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Predation risk impairs diet selection in juvenile salmon
Short Communications
ling turtles affects their subsequent behaviour. An important factor apparent in this experiment is that the imprinting required a relatively long exposure (food imprinting studies using Chelydra serpentina required 2 weeks of exposure to affect a response; Burghardt & Hess 1966). The present study exposed turtles to chemical cues during incubation and for 3 months after entering the water. Either we are dealing with an imprinting process with a prolonged critical period or we may be dealing with some other developmental psychobiological mechanism that differs slightly from imprinting. Turtles were collected and maintained under U.S. Fish and Wildlife permit PRT-3456, Florida Department of Natural Resources permit TP-72 and Texas Parks and Wildlife permit 480. Research was funded by the Texas A&M University Sea Grant College Program; NA83AA-D-00061. MARK GRASSMAN* DAVID OWENS Department o f Biology, Texas A & M University, College Station, T X 77830, U.S.A.
* Present address: Institute of Reproductive Biology, Department of Zoology, University of Texas, Austin, TX 78712, U.S.A. References Burghardt, G. M. & Hess, E. A. 1966. Food imprinting in the snapping turtle, Chelydra serpentina. Science, N. Y., 151, 108 109. Carr, A. 1967. So Excellente a Fishe. New York: Natural History Press. Grassman, M. A. 1984. The chemosensory behavior of juvenile sea turtles: implications for chemical imprinting. Ph.D. thesis, Texas A&M University. Grassman, M. A., Owens, D. E., McVey, J. P. & Marquez, M. R. 1984. Olfactory-based orientation in artificially imprinted sea turtles. Science, N. Y., 224, 83 84. Hasler, A. D. & Scholz, A. T. 1983. Olfactory Imprinting in Salmon. Berlin: Springer-Verlag. Manton, M. L. 1979.Olfaction and behavior. In: Turtles: Perspectives and Research (Ed. by M. Harless & H. Morloek), pp. 28%301. New York: John Wiley. Owens, D. W., Crowell-Comuzzie,D. & Grassman, M. 1986. Chemoreception in the homing and orientation behavior of amphibians and reptiles with special reference to sea turtles. In: Chemical Signals in Vertebrates, Vol. IV." Ecology, Evolution and Comparative Biology (Ed. by D. Duvall, D. Muller-Schwarze& R. M. Silverstein),pp. 341-355. New York: Plenum Press. Owens, D. W., Grassman, M. A. & Hendrickson, J. R. 1982. The imprinting hypothesis and sea turtle reproduction. Herpetologica, 38, 124~135. (Received 14 August 1986; revised 8 September 1986; MS. number:As-420)
931
Predation Risk Impairs Diet Selection in Juvenile Salmon
It is increasingly evident that the nervous systems of higher animals may be limited in their ability to process visual information of different forms simultaneously. One circumstance in which this limitation leads to a conflict of priorities is when foraging animals are required to concentrate their visual attention both on the food itself and on potential predators. Recent experiments have shown that the greater the attention required for the feeding task, the less the forager is able to detect an approaching predator (Milinski 1984; Lawrence 1985). Here we examine the same problem from a different angle, and test how an increase in predation risk affects the attention devoted to food by juvenile salmon, Salmo salar L. These fish are sitand-wait predators, orientating from a resting position to food items passing in the water current and then darting out to intercept them. They must discriminate quickly between suitable and unsuitable items, since current velocities are often high. Juvenile salmon are slower to respond to approaching food items when under an increased threat of predation (Metcalfe et al. 1987), suggesting that this recognition process may be impaired by the need to watch for predators. We now examine this conflict in more detail, by testing whether the ability of the salmon to distinguish between two types of food (one suitable, the other unsuitable) varies with predation risk. The foods used were commercial salmon pellets (manufactured by EWOS-Baker), which are dark brown, roughly spherical and made in a range of sizes. These pellets were sieved to produce two size classes. The smaller (0.7-0.9 mm in diameter) was that recommended to produce optimal feeding responses and growth for the size of fish tested (Wankowski & Thorpe 1979), whereas the larger pellets (1.4-1.6 mm) were too big for the fish to swallow. The validity of these categories was confirmed by the responses of the fish: during the tests they swallowed most (91-4%) of the small pellets that they attacked, but none of the large. Fish therefore gained nothing from attacking large pellets, but still incurred the costs (in terms of both energy and increased vulnerability to predators) of moving to intercept them. The fish (mean length 37.) mm, SE 0"74, N = 14) were the progeny of wild sea-run salmon, and were raised on the same pelleted food at the Department of Agriculture and Fisheries, Scotland, Pitlochry Fisheries Laboratory. The experiments were carried out using experimental flume tanks at the Universities' Field Station, Loch Lomon& These tanks, described in detail in Metcalfe et al. (1987), each held a single fish. The fish was induced to
932
Animal Behaviour, 35, 3
