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Conditioning the pecking motions of pigeons

Wolfgang Horster, Ernst Krumm, Chri stine Mohr, Juan D. Delius *

Allgemeine Psychologie, Universiliil KOlIslanz, 78457 Konsl{lnz, Germany

Abstract

The spatio-temporal courses of head and neck motions of pigeons while pecking at small grains are described.

Single and serial pecks are distinguished but the inter- and intraindividual variability of the peck kinetics is stressed.

Pigeons were then trained with instrumental conditioning procedures to speed-up their pecking. A partial reinforce- ment schedule where pigeons had to peck repeatedly before receiving reward led to a mild shortening of inter-peck intervals at lower reinforcement rates but surprisingly, a lengthening at higher rates. A schedule where short inter-peck intervals were differentially rewarded yielded a pronounced abbreviation of the inter-peck intervals, but this was achieved by a reduction of the movement path rather than an increase in motion velocity. A schedule whereby increased approach velocities were differentially rewarded yielded marked movement accelerations. When pigeons were rewarded for diminished approach speeds they also showed significant movement decelerations. Finally, it is shown that pigeons could learn to reliably abort their peck approach movement when a visual stimulus signalling a penalty was occasionally presented during the approach movement. The proportion of successful peck interruptions decreased as these interruption signals occurred later during the approach phase. It is concluded that the pecking of pigeons is neither an innately fixed nor a visually ballistic movement. It is instead a multiply controlled and flexibly adaptable response pattern.

Ke)llvords: Pigeon pecking; Fixed pattern; Ballistic motion; Movement speed; Instrumental conditioning

1. Introduction

Over recent years the forage pecking behaviour of pigeons and some other birds has attracted considerable interest. Compared with the much investigated reaching and grasping by the pri- mate/human arm and hand the pecking of pigeon

<:l'ppeared to be a relatively simple action con- trolled by a comparatively small brain (Jean-

"Corresponding author. Fax: +49-7531-88-3184.

£-rnail address: juan.delius@uni-konstanz.de (J.D. Delius).

nerod, 1988, 1997; Zeigler, 1997). Earlier, ethologists had assumed that the pecking re- sponses of birds were fixed action patterns (Lorenz, 1937; Tinbergen, 1951). Such patterns were characterized as species-specific behavioural responses of a markedly invariant course and with an innate developmental origin (Thorpe, 1951).

Later this stringent definition was relaxed in sev- eral respects (Schleidt, 1974; Barlow, 1977) but even so, the concept of fixed action patterns is nowadays no longer considered to be theoretically or empirically useful (Roth and Dicke, 1994).

http://dx.doi.org/10.1016/S0376-6357(01)00208-X

Konstanzer Online-Publikations-System (KOPS) URN: http://nbn-resolving.de/urn:nbn:de:bsz:352-202909

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Nevertheless, the considerations on which the concept was founded have continued to play a heuristic role in investigations on pecking move- ments. Much of the earlier research on the forage pecking of pigeons has referred to the relative immutability of this action (Zeigler et aI., 1980;

Zweers, 1982b; Goodale, 1983). As the research progressed pecking has, however, revealed itself as a flexible behavioural pattern that adapts itself multifariously to a variety of situations (Siemann and Delius, 1992). It is furthermore the prod uct of several interleaved component responses involving the neck, yaw and eye musculature with each being influenced by a number of sensory inputs (Schall and Delius, 1991; Wohlschl~iger et aI., 1993; Zeigler et aI., 1993; Ploog and Zeigler, 1995;

Hbrster, 1997). Over and above that pecking-like motions of different kinds are shown by pigeons in several other behavioural contexts than just food uptake (exploration, grooming, aggression, courtship, nesting, incubation, hatching, parent- ing, etc.; cf. Ramirez and Delius, 1978; Palya and Zacny, 1980; Zweers, 1982a; Delius, 1985, 1988;

Haag, 1991; Hansell, 2000). Repetitive pecking can also be triggered by the administration of a dopaminergic drug, apomorphine (Brunelli et aI.,

1975; Keller and Delius, 200 I).

