Short Communications 927

undeprived rats had simultaneous access to sun- flower seeds with and without husks in their home cages, they chose seeds without husks almost exclusively. We found the same results when hungry rats were offered two dishes of seeds in our apparatus. These results suggest that the rats should have chosen seeds without husks over- whelmingly in condition 3. However, the main determinant of feeding rate in our procedure was the time to travel between feeder and lever and complete the F R 3. Research on reinforcer delays and on diet choice (Fantino & Abarca 1985) suggests that the difference in feeding rate due to choice of husks or no husks would not have as much effect on choice here as when husking time was the main component of delaY between food items. In addition, rats tended to develop strong side preferences in our apparatus. A more sensitive test might reveal significant preferences in condi- tions 2 and 3. That the rats did prefer husking to waiting in condition 1, despite the failure to detect any preferences in the other conditions, shows that the contents of handling must be of crucial impor- tance. In a second experiment, with three rats and a slightly different procedure, we found the same pattern of results.

There are good mechanistic reasons why rats should prefer husking seeds to waiting for them. The delay signal and the husk of the seed are both secondary reinforcers presented immediately on completion of the FR, but the husk should be a stronger reinforcer because it is closer to the primary reinforcer, the seed itself (Rescorla & Cunningham 1979). Seeds with husks may also appear larger than seeds without husks and be preferred for that reason. Handling and gnawing the husk may be reinforcing in itself, and the gradual emergence of the seed, even if the rat cannot eat it until the shell is off, would be expected to be a more powerful reinforcer than the unmarked passage of time in the delay condition. However, we must also consider that the real handling time was somewhat variable, while the delay was fixed. In operant studies, at least, animals tend to prefer variable to fixed delays (Fantino & Abarca 1985). Yet the fact that husking constituted a delay before eating did not cause the rats to reject seeds with husks in conditions 2 and 3, so it seems unlikely that the variability of that delay was of much importance. And, finally, a functional reason why rats should prefer husking seeds to waiting for them is presumably that 'a seed in the paw is more certain than one in the feeder'.

This research was supported by a grant to the first author from the Natural Sciences and Engi- neering Research Council of Canada. Some of the work formed part of a B.S. thesis by the second

author. We thank Cynthia Thomas and Nancy Mingo for help with the experiments and L.-A. Giraldeau for comments.

SARA J. SHETTLEWORTH VALERIE JORDAN

Department o f Psychology, University o f Toronto, Toronto, Ontario M 5 S 1A1, Canada.

References Fantino, E. & Abarca, N. 1985. Choice, optimal foraging,

and the delay reduction hypothesis. Behav. Brain Sci., 8, 315-362.

Kaufman, L.W. & Collier, G. 1981. The economics of seed handling. Am. Nat., 118, 46-60.

Krebs, J. R., Erichsen, J. T., Webber, M. I. & Charnov, E. L. 1977. Optimal prey selection in the great tit. Anita. Behav., 25, 30-38.

Lea, S. E. G. 1979. Foraging and reinforcement schedules in th~ pigeon: optimal and non-optimal aspects of choice. Anita. Behav., 27, 875 886.

Rescorla, R. A. & Cunningham, C. L. 1979. Spatial contiguity facilitates Pavlovian second-order condi- tioning. J. Exp. Psycho/.: Anita. Behav. Proc., 5, 152 161.

(Received 29 July 1985; revised 30 October 1985; MS. number: As-353)

Western Meadowlarks Switch Song Types when Matched by Playback

Most singing birds have more than one song or phrase type (Dobson & Lemon 1975). In some species, each song type is associated with different kinds of behaviour (Lein 1978; Smith et al. 1978). In many species, however, song types seem to be used in equivalent situations (Krebs 1976; Smith & Reid 1979). Many of these species show a tendency to match songs. That is, they answer a particular song type, broadcast by another bird or by play- back, with a song of the same type.

The western meadowlark is a territorial open country icterid with a repertoire of about seven song types that differ greatly in structure. Western meadowlarks repeat each song type to form a bout before switching to the next song type. For exam- ple, if a meadowlark with song types A, B and C sang the sequence A,A,A,A,A,B,B,B,B, C,C,C,C,C,C, it would have sung three bouts which were five, four and six songs 10ng.

Western meadowlarks tend to switch song types when a playback begins on their territory. I f the recording is a song type in their repertoire, these birds tend to match the recording (Falls 1985). A song that matches the song of a territory holder is

928 Animal Behaviour, 34, 3

presumably more directly addressed to him than a song that is not a match (Br6mond 1968). Thus a territory holder might be expected to respond sooner to an intruder that matches him. He could respond by matching the intruder, but this would not involve a change in his behaviour, and there- fore it would not tell the intruder that the intruder has been detected. Therefore, we predict that territory holders should switch sooner when matched by playback.

We conducted experiments from 22 May to 8 June 1984, between 0430 and 0830 hours Central Standard Time. The territories used were along the Assiniboine River Diversion near Oakland, Mani- toba. We usually determined boundaries by flush- ing or by observing neighbour-neighbour interac- tions. Where this was not possible, we conducted trials far enough apart to ensure that the same bird was not tested twice.

To avoid having to change the tape loops for each trial, we used only the four most widely shared song types in the study area. The actual songs we used were recorded outside the study area. Each song was recorded on a 2.5 s loop played on the territory from a Uher Stereo Report IC tape recorder, at about 25 dB less than natural sound levels (126 dB at source; S. Cosens, unpublished data).

Each trial of the experiment consisted of two treatments. In one treatment the song type was the same as the bird's (match); in the other it was not (non-match). The same loop was used in both treatments. Each trial proceeded as follows. We let the subject finish the bout of songs it was singing when we arrived. Immediately after the first song in the bird's next bout, we played the single song on the tape. We played the tape immediately after each of the bird's songs, until the bird switched songs, then we stopped the playback. At least one bout after the first treatment ended, we applied the second playback treatment in a similar manner. If the first playback was a match of the singing bird, the second playback began only when the bird switched to a song type which the playback did not match. If the first playback was a non-match, the second playback began when the bird switched to a song of the same type as the playback.

We completed six trials of each treatment order, yielding 12 trials involving 12 subjects. The matched bouts were significantly shorter than the non-matched bouts (median, range: 4.5, 1-7 versus 6, 2-14, P<0'010, Wilcoxon signed-ranks test, one-tailed). Since birds did not always switch to a matching song type after one bout, the number of intervening bouts was greater and more variable when the trial began with a non-match than when it began with a match. However, because we con-

ducted both treatment orders an equal number of times, this does not affect our result.

As predicted, territory holders switched sooner when matched by playback; matched bouts were shorter than unmatched bouts. Therefore, match- ing and switching do interact in the control of western meadowlark singing. The nature of this interaction agrees with the model of bout singing proposed by Hinde (1958). According to this model, song types are repeated in bouts because each repetition of a song type facilitates the next repetition up to a point at which they inhibit further repetitions, so the bout ends. Falls (1985) suggested that birds match songs they hear because the playback songs stimulate the facilitatory phase of this internal mechanism. His results show that western meadowlarks match playbacks of their own songs more often than other songs of the same type, perhaps because the former more closely resemble the normal stimulus for the mechanism. Also, the latencies between playback songs and respondents' songs are less variable when the respondents match than when they do not, again suggesting that the bird's mechanism treats the playback's songs as its own (Falls 1985). The present results are also consistent with this mecha- nism. Matched bouts may have been shorter than unmatched bouts because subjects treated the playback's songs as their own.

This may be the mechanism of matching, but what is its function? The present result is consistent with the hypothesis that matching indicates a readiness to interact. When western meadowlarks match a playback, they do not respond more strongly than when they do not match a playback, according to conventional response measures like distance to the speaker, latency to closest approach, and the number of flights (Falls 1985). However, the present result shows that when playback matches western meadowlarks, the birds switch song types sooner. Switching appears to be a graded signal of readiness to interact in meadow- larks, as they shorten their bouts during interac- tions, switch at the onset of playback or an intrusion, and respond more strongly to playback of shorter bouts (Falls & d'Agincourt 1982; Horn, unpublished data). Therefore, even ifa match is not predictive of the sender's behaviour, it may chal- lenge a receiver to signal how prepared he is to interact. By switching in response to a match, the territory holder directs his switch to the bird that matched.

ANDREW HORN J. BRUCE FALLS

Department of Zoology, University of Toronto, Toronto, Ontario M5S 1A1, Canada.

Short Communications 929

References Br6mond, J. C. 1968. Recherches sur la semantique et les

elements vecteurs d'information dans les signaux acoustiques du rouge-gorge (Erithacus rubecula L.). Terre Vie, 2, 109-220.

Dobson, C. W. & Lemon, R. E. 1975. Re-examination of monotony threshhold hypothesis in bird song. Nature, Lond., 257, 126~128.

Falls, J. B. 1985. Song matching in western meadowlarks. Can. J. Zool., 63, 2521~2524.

Falls, J. B. & d'Agincourt,L. G. 1982. Why do meadow- larks switch song types? Can. J. Zool., 60, 3400-3408.

Hinde, R. A. 1958. Alternative motor patterns in chaf- finch song. Anita. Behav., 6, 211~18.

Krebs, J. R. 1976. Habituation and song repertoires in the great tit. Behav. Ecol. Sociobiol., 1, 215~27.

Lein, R. 1978. Song variation of chestnut-sided warblers: its nature and suggested significance. Can. J. Zool., 56, 1266-1283.

Smith, D. G. & Reid, F. A. 1979. Roles of the song repertoire in red-winged blackbirds. Behav. Ecol. Sociobiol., 5, 279-290.

Smith, W. J., Pawlukiewicz, J. & Smith, S. T. 1978. Kinds of activity correlated with singing patterns of the yellow-throated vireo. Anim. Behav., 26, 862-864.

(Received 8 August 1985; revised 25 October 1985; MS. number: As-354)

Female Response Thresholds of Two Stonefly (Piecoptera) Species to Computer-simulated and

Modified Male Drumming Calls

Artificially synthesized and modified male cricket calls have been used to determine female response levels over an artificially expanded range of call variations, and to test the specificity of female response to calls of related species (Zaretsky 1972; Pollack & Hoy 1979; Hoy et al. 1982). Stonefly drumming especially lends itself to such an approach. The principal mode of drumming exchange involves substratum-borne vibrations produced when the insects strike or rub the sub- stratum with the distal ventral portion of their abdomen (Rupprecht 1967; Zeigler & Stewart 1977). The number and spacing of the resultant pulses provide information for species and sex recognition and mate location (Stewart & Zeigler 1984). Similar pulse patterns can be programmed using a variety of microcomputers. Experiments using simulated and modified male calls with live female test groups could establish the limits of female response to variations of key parameters which extend beyond the variation observed in small population samples. Such experiments should indicate the relative importance of different call parameters in species recognition, as well as giving an idea of the intraspecific variation of calls

for the population sampled. The latter would be important if the behaviour is to be used as evidence to infer phylogeny within the group, or to resolve systematic questions such as identification of sib- ling species.

Virgin adults were reared from mature nymphs of two stonefly species known to be prolific drum- mers (Zeigler & Stewart 1977). They were collected as follows: (1) Pteronarcella badia (Hagen), at Conejos River, Conejos County, Colorado, on 7 May 1983; (2) Perlinella drymo (Newman), at Middle Fork/Bosque River, McLennan County, Texas, on 21 February 1983. The calls of a few males were recorded and compared with the pub- lished descriptions before synthesized calls were produced.

Programs were written in TI Basic on a Texas Instruments 99/4A microcomputer to approximate the pulse patterns of typical male calls (Fig. 1). The resulting sounds were recorded on a Superscope C- 202 LP two-speed cassette recorder. The limits of the Basic language (processing speed) sometimes made additional temporal modifications necessary. This was done using the variable speed functions of the recorder in order to achieve the desired results. Variations from the typical call were achieved by changing the program and/or varying the playback speed with the recorder controls.

The playback to live females was done using a small 8-ohm speaker with a short wooden rod that was glued perpendicular to the centre of the speaker cone and positioned so that the rod made contact with a dried, mounted aspen branch (approximately 75 cm long) on which the female was positioned. The linear distance from the speaker contact point to the live female was 40-50 cm. Females ceased to locomote after they sensed drumming signals, so the distance from speaker to female was held constant for each female. Although the intensity of the vibrations over this distance was not measured for live or synthesized male calls, the female answering rates to typical call signals (below) indicated that, with the low play- back volume used in this study, call transfer was effective over this distance. In other experiments (Stewart & Zeigler 1984), answering rates of 100% were obtained using the same aspen branch with live pairs.

Variant calls were always played to females before the typical calls, because in preliminary experiments, females lowered their call-recognition requirements (answered highly variant calls) just after having answered an acceptable call. A similar phenomenon has been observed in some cricket species (Zaretsky 1972). Different 2-3-day-old (age since moulting to the adult stage) females were used for each comparison experiment.