remain in one position in the tank by providing a small overhead shelter, u n d e r which it maintained a position facing into the water flow (approximately 5 cm/s). Pellets were introduced into the current 20 cm in front of the shelter using a feeding pipe. They were thus carried past (and to within 0-2 cm of) the waiting fish, well within their capture range (up to 10 body lengths, Wankowski 1981). Predation risk was varied using a fibreglass model, 28 cm long, of a brown trout, Salmo trutta, placed in an adjacent compartment of the flume tank and moved to within 25 cm of the resting salmon, which could see it through a transparent partition. Observations were made through a small shielded peephole. The experimental protocol was as follows. Each fish was tested for 2 days, after a 2-day settling down period. Both days of testing consisted of five hour-long observations, with 0.75 h between each. During each observation period the fish received a pellet every 10 min; the six pellets in each hour consisted of three of each size category presented in an unpredictable sequence. The first observation period each day was a training trial, which allowed the fish to experience both sizes of particle in the absence of the predator. The remaining four periods each day were alternately High and Low Predation Risk trials, half the fish first receiving High Risk trials, and half first receiving Low Risk trials. In Low Risk trials the predator was not visible. Predation risk as perceived by the salmon was increased for each High Risk trial by moving the model trout into view for 30 s immediately before the trial began; juvenile salmon respond to this brief frightening stimulus by altering their foraging strategy over the next hour, but recovering within a further hour (Metcalfe et al. 1987). For each pellet a note was made of whether the fish orientated (i.e. gave a twitch of the head towards the pellet, see Metcalfe et al. 1987) or attacked, and whether the pellet was swallowed or rejected. The experiments were carried out from 10 to 15 July 1986, when the mean midday water temperature was 17.0~ (range 16.0-17.5~ Under the Low Risk conditions only a quarter of the attacks attempted by the salmon were on the inedible pellets (i.e. were 'mistaken' attacks, Fig. la), although the two pellet sizes were presented at equal rates. However, this ability to discriminate was absent in the High Predation Risk trials; in the hour after seeing a predator, the salmon attempted captures of both pellet sizes with equal frequency (Fig. la). This same trend was found when data for individual fish were analysed separately: of those fish that made at least three attacks in both types of trial, all were more likely to make mistaken attacks during the High Risk trials (Wilcoxon signed-ranks test, Ts = 0, N = 7, two-tailed P < 0.02).
(a) P
z uJ v' < I.cO
60-
40-
20-
63
o L O W RISK
HiGH RISK
(b) P(0"05
80 <
I
i
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47
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Ined
O
~- 60 (J _z Q <
w 40
-J G9 Z
_o
h,< I.z ILl ~: O
20-
o
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38
Ed
Ined
L O W RISK
HIGH RISK
Figure 1. (a) Attacks on the inedible pellets ('mistaken attacks') as a percentage of all attacks made by the salmon. The proportion of mistaken attacks increased significantly with an increase in predation risk. (b) The percentage of orientations to edible (Ed) and inedible (Ined) pellets that led to attack. P from (a) chi-squared test and (b) Wilcoxon signed-ranks test; sample sizes shown in figure. When predation risk was low, the salmon were apparently able to make assessments of the suitability of pellets at several stages in the feeding sequence from before orientation through to pellet capture. Thus they were more likely to orientate to the edible than the inedible pellets (orientating to 45.7% of pellets as against 22.6%; Wilcoxon signed-ranks test, T~= 4, N = 13, two-tailed P<0-01), and orientations to edible pellets were more likely to lead to an attack (Fig. lb). However, these differences in response to the two pellet types disappeared under conditions of high predation risk, when the need for vigilance was greater (Fig. lb). There was a significant reduction in the number of orientations to edible pellets with increasing predation risk (Wilcoxon signed-ranks
Short Communications test, T~=5, N=14, two-tailed P<0-01), but no change in the number of orientations to inedible pellets, possibly indicating that the smaller pellets were more often overlooked when predation risk was high. It would appear that an increase in predation risk leads to a decrease in the attention paid to the selection of food items, resulting in an increased frequency of errors. The need to watch for the possible return of a predator may therefore reduce food intake not only by a reduction in the rate of feeding (e.g. Metcalfe et al. 1987), but also through reducing the efficiency of prey selection. Predators may thus influence the habitat use of their prey species through a variety of subtle effects as well as those more obvious. We thank A. McQueen for rearing the fish, C. E. Adams and D. Brown for their help with the water supply, and P. Monaghan and two referees for commenting on the manuscript. The study was funded by a NERC project grant. NElL B. METCALFE* FELICITY A. HUNTINGFORD*
JOHN E. TnORP~t * Department of Zoology, Glasgow University, Glasgow G12 8QQ, U.K. t DAFS Freshwater Fisheries Laboratory, Faskally, Pitlochry, Perthshire, PH16 5LB, U.K.
References Lawrence, E. S. 1985. Vigilance during 'easy' and 'difficult' foraging tasks. Anim. Behav., 33, 1373-1374. Metcalfe, N. B., Huntingford, F. A. & Thorpe, J. E. 1987. The influenceof predation risk on the feeding motivation and foraging strategy of juvenile Atlantic salmon. Anita. Behav., 35, 901-911. Milinski, M. 1984. A predator's costs of overcoming the confusion-effect of swarming prey. Anim. Behav., 32, 115%1162. Wankowski, J. W. J. 1981. Behavioural aspects of predation by juvenile Atlantic salmon (Salmo salar L.) on particulate, drifting prey. Anim. Behav., 29, 557571. Wankowski, J. W. J. & Thorpe, J. E. 1979. The role of food particle size in the growth of juvenile Atlantic salmon (Salmo salar L.). J. Fish Biol., 14, 351-370. (Received 18 September 1986; revised 20 October 1986; MS. number: sc-337)
Distress Call Alternation in Ducklings and Goslings: A Reply to Lamprecht Recently, Lamprecht (1985) demonstrated that pairs of young bar-headed goslings, Anser indicus,
933
isolated from their brood alternate their distress calls with one another, with each bird tending to inhibit its calls while its partner is calling. This appears to replicate work from my laboratory (Gaioni 1982; Gaioni & Platte 1982; Gaioni & Evans 1985) demonstrating a similar effect in mallard ducklings, Anas platyrhynchos. We have suggested that this alternation response may facilitate maternal retrieval of the separated ducklings by enabling them to broadcast a fairly continuous signal to the mother in an energy-efficient fashion without masking the temporal patterning of the notes within each other's calls. In contrast, Lamprecht suggests that call alternation may be an artefact of a 'fear response' in which the young birds become silent whenever they hear a sound indicating possible danger, and then resume calling to the parent(s) whenever this sound ceases. It is not clear that the mallard ducklings' and the barheaded goslings' alternation responses have the same functional significance. Further, I believe that there are problems with Lamprecht's fear-response account. Finally, several points need to be clarified concerning the means by which call alternation is quantified, and the difficulty that is likely to arise when attempting to observe an alternation response among brood-mates in the wild. Lamprecht observes that when goslings are separated from their brood and begin to give distress calls, they alternate their own calls more strongly with adult excitement calls or with artificial sounds, than they do with the calls of other goslings. In contrast, I had previously reported (1982) that mallard ducklings alternate more strongly with other duckling calls than with pure tones or broadband mechanical clicks. Lamprecht suggests that this opposite pattern of results may have been obtained because my artificial sounds lacked low frequency components that would have made them more fearful than the sibling calls, by making them more similar to adult calls. (Lamprecht notes that when a gosling approaches a strange adult that is calling, it is often attacked by the adult. This might make the adult calls frightening to the gosling.) There are two major problems with this analysis. (1) The low frequency mallard hen's call with the temporal pattern most similar to the distress call is the assembly call, which ducklings find attractive, not fearful. (2) Recently, using synthesized distress calls, Evans and I have shown that either increasing or decreasing the dominant frequency of distress notes results in a significant decrement in the alternation response (Fig. 1). The latter finding is consistent with a growing body of literature from our laboratory which indicates that the alternation response of mallard ducklings is quite narrowly tuned to the features of conspecific
ling turtles affects their subsequent behaviour. An important factor apparent in this experiment is that the imprinting required a relatively long exposure (food imprinting studies using Chelydra serpentina required 2 weeks of exposure to affect a response; Burghardt & Hess 1966). The present study exposed turtles to chemical cues during incubation and for 3 months after entering the water. Either we are dealing with an imprinting process with a prolonged critical period or we may be dealing with some other developmental psychobiological mechanism that differs slightly from imprinting. Turtles were collected and maintained under U.S. Fish and Wildlife permit PRT-3456, Florida Department of Natural Resources permit TP-72 and Texas Parks and Wildlife permit 480. Research was funded by the Texas A&M University Sea Grant College Program; NA83AA-D-00061. MARK GRASSMAN* DAVID OWENS Department o f Biology, Texas A & M University, College Station, T X 77830, U.S.A.
* Present address: Institute of Reproductive Biology, Department of Zoology, University of Texas, Austin, TX 78712, U.S.A. References Burghardt, G. M. & Hess, E. A. 1966. Food imprinting in the snapping turtle, Chelydra serpentina. Science, N. Y., 151, 108 109. Carr, A. 1967. So Excellente a Fishe. New York: Natural History Press. Grassman, M. A. 1984. The chemosensory behavior of juvenile sea turtles: implications for chemical imprinting. Ph.D. thesis, Texas A&M University. Grassman, M. A., Owens, D. E., McVey, J. P. & Marquez, M. R. 1984. Olfactory-based orientation in artificially imprinted sea turtles. Science, N. Y., 224, 83 84. Hasler, A. D. & Scholz, A. T. 1983. Olfactory Imprinting in Salmon. Berlin: Springer-Verlag. Manton, M. L. 1979.Olfaction and behavior. In: Turtles: Perspectives and Research (Ed. by M. Harless & H. Morloek), pp. 28%301. New York: John Wiley. Owens, D. W., Crowell-Comuzzie,D. & Grassman, M. 1986. Chemoreception in the homing and orientation behavior of amphibians and reptiles with special reference to sea turtles. In: Chemical Signals in Vertebrates, Vol. IV." Ecology, Evolution and Comparative Biology (Ed. by D. Duvall, D. Muller-Schwarze& R. M. Silverstein),pp. 341-355. New York: Plenum Press. Owens, D. W., Grassman, M. A. & Hendrickson, J. R. 1982. The imprinting hypothesis and sea turtle reproduction. Herpetologica, 38, 124~135. (Received 14 August 1986; revised 8 September 1986; MS. number:As-420)
931
Predation Risk Impairs Diet Selection in Juvenile Salmon
It is increasingly evident that the nervous systems of higher animals may be limited in their ability to process visual information of different forms simultaneously. One circumstance in which this limitation leads to a conflict of priorities is when foraging animals are required to concentrate their visual attention both on the food itself and on potential predators. Recent experiments have shown that the greater the attention required for the feeding task, the less the forager is able to detect an approaching predator (Milinski 1984; Lawrence 1985). Here we examine the same problem from a different angle, and test how an increase in predation risk affects the attention devoted to food by juvenile salmon, Salmo salar L. These fish are sitand-wait predators, orientating from a resting position to food items passing in the water current and then darting out to intercept them. They must discriminate quickly between suitable and unsuitable items, since current velocities are often high. Juvenile salmon are slower to respond to approaching food items when under an increased threat of predation (Metcalfe et al. 1987), suggesting that this recognition process may be impaired by the need to watch for predators. We now examine this conflict in more detail, by testing whether the ability of the salmon to distinguish between two types of food (one suitable, the other unsuitable) varies with predation risk. The foods used were commercial salmon pellets (manufactured by EWOS-Baker), which are dark brown, roughly spherical and made in a range of sizes. These pellets were sieved to produce two size classes. The smaller (0.7-0.9 mm in diameter) was that recommended to produce optimal feeding responses and growth for the size of fish tested (Wankowski & Thorpe 1979), whereas the larger pellets (1.4-1.6 mm) were too big for the fish to swallow. The validity of these categories was confirmed by the responses of the fish: during the tests they swallowed most (91-4%) of the small pellets that they attacked, but none of the large. Fish therefore gained nothing from attacking large pellets, but still incurred the costs (in terms of both energy and increased vulnerability to predators) of moving to intercept them. The fish (mean length 37.) mm, SE 0"74, N = 14) were the progeny of wild sea-run salmon, and were raised on the same pelleted food at the Department of Agriculture and Fisheries, Scotland, Pitlochry Fisheries Laboratory. The experiments were carried out using experimental flume tanks at the Universities' Field Station, Loch Lomon& These tanks, described in detail in Metcalfe et al. (1987), each held a single fish. The fish was induced to
932
Animal Behaviour, 35, 3
remain in one position in the tank by providing a small overhead shelter, u n d e r which it maintained a position facing into the water flow (approximately 5 cm/s). Pellets were introduced into the current 20 cm in front of the shelter using a feeding pipe. They were thus carried past (and to within 0-2 cm of) the waiting fish, well within their capture range (up to 10 body lengths, Wankowski 1981). Predation risk was varied using a fibreglass model, 28 cm long, of a brown trout, Salmo trutta, placed in an adjacent compartment of the flume tank and moved to within 25 cm of the resting salmon, which could see it through a transparent partition. Observations were made through a small shielded peephole. The experimental protocol was as follows. Each fish was tested for 2 days, after a 2-day settling down period. Both days of testing consisted of five hour-long observations, with 0.75 h between each. During each observation period the fish received a pellet every 10 min; the six pellets in each hour consisted of three of each size category presented in an unpredictable sequence. The first observation period each day was a training trial, which allowed the fish to experience both sizes of particle in the absence of the predator. The remaining four periods each day were alternately High and Low Predation Risk trials, half the fish first receiving High Risk trials, and half first receiving Low Risk trials. In Low Risk trials the predator was not visible. Predation risk as perceived by the salmon was increased for each High Risk trial by moving the model trout into view for 30 s immediately before the trial began; juvenile salmon respond to this brief frightening stimulus by altering their foraging strategy over the next hour, but recovering within a further hour (Metcalfe et al. 1987). For each pellet a note was made of whether the fish orientated (i.e. gave a twitch of the head towards the pellet, see Metcalfe et al. 1987) or attacked, and whether the pellet was swallowed or rejected. The experiments were carried out from 10 to 15 July 1986, when the mean midday water temperature was 17.0~ (range 16.0-17.5~ Under the Low Risk conditions only a quarter of the attacks attempted by the salmon were on the inedible pellets (i.e. were 'mistaken' attacks, Fig. la), although the two pellet sizes were presented at equal rates. However, this ability to discriminate was absent in the High Predation Risk trials; in the hour after seeing a predator, the salmon attempted captures of both pellet sizes with equal frequency (Fig. la). This same trend was found when data for individual fish were analysed separately: of those fish that made at least three attacks in both types of trial, all were more likely to make mistaken attacks during the High Risk trials (Wilcoxon signed-ranks test, Ts = 0, N = 7, two-tailed P < 0.02).
(a) P
z uJ v' < I.cO
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o L O W RISK
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(b) P(0"05
80 <
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i
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~- 60 (J _z Q <
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-J G9 Z
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h,< I.z ILl ~: O
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Figure 1. (a) Attacks on the inedible pellets ('mistaken attacks') as a percentage of all attacks made by the salmon. The proportion of mistaken attacks increased significantly with an increase in predation risk. (b) The percentage of orientations to edible (Ed) and inedible (Ined) pellets that led to attack. P from (a) chi-squared test and (b) Wilcoxon signed-ranks test; sample sizes shown in figure. When predation risk was low, the salmon were apparently able to make assessments of the suitability of pellets at several stages in the feeding sequence from before orientation through to pellet capture. Thus they were more likely to orientate to the edible than the inedible pellets (orientating to 45.7% of pellets as against 22.6%; Wilcoxon signed-ranks test, T~= 4, N = 13, two-tailed P<0-01), and orientations to edible pellets were more likely to lead to an attack (Fig. lb). However, these differences in response to the two pellet types disappeared under conditions of high predation risk, when the need for vigilance was greater (Fig. lb). There was a significant reduction in the number of orientations to edible pellets with increasing predation risk (Wilcoxon signed-ranks
Short Communications test, T~=5, N=14, two-tailed P<0-01), but no change in the number of orientations to inedible pellets, possibly indicating that the smaller pellets were more often overlooked when predation risk was high. It would appear that an increase in predation risk leads to a decrease in the attention paid to the selection of food items, resulting in an increased frequency of errors. The need to watch for the possible return of a predator may therefore reduce food intake not only by a reduction in the rate of feeding (e.g. Metcalfe et al. 1987), but also through reducing the efficiency of prey selection. Predators may thus influence the habitat use of their prey species through a variety of subtle effects as well as those more obvious. We thank A. McQueen for rearing the fish, C. E. Adams and D. Brown for their help with the water supply, and P. Monaghan and two referees for commenting on the manuscript. The study was funded by a NERC project grant. NElL B. METCALFE* FELICITY A. HUNTINGFORD*
JOHN E. TnORP~t * Department of Zoology, Glasgow University, Glasgow G12 8QQ, U.K. t DAFS Freshwater Fisheries Laboratory, Faskally, Pitlochry, Perthshire, PH16 5LB, U.K.
References Lawrence, E. S. 1985. Vigilance during 'easy' and 'difficult' foraging tasks. Anim. Behav., 33, 1373-1374. Metcalfe, N. B., Huntingford, F. A. & Thorpe, J. E. 1987. The influenceof predation risk on the feeding motivation and foraging strategy of juvenile Atlantic salmon. Anita. Behav., 35, 901-911. Milinski, M. 1984. A predator's costs of overcoming the confusion-effect of swarming prey. Anim. Behav., 32, 115%1162. Wankowski, J. W. J. 1981. Behavioural aspects of predation by juvenile Atlantic salmon (Salmo salar L.) on particulate, drifting prey. Anim. Behav., 29, 557571. Wankowski, J. W. J. & Thorpe, J. E. 1979. The role of food particle size in the growth of juvenile Atlantic salmon (Salmo salar L.). J. Fish Biol., 14, 351-370. (Received 18 September 1986; revised 20 October 1986; MS. number: sc-337)
Distress Call Alternation in Ducklings and Goslings: A Reply to Lamprecht Recently, Lamprecht (1985) demonstrated that pairs of young bar-headed goslings, Anser indicus,
933
isolated from their brood alternate their distress calls with one another, with each bird tending to inhibit its calls while its partner is calling. This appears to replicate work from my laboratory (Gaioni 1982; Gaioni & Platte 1982; Gaioni & Evans 1985) demonstrating a similar effect in mallard ducklings, Anas platyrhynchos. We have suggested that this alternation response may facilitate maternal retrieval of the separated ducklings by enabling them to broadcast a fairly continuous signal to the mother in an energy-efficient fashion without masking the temporal patterning of the notes within each other's calls. In contrast, Lamprecht suggests that call alternation may be an artefact of a 'fear response' in which the young birds become silent whenever they hear a sound indicating possible danger, and then resume calling to the parent(s) whenever this sound ceases. It is not clear that the mallard ducklings' and the barheaded goslings' alternation responses have the same functional significance. Further, I believe that there are problems with Lamprecht's fear-response account. Finally, several points need to be clarified concerning the means by which call alternation is quantified, and the difficulty that is likely to arise when attempting to observe an alternation response among brood-mates in the wild. Lamprecht observes that when goslings are separated from their brood and begin to give distress calls, they alternate their own calls more strongly with adult excitement calls or with artificial sounds, than they do with the calls of other goslings. In contrast, I had previously reported (1982) that mallard ducklings alternate more strongly with other duckling calls than with pure tones or broadband mechanical clicks. Lamprecht suggests that this opposite pattern of results may have been obtained because my artificial sounds lacked low frequency components that would have made them more fearful than the sibling calls, by making them more similar to adult calls. (Lamprecht notes that when a gosling approaches a strange adult that is calling, it is often attacked by the adult. This might make the adult calls frightening to the gosling.) There are two major problems with this analysis. (1) The low frequency mallard hen's call with the temporal pattern most similar to the distress call is the assembly call, which ducklings find attractive, not fearful. (2) Recently, using synthesized distress calls, Evans and I have shown that either increasing or decreasing the dominant frequency of distress notes results in a significant decrement in the alternation response (Fig. 1). The latter finding is consistent with a growing body of literature from our laboratory which indicates that the alternation response of mallard ducklings is quite narrowly tuned to the features of conspecific