The fixed action concept patterns focused on two response characteristics. One of these charac- teristics was the invariability of the behaviour patterns. As already pointed out, this feature has turned out to be less pronounced with regard to pecking than it was once thought. The other characteristic of fixed action patterns was that they were assumed to be of innate origin. Indeed, concerning avian pecking it was this latter charac- terisation that early on led to the objection that pecking might be a co-ordination that is learned embryonically (Lehrman, 1953). While this latter hypothesis has turned out to be improbable, there is no doubt that after hatching birds learn much about what is worth pecking and what is not, and about how to beak-handle different food kinds of food to achieve successful ingestion (Hogan, 1984;

Balsam et aI., 1992; Deich and Balsam, 1994). A search pecking with which free-ranging pigeons normally uncover grains among grit by either scattering or removing the latter is an appetitive

response that often precedes a consumatory food pecking. This search pecking can be freely condi- tioned even in adult pigeons. They can be easily taught to peck away one type of grit covering grains but not to peck at another, different grit not containing grains (Wright and Delius, 1994;

Siemann et aI., 1996). It is reasonable to view the key pecking studied by behaviourists as a variant of such food search pecking (Siemann and Delius, 1992). It has of course been more than amply established that the emission of key-pecking for food reward can be brought under the control of a variety of cues and that more generally the location, timing and frequency of key-pecking can be easily conditioned both classically and instru- mentally (Mackintosh, 1974; Delius, 1983). How- ever, the individual motor act of pecking was for a long time considered by behaviourists as a largely immutable, unconditionable response; the reinforceable unit was considered instead to be the inter-peck interval (Zeiler, 1977). A restricted variability of the key-pecking action was only recognized when the duration of thoroughly con- ditioned key pecks were found to be of longer duration and more forceful than not yet properly conditioned key pecks (Schwartz, 1977; Ziriax and Silberberg, 1978). Earlier it had been found that key-pecks rewarded with grain differed from those rewarded with water by the fonner evincing a bill-gape opening and being more forceful than the latter (Wolin, 1968; Jenkins and Moore, 1973;

Spetch et aI., 1981; Stanhope, 1992). More re- cently it was also established that key-pecks re- warded by large grains were characterized by a wider beak-gape than key-pecks rewarded by small grains (Allan and Zeigler, 1994; Jager et aI., 2002).

Note, however, that while the above key peck- ing variants arose during conditioning experi- ments, they did not emerge as a consequence of reinforcement being made contingent on a partic- ular variant of key-pecking. Thus, it was still largely uncertain whether the pecking variants spontaneously shown by adult pigeons were possi- bly due to operantly conditioned modifications or whether they were simply being selectively elicited out of the pigeon's established repertoire of peck- ing modes by varying stimulus contexts, these

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including contingencies that led to classical condi- tioning. However, it was then demo;~strated by Zeiler et al. (1980) that pigeons can be trained to lengthen the duration of pecking-key activations through a differential food reward procedure.

Soon afterwards it was shown that the amplitude of the bill gaping component of pecking was easily amenable not only to classical but also to instrumental conditioning (Mallin and Delius, 1983; Deich et aI., 1988; Jager, 1993; Remy and Zeigler, 1993). It is the case, though, that much earlier Skinner (1960) had already reported in a by-line about having successfully conditioned pi- geons to key-peck so forcefully that the base of their beaks become inflamed. The present paper's purpose is to show that other parameters of the pecking action are equally susceptible to apprecia- ble modification by instrumental conditioning.

However, we first document descriptively that the temporo-spatial course of the approach phase of pecks is considerably more variable than some earlier accounts have suggested. We then report the results of five conditioning experiments aimed at modifying the course of pecks through instru- mental contingencies. The first two of these exper- iments attempted at speeding-up the pecking motion generally, the middle experiment focused on the speeding-up of the approach phase spe- cially and the last two experiments were aimed at slowing-down and even arresting the approach phase of pecking.

2. Experiment I: peck approach action

To establish a background against which to assess the success of the experiments that follow, we first analysed in some detail the kinematics (the so-called topography) of feeding pecks that pi- geons produce during the ingestion of small grains or seeds. We focused on the approach phase of the pecks and examined them while the pigeons fed on single grains or fed from a heap of grains.

2.1. Met/wei

Adult domestic pigeons (Columba Livia) of local homing stock were used throughout this study.

They were housed in individual cages (40 x 45 x 35 cm) located in a well ventilated and brightly illuminated animal room. Lights were on daily from 06:00 to 20.00 h. All experimental treat- ments were in accordance with the rules and regulations of the German animal protection law.

For the duration of the experiments the birds were kept food deprived to between 90 and 80%

of their ad-libitum feeding weight.

Five experimentally naive pigeons were used for this first experiment. The right side of their head and neck was de-feathered under general anaes- thesia (Mallin and Delius, 1983) and five white circular marks (diameter 2 mm) were attached to the tip of the beak and onto the skin about 5, 25, 45 and 55 mm caudal to the eye and along towards the base of the neck (Fig. I). Five once- daily sessions lasting about 20 min were run. The pigeons were placed into a narrow experimental cage (22 cm wide, 50 cm long and 40 cm high) with a transparent side panel. The dark back wall bore a 10 mm sq uare grid of white lines. The pigeons were fed with either single millet grains dropped through a tube into a small bowl placed on the cage floor at the head end of the cage or alternatively with about 100 grains of millet filling the same bowl. The pigeon's side view was videotaped at 25 frames per s with the camera positioned at pigeon level.

2.2. Results

Approximately 50 randomly selected pecks per pigeon under both the single grain and the many grain condition were examined using the video recorder's frame by frame option. Out or these 20 typical pecks for each pigeon were traced off the monitor's screen for a detailed analysis. The spatio-temporal course of a modal single peck approach motion is sketched in Fig. I a. The position from which the movement towards the grain started was labelled with 0 ms. This position had been previously held for several frames and probably involved a first visual fixation of the target grain. Until contact with the grain the movement took 920 ms, including two intermitted stops, each lasting 240 and 160 ms. These stops

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probably involved renewed fixatiQJ1s of the grain with convergent eye movements (Goodale, 1983;

Wohlschlager et aI., 1993). Overall the beak tip moved over a distance of 110 111m while the neck base did so only over a distance of about 30 111111. Some of the latter 1110vement was due to a down- ward bow of the body itself. The overall speed of the beak tip was about 12 cm/s but this included

Fig. I. Experiment I, spatio-temporal courses or the api roach phase 01" six exemplary pecks; a and b, single pecks; e and r, serial pecks; c, peck with intermediate ehanlcteristics; d, peck with aberrant characteristics. The drawings arc based on frame by frame (25 frames per second) video analyses. The tillle course or the approach movements is indicatcd in units or 10

illS.

the two stops. When these were excluded the mean speed maintained over the actual movement stages was about 20 cm/s. The beak tip velocity increased from about 10 cm/s during the initial movement stage to about 40 cm/s during the final movement stage. Another example of an ap- proach motion of a single peck though an unusu- ally brief one, is shown in Fig. I b. In this case there was a single short stop, the beak tip trav- elled over about 850 mm in 360 ms and the beak's overall velocity was thus somewhat higher, namely about 25 cm/s.

More generally the amplitudes of the approach phases of single pecks differed over a considerable range, but in more than 80'% of the cases the movement of the beak tip started from a position above 70 mm frol11 the ground. The duration of the downward movel11ent also varied frol11 one peck to the next, but more than 80% of them lasted longer than 600 ms. All single pecks exhib- ited at least one pause but more than 80'1£, were interruptcd by two pauses, not counting the fixa- tion pause before the downward movcment be- gan. The direction of the beak tip movement was mainly vertical but there were same horizontal components depending on the varying positions of the target grains relative to the pigeons posi- tion. Almost invariably the velocity of the head movement increased while approaching towards the ground and was highest during the last stage of the approach phase.

Fig. I

r

illustrates the course of a typical serial peck directed at grains offered in a heap. From the beginning of the peck movement until grain contact the beak tip travelled ovcr a distance of about 40 mm, did not exhibit any stops, and la ted 120 ms. The overall speed was thus about 34 cm/s. The mark close to the neck base re- mained nearly immobile but was nearer to the ground right from the beginning of the peck. An example of an approach phase of an unu ually slow serial peck including a long stop is illustrated in Fig. Ie. The beak tip moved over about 50 mm in 240 m5 with an overall speed of about 20 cm/s.

At an intermediate stage of the approach, how- ever, it momentarily reached a peak velocity of almost 60 cm/s. The repetitiveness of serial pecks

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was of course their most salient cI~~racteristic.

The pigeons maintained a bowed down body po- sition. The upper position of the head from which the repeated approach movement started was only about half of that corresponding to the start position of single pecks. More than 90% of erial pecks started at heights less than 60 mm. All approach phases of single pecks included no or at most one fixation stop, not counting the stop occurring immediately before each peck was ini- tiated. This latter stop was as a rule inserted immediately after the withdrawal movement of the previous peck. The frequency of this rhythmic serial pecking varied individually between 2.7 and 3.5 pecks per second with the complete peck cycles each lasting on average about 330 ms (cf.

Horster et aI., 2002).

However, pecks of an intermediate kind oc- curred often and also pecks of unusual spatio- temporal courses occurred occasionally even under the two unnaturally uniform laboratory conditions used here. The approach courses illus- trated in Fig. Ic and d present examples of ap- proach phases of an intermediate I eck and an aberrant peck. Although not examined in the present study we know that when grain are offered in a scattered manner they induce a higher proportion of 'uch intermediate pecks and that when grains are offered on broken ground they elicit more such aberrant pecks (cf. Siemann and Delius, 1992). We should note, however, that, as described by others, virtually all pecks that we examined involved an incomplete eye lid closure and a beak gape opening setting in towards the end of the approach phase.

In the remainder of the paper wc refer mainly to instrumentally conditioned pecks directed at a pecking disk (see below) which were subsequently rewarded with grains but were not themselves directly aimed at grains. However, as the frame by frame examination of several hundred such oper- ant pecks videorecorded before any special peck modifying training schedules came into operation showed, the approach phases of disk-pecks did not differ in any fundamental way from the grain pecks described above. Note that different from the peck-key vertically mounted at about eye-

height commonly employed in conditioning exper- iments, the present peck-disk was horizontally mounted at nearly ground level. This without doubt helped to further augment the similarity between grain and grain-rewarded key-pecks al- ready noted by earlier authors (e.g. Jenkins and Moore, 1973; Remy and Zeigler, 1993). When each individual disk-peck was rewarded with grain the disk-pecks took the form of single pecks.

The downward movement duration of these single disk-pecks was on average somewhat shorter than that of single grain pecks. Whereas only about 20'% of the approach durations of the latter were briefer than 600 ms, as many as about 50% of the approach durations of the former lasted less than that. This was principally caused by a smaller mean number of stops by a somewhat briefer mean duration of tops and by some shortening of the mean overall path. The overall shorter duration of the approach phase could be ascribed to the circumstance that the pecking-disk was a much more extensive target than the millet grains and was located at an absolutely fixed position (see below). When several consecutive disk-pecks were demanded for a reward, the disk-pecks took the characteristics of serial pecks. These were by all criteria indistinguishable from the serial grain pecks. Incidentally, as it has already been de- scribed by the above cited authors, the eye lid part-closure and the grasp-preparing gape open- ing were also reliable constituents of all of the operant pecks examined (cf. Ostheim, 1997a; Os- theim et aI., 2002; J~iger et aI., 2002).

3. EXI)eriment II: fixed ratio pecking

It is well established that pigeons can be trained to repeatedly peck at a disk when grain reward is only given after a fixed number of responses (Domjan, 1993). One could expect that in an effort to maintain a high ingestion rate, pigeons might peck at higher speed as the number of responses leading to reward is progressively in- creased. This experiment examined whether in- creasing response demands would actually lead to shorter intervals between successive disk-pecks.

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Fig. 2. Experiment II, Conditioning apparatus, schematic, with a piezoceramic pecking disk, a small reeding bowl, barriers, an automatic grain dispenser and an inrrared photoelectric gate.

The gate was used in later experiments (see text).

3.1. Method

Six new pigeons were used. No neck and head markers were affixed onto these pigeons. The experimental chamber was widened to 30 cm. A 20 mm diameter pecking disk was cemented onto a 15 mm diameter piezoceramic element mounted on a 6 x 15 x 0.5 cm iron slab placed at the head end of the cage. The pecking disk surface lay 10 mm above the cage floor level. The peck-impact signals were amplified and fed into an analogue to digital interface of a home computer. A small feeding bowl was glued onto to the iron slab 6 cm sidewards from, and level with the pecking disk.

A solenoid operated food dispenser located out- side of the cage delivered 2-4 grains of millet into it through a tube. A small 2 W bulb installed 20 cm above the disk-bowl arrangement served as a house light (Fig. 2). The computer was pro- grammed so as to control and record the course of the sessions (Xia et aI., 1991). The twice daily

(morning and evening) sessions lasted about 30 min each.

The pigeons were first rewarded by the experi- menter for approaching the disk and subsequently for pecking it. Then every peck to the disk led to an automatically delivered grain reward. When the pigeons had completed two sessions lasting about 25 min under this condition the reinforce- ment schedule was modified so that only every second peck onto the key was rewarded. A reward thus followed whenever an interpeck interval be- tween two consecutive disk contacts had been registered. The number of interpeck intervals re- quired before reward delivery was afterwards suc- cessively increased every four sessions to 2, 5, 10, 20, 50, 100, 200, and 300 inter-peck intervals per reward. These schedules were thus far equivalent to fixed ratio (FRn) schedules (Oomjan, 1993).

However, deviating from these, if any interpeck interval exceeded 3 s there was a 10 s time-out penalty during which the houselight was extin- guished, no rewards were available and further- more the interpeck interval counter determining the delivery of the next reward was reset to O. The durations of all interpeck intervals produced dur- ing the two last sessions at each schedule value were stored for later analysis. The birds were videotaped during every second session.

3.2. Results

At the beginning, the pigeons adapted quickly to the increasing schedule demands. One to two sessions under each newly instituted schedule sufficed to induce regular pecking with no more than 5% of all interpeck intervals longer than 3 s.

However, as the higher response ratios were intro- duced one pigeon ceased to peck altogether when the 100 intervals schedule came into operation, two when the 200 intervals schedule was instituted and another two when the 300 intervals schedule become operative. Only one pigeon continued to peck with the 300 intervals schedule. This reflects the effect of the so-called ratio strain (Oomjan, 1993) that presumably operates towards prevent- ing foraging pigeons from incurring a too large effort for a too small return. Nevertheless, the terminal ratios achieved by our pigeons fell well

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short of those that pigeons have been reported to cope with in the reinforcement schedule literature (Ferster and Skinner, 1957). However, in the rele- vant studies the pigeons were kept much hungrier, received larger rewards and were not penalized for longer breaks of pecking.

Fig. 3 shows that under the initial one inter- peck-interval per reward condition each pigeon exhibited individually typical mean interpeck in- tervals ranging from 255 to 352 ms (average 321 ms). The figure also shows that as the ratio was increased to 2 intervals per reward the average interpeck intervals decreased in duration from an average 321 ms to a average 292 ms (I.-test, P <

0.05). This shorter duration was retained with the ratio of five intervals per reward but after that the interval durations rose as the ratio was increased to 50 and beyond. With the ratio of 50 the mean interpeck intervals was already tendentially longer over those shown under the initial one interval per reward ratio (mean 340 ms, P < 0.06). This was mainly because occasional markedly longer than usual interpeck intervals became more frequent at the higher ratios. The video recordings indicated that these longer intervals represented definite in- terruptions of serial pecking. However caused (Zeiler, 1977), these pauses appeared to serve a vigilance function ince while they lasted the pi- geons took up a relatively upright posture and often showed head- canning movements (cf. Hart and Lendrem, 1984).

400 ms D

~ A

350 II A A

0 0 0

a • +

0 >

.

:;; 300 +

C +

+

250 + +

+ +

200 I I I I I I I I

2 5 10 20 50 100 200 300 ratio

Fig. 3. Experiment II, Interpeck interval durations as a func- tion or progressivciy increasing number of intervals required to obtain reward. Symbols: means of six individual pigeons; thick line: averages.

It is well established that higher FR ratios lead to so-called post-reinforcement responding pauses (Ferster and Skinner, 1957; Felton and Lyon, 1966). To check whether such an initial pausing and whether a hypothetical final slowing due to exhaustion might contribute to the above men- tioned longer mean intervals we examined whether the first five intervals, and separately the last five intervals of the pecking runs were longer than the overall mean intervals but this was not found to be the case. Note that the reinforcement schedule that was used included a longer than 3 pause penalization element and this might have suppressed the emergence of any extended post- reward pauses. The finding that even prolonged bouts of pecking did not elicit any outwardly obvious signs of exhaustion agrees with findings reported by H6rster et al. (2002). These latter authors, incidentally, also reported that pecking tended to be slightly but significantly slower in the afternoon than in the morning. Thi could be confirmed using the present data. On average, the afternoon mean intervals were slightly (6 ms) but nevertheless significantly longer than morning mean intervals (I-test, P < 0.05).

4. Experiment III: shortened pecking

Since imple fixed ratio conditioning had not led to a consistent shortening of the interpeck intervals it was now attempted to achieve such an effect with a more exacting conditioning proce- dure that involved an explicitly differential rein- forcement of shorter inter-response times.

4.1. Mel/lOci

Six new pigeons were used. The same apparatus as before was used. Daily sessions lasted about 20 min each. The pigeons were pre-trained in the same way described in the previous section. When their pecking performance had stabilized at the one reward per two consecutive pecks (FR2 schedule) the interpeck intervals of 1000 pecks (10 sessions

a

100 pecks) were recorded as a baseline for each pigeon. During the following sessions the pigeons were only grain rewarded 'when the inter-

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400 ms

0

350

.

0

-0

.

>

2

300

c

250

200 $

i i i i

0 2 3 4

round

Fig. 4. Experiment Ill, lnterpeck interval durations achieved during successive shortening conditioning rounds. Symbols, means or six individual pigeons; thick line: averages.

vals between the two consecutive key pecks were at least 10% shorter than their individual mean baseline intervals. Intervals longer than these threshold durations were penalized with a time- out period lasting 10 s. That is, a differential reinforcement of short inter-response times was enforced (JRT < t schedule; Zeiler, 1977). When the individual pigeons produced 75% shorter than criterion intervals within each of 10 successive sessions of 100 pecks the mean duration of these 1000 inter-peck intervals was set as threshold for the next round of sessions. Each pigeon was videotaped during at least one session both under the baseline condition and under each of the subsequent speeding-up training rounds.

4.2. Results

The mean interpeck intervals during the base- line condition varied individually between 267 and 368 ms, average 318 ms (Fig. 4). The pigeon starting from the shortest baseline mean interval did not achieve the 75% shorter interval level in any of 12 sessions of the first reduction training round. One pigeon did so during its seventh ses- sion, three pigeons during their second or third session and one pigeon did so in its very first session. Four of these latter five pigeons pro- gressed up to a third round of interval reduction training and one even up to a fourth round. The final mean intervals achieved by the six pigeons ranged between 202 and 240 l11S (average 221 ms).

The reductions of interpeck interval durations achieved by the various pigeons ranged between 10 and 39% (average 29%). A t-test showed that the overall interpeck interval reduction achieved was significant at the P < 0.05 level. This result generally agrees with the findings of earlier studies employing TRT < t schedules (Zeiler, 1977).

Twenty baseline condition pecks and 20 final round pecks of one typical pigeon (# 642) were analysed frame by frame from the videotapes. The results of this analysis are summarized in Fig. 5.

They confirm that the pigeon managed to consid- erably shorten the inter-peck intervals, the mean duration of the pecks sampled diminishing from 326 to 204 ms. This reduction was mainly due to a shortening of the withdrawal phase duration and a reduction of pause durations while the approach phase was only slightly abbreviated.

The time reduction was achieved through a short- ening of the overall beak tip path from 86 to 27 mm. The actual movement velocities somewhat paradoxically decreased from 42 to 23 cm/s. A less detailed examination of the video recordings of the other pigeons showed that they too had achieved the shortening of the interpeck intervals by markedly decreasing the path length of their pecks, that is by keeping their body bowed and their heads closer to the ground. Thus, although reducing the interpeck interval durations as re- quired by the schedule, the pigeons did not man- age to retain the velocities shown under ba eline conditions. The new peck paths were presumably

360 90 45

ms mm em

Is

270 a a

60 30

180 p

m

30 w

tB

15 wa t

~

90 w

0 0 0

bas. con. bas. con. bas. con.

duro path vel.

Fig. 5. Experiment Ill, Mean durations (dur.), path and velocities (veL) or pecks of one pigeon (# 642) berore (base- line) and after conditioning ror shorter inter-peck intervals.

The total (/) intervals are divided into withdrawal (w), pause (p) and approach (a) phases.

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too short to achieve the requIsite he~d accelera- tions. This indicates that the pigeons were re- sponding to the IRT < t schedule demands by producing pecks that from their kinematics were more similar to serial pecks than to single pecks described in Experiment 1.

5. Experiment IV: approach SIJeed-up

Although the previous experiment had achieved a conditioned shortening of the interpeck intervals it had failed to yield any major velocity speed-up of the peck motions. In fact, rather the reverse had happened. The present experiment involved an attempt to obtain a true motion velocity in- crease specifically affecting the approach phase of pecking.

5.1. Mellwel

Seven new pigeons were trained. The apparatus used was the same as before except that an in- frared photoelectric gate spanning a 7.5 cm deep by 15 cm wide surface over the pecking disk and the reward bowl was mounted on the support iron slab. The height of the gate over this platform was adjustable and it yielded a signal when its virtual surface was broken anywhere by the pigeon's beak. The birds were pre-trained to peck the disk for an automatic grain reward after each disk- peck. The infrared gate was then positioned 65 mm above the key. Adjustable barriers made of dark plastic board placed in front of the gate and between the disk and the bowl ensured that the pigeons lifted their head and beak to an above- gate level before initiating pecks towards the disk. The duration of the downward motion of each peck was derived from the instant when the gate was broken until the instant when the impact on the disk was registered. Note that pecking disk responded to the impact of the tip of the beak while the infrared-gate only responded to the transit of the shaft of the beak. The veridical length of the course over which the duration of the approach movement was measured was, there- fore, closer to 60 111m instead of the nominal 65 mm. All the velocity estimates given later take this circul11stance into account.

Daily sessions with each pigeon lasted until a 100 such intervals had been recorded. This took less than 15 min. The individual mean of the approach durations recorded during 10 such ses- sions served as baseline. In the sessions that fol- lowed the pigeons only received grain rewarded for approach durations that were at least by 10%

briefer than their baseline approach duration. Du- rations longer than this criterion where followed by a 10 s timeout penalty. When a pigeon pro- duced 75'% rewarded peck approaches within each of ten consecutive sessions of 100 pecks each, the mean approach duration it had produced during these 10 sessions was used as the new threshold duration for the its next sessions. The last 1000 approach durations under each successive threshold condition were evaluated. This proce- dure continued until the individual pigeons failed to reach the 75% rewarded criterion with a new threshold duration within 20 sessions.

5.2. Results

The pigeons yielded mean approach baseline durations of between 199 and 270 ms with an average duration of 234 ms. They all managed to shorten the approach duration of their pecks. They did this maximally over one (one pigeon), two (one pigeon), three (one pigeon), four (two pigeons) or five (two pigeons) rounds of threshold reduction. After having achieved a given speed-up criterion and upon having been switched to the next threshold reduction round, the pigeons that succeeded first exhibited approach durations around the previous mean but by about the sec- ond to fourth session they began to exhibit the newly required shortened approach durations. Overall they achieved final approach durations of an average of 165 ms with the individual means varying between 129 and 210 ms (Fig. 6). A l-test showed this red uction to be significant at the P < 0.01 level. These ·final approach durations were shorter than baseline by between 10 and 76%

and on average by 49%. The pigeons thus suc- ceeded to speed-up the approach pecking move- ment from an average overall velocity of about 26 cm/s to one of about 36 cm/s.

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280 ms ~

240 'i!

0 +

£ I.l

a ~ 200

B +

a. a. 0

a 0

160

I 0

120 r r r

0 2

round 3 4 5

Fig. 6. Experiment IV, Average approach durations achieved in successive rounds of shortening conditioning. Symbols, means of seven individual pigeons; thick line: average dura- tions.

6. Experiment V: approach slow-down

The preceding experiment demonstrated that a true speeding up of the peck approach motion could be conditioned. The present experiment aimed at conditioning the reverse, a decrease of the approach velocity, that is a lengthening of the approach durations. A more direct conditioning procedure than that used for the preceding exper- iments was tried out.

6.1. Method

Five new pigeons were trained. The same ap- paratus as above was employed. The pigeons were pre-trained as described before until every disk- peck was automatically grain rewarded. The in- frared gate was now placed 70 mm above the key.

During 10 daily sessions, each of which lasted until about 100 pecks had been issued, approxi- mately 1000 peck approach durations were recorded for each pigeon. From these we derived individual mean baseline durations. Approach du- rations that were longer by 10 (two pigeons), 15, 25 and 30% (one pigeon each) were then calcu- lated. In the following sessions only disk-pecks preceded by approach durations as long or longer than the corresponding threshold were rewarded with grain. Shorter than threshold approaches were penalized by timeouts. The experiment ended when the pigeons completed 24 sessions.

6.2. Results

The detailed course of the conditioning was first examined using the data of two pigeons (Fig. 7).

Pigeon # 336 needed a mean 254 ms for the approach movement from the gate to the key during the before-conditioning baseline phase. £t was then only rewarded for approaches as long or longer than the calculated threshold of 279 ms . The mean approach duration increased to 321 ms during the first session. The mean duration corre- sponding to all 24 sessions, each lasting until about 100 pecks had been issued, was 370 ms.

This entailed an approach duration increase by 46%, well above the imposed threshold duration.

The proportion of longer than threshold ap- proaches was close to 50% during the first two blocks of sessions but subsequently it increased to about 85% (mean of all 24 sessions was 81 %).

Even though pigeon # 337 started with the same mean 254 ms baseline approach duration as the previous bird and was imposed with the same 10%

increased threshold duration of 279 ms, it yielded relatively shorter approach durations and thus obtained a lower 55% rate of rewards. Neverthe-

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Fig. 7. Experiment V, Conditioned lengthening of the peck approach durations in two pigeons (# 336, # 337) over suc- cessive blocks of two sessions. Circles, mean approach dura- tions; arrows: set threshold durations; crosses, percent rewa rded responses.

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Fig. 8. Experiment V, Approach durations before (front columns) Hnd after conditioning for approach slowing (rear columns) cxhibitcd by five pigeons.

less, the mean approach durations always ex- ceeded the threshold duration. The mean ap- proach duration across all sessions was 316 ms and thus by 24% longer than the mean baseline duration.

The baseline approach phase duration ranged between 254 and 319 ms for the various pigeons and averaged 283 ms. Over the 24 conditioning sessions all pigeons increased their individual mean approach durations to between 316 ms and 374 ms with an average of 354 ms. This involved conditioned increases ranging between 18 and 46(% with an average duration increase of 26%

(Fig. 8). The video-recordings indicated that the different birds used somewhat different strategies to achieve the slow-downs. A more frequent sec- ond midway stop, a lengthening of the stops and an actual reduction in movement speeds were involved. The overall average approach motion velocities calculated on the basis of an actual 65 mm beak-tip path was in any case reduced from an average 23 cm/s before conditioning to an average 18 cm/s after conditioning. It must be remarked that the two pigeons which had been set the proportionally highest thresholds demanding duration increases by 25 and 30°/., did not manage to exceed these on a regular basis. They merely a,chieved mean duration increases of 18 and 23<10 and accordingly only received sporadic rewards for their pecking efforts. The all-at-once method used in this experiment (it represented an attempt to abbreviate the proceedings) was undoubtedly

suboptimal compared with the more gradual step- by-step conditioning used in earlier experiments.

Nevertheless, the mean increase in duration was significant (Hest, P < 0.05). Using a different conditioning procedure the next experiment will demonstrate that pigeons can achieve a more pronounced lengthening of the approach phase.

7. Experiment VI: approach interruption

The purpose of the final experiment was to find out whether or not the approach phase of pecks was visually ballistic. Lt investigated whether the pigeons could be taught to interrupt a peck ap- proach once it had already started and if so, up to which stage of the peck approach pha e they could still do so.

7.1. Method

Fifteen naive pigeons were trained. The appara- tus was the ame a before but a red and a green light emitting diode were centrally cemented 5 mm apart onto the pecking disk. The birds were pre-trained as before to peck the now red diode-lit disk. Then they were trained to continue pecking the red lit disk but to withhold from disk contacts when the disk was green diode lit. We first de- scribe the procedure as it applied to a first group of eight pigeons. Either the red and green light were on during quasi-randomly alternating 3 s periods with in-between intervals lasting 5 s when the disk was dark and deactivated. Pecks upon the onset of the red light periods were grain rewarded. Pecks upon the onset of the green light periods were penalized by 5 s timeouts. When the pigeons had learned this go/no-go discrimination task and issued not less than 80% of all their pecks at red-lit disk the procedure changed. All trials started with the red diode being lit. In 70%

of the trials the red diode stayed on until the pigeons pecked and were rewarded. During a 30%

randomly selected trials the red diode was switched off and the green diode came on midway during the peck approach movement. The color change was triggered by the pigeons' breaking of the infrared gate set at 70 mm above the pecking

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Fig. 9. Experiment VI, Approach movement interruptions. Circles (right side scale): mean S.D.) average correct peck interruptions at different infrared gate heights (15 pigeons). Triangles (left side scale): average approach durations S.D.) measured with a standard 4.5 cm gate height after each gate-height stage (seven pigeons). The entries designated refer to mean approach durations obtained prