.2 m wide, with walls 1.8 m high. The subject was confined to a small cage (1.0 x 0.6 x 0.6 m) in the centre of the chamber. When required, a second bird was introduced into another cage immediately adjacent. Two tracks of double nylon monofilaments were set at 2.3 m above ground, located 40 cm on either side of the middle of the long axis of the chamber. The models, pieces of black cardboard shaped like a raptor, measured 5.0 and 7-5 cm across. As presented, they mimicked a hawk with a wingspread of 120 cm flying either at 48 or 32 m above the ground. An electric motor pulled the models by means of a third filament at 0.5 m/s and 0.7 m/s, respectively, giving an apparent speed of approximately 45 kin/ h, the cruising speed of several raptors reported by Meinerthagen (1955). We tested each male in two experiments separated by 4 weeks. During the week preceding each experiment, the birds were placed alone in the chamber for two half-hour intervals for acclimation without model presentation. Each experiment was divided into three sessions, the order of which was varied systematically. In one session the male was tested alone; in another, with its mate. In the remaining session the social context differed between the two experiments. In experiment 1, the males were tested with a hen mated to another male and housed in the same room as the subjects for several weeks prior to the study. A different female was used for each male. In experiment 2, each of the subjects was tested with a single male which was unmated and housed in another building prior to the experiment. During a 30-min period immediately preceding testing, the subjects and the audience birds were familiarized with the chamber and each other. Then large and small models of flying raptors were each presented once with a 20-min interval between them to each of the six cockerels, giving a total of 12 presentations for each social context. We avoided habituation by varying the size, direction of transit and track of the model and by never repeating the same configuration to a given bird in the same experiment. Separate sound recordings of the subject and audience bird were made by means of a biotelemetric system developed by C. Clark in conjunction with the electronics laboratory of the Rockefeller University.
1571
Vocal responses to the models were highly predator-specific. In response to a total of 72 presentations, we recorded 99 aerial predator alarm calls and no ground predator alarm calls at all. There was a significant overall effect of the presence of an audience on the number of alarm calls given by the males in each experiment (Table I, Friedman two-way ANOVA, experiment 1: Z2= 9.75, df= 2, P < 0.0055; experiment 2: Z~= 9.0, df=2, P = 0.0081). In experiment 1 (Table I), paired comparisons of solitary and social situations showed significant differences. With their mate or another female as an audience, males increased their alarm calling rates significantly. In experiment 2 (Table I), the same relationship was found. Males called significantly more frequently when accompanied by either their mate or another male than when alone. Males called more frequently to an unfamiliar female than to their mate, but this difference was not significant (experiment 1), nor was there any significant difference in the number of calls when the audience was another male as compared to their mate (experiment 2). The audience birds could also see the hawk model and were free to call. There was a clear sexual dimorphism in alarm calling. Females seldom gave alarm calls. We recorded only five instances of alarm calling by a hen in a total of 36 presentations and their vocalizations always fol-
Table I. Effect of social context on male alarm call production Male Condition Experiment 1 Alone Mate Other female Experiment 2 Alone Mate Unfamiliar male
1 2 3 4 5 6 Median Range 2* 0 0 0 0 0 3 3 2 3 3 4 5 6 6 5 2 4
0 3t 5t$
1 3 8
0 0-1 3t 18 3.5~'w 1-8
0 0 0 0 0 8 8 3 1 3 1 7 3 1 4
0~ 2~4 2-6
* Each number represents the sum of alarm calls for two model presentations. t Comparison with the alone condition, Wilcoxon matched-pairs signed-ranked two-tailed test, N=6, T=0, P<0.05. $ Comparison between mate and other female condition, two-tailed sign test, N= 5, x= 1, P= 0.376. wComparison between mate and other male condition, two-tailed sign test, N=4, x=2, P= 1.0.
1572
Animal Behaviour, 34, 5
lowed the alarm calls of the cockerels. We demonstrated that the cockerels were not being cued by their female companions by presenting the models in a subsequent experiment in such a way that the female could see them and the males could not. In no case did a male alarm call despite the fact that the females responded to the models with evasive behaviours such as crouching, freezing, monitoring and, rarely, with an alarm call of their own. This is the first demonstration in a controlled environment that alarm calling in an avian species varies according to the presence or absence of another bird. This suggests that an audience could be an important factor eliciting alarm calls. However, an alternative hypothesis does not require the presence of a conspecific receiver to account for an audience effect. Alarm calls could inform the predator that it has been detected by an alert prey. Call frequency might be higher when the cockerel is accompanied by another potential prey, conspecific or otherwise, than when he is alone because of the sharing with other prey of the residual risk that the predator may attack nonetheless. In either case the cockerels might be expected to act so as to minimize the risk to their mates as opposed to other conspecifics, and in this respect the two hypotheses make different predictions. If calling functions to warn, cockerels should call more frequently in the presence of their mates than with other adult males. Alternatively, if calling signals to the predator that it has been detected, and entails some shared risk, cockerels should call more frequently when with other males than when with their mates. Since in our experiments no significant difference was found in the frequency of calling with mates as compared to other adult males, the question of calling frequency as a function of audience identity bears further investigation. Cockerels also modulate their food calling according to the social context (Marler et al. 1986). In contrast to alarm calling, the gender of the audience is important for food calling. Cockerels inhibit food calling in the presence of another male. Accordingly, the alarm call data cannot be explained by general arousal effects that are due to the presence of a conspecific companion, or by assuming that cockerels make no functional distinction between males and females as a communicative audience. Chickens thus have the capacity to modulate their vocal output with reference to their social environment. As evidenced by the comparison between food and alarm calling, the nature of this modulation varies with the type of communication involved. The fact that vocal systems are subject to the influence of an audience in animals as diverse as the domestic chicken and the vervet monkey (Cheney
& Seyfarth 1985) provides significant opportunities for a comparative analysis of phenomena lying along the continuum ranging from reflexive signal production to communicative intentionality. We thank E. Balaban, G. Ball, W. Buskirk, S. Clark, D. Griffin, R. Hegner, G. Klump, J. Mitani, D. Nelson, S. Nowicki, C. Ristau and R. Suter for helpful comments on an earlier draft. Financial support was provided to M. Gyger by the Swiss National Science Foundation (No. 83.115.0.83) and by the Winston Foundation, to P. Marler by the National Science Foundation (BNS No. 8416451) and by BRSG S07 RR07065. MARCEL GYGER STEPHEN J. KARAKASHIAN PETER MARLER
Rockefeller University, Field Research Center for Ethology and Ecology, Millbrook, N Y 12545, U.S.A.
References Cheney, D. L. & Seyfarth, R. M. 1985. Vervet monkey alarm calls:manipulation through shared information? Behaviour, 94, 150-166. Guyomarc'h, J.-C. 1974. Les vocalisationsdes gallinac6s. Structure des sons et des r6pertoires. Ontogen~se motrice et acquisition de leur s~mantique. Th6se d'Etat, Universit~ de Rennes. Klump, G. M. & Shalter, M. D. 1984.Acoustic behaviour of birds and mammals in the predator context. Z. Tierpsychol., 66, 189526. Konishi, M. 1963. The role of auditory feed-back in the vocal behaviour of the domestic fowl. Z. Tierpsychol., 20, 349-367. Marler, P., Dufty, A. & Pickert, R. 1986.Vocal communication in the domestic chicken: II. Is a sender sensitive to the presence and nature of a receiver?Anim. Behav., 34, 194-198. Meinerthagen, R. 1955. The speed and altitude of bird flight. Ibis, 97, 81 117. Schleidt, W. M. 1961. Reaktionen von Truthfihnern auf fliegende Raubv6gel und Versuche zur Analyse ihrer AAM's. Z. Tierpsychol., 18, 534-560. Sherman, P. 1985. Alarm calls of Belding's ground squirrels to aerial predators: nepotism or self-preservation? Behav. Ecol. Soeiobiol., 17, 313-324. Sullivan, K. 1985. Selective alarm calling by downy woodpeckers in mixed-species flocks. Auk, 102, 184187. (Received 2 January 1986; revised 17 March 1986: MS. number As-379)
Song Development by Castrated Marsh Wrens Gonadal steroids play a role not only in controlling adult song but also in the ontogeny of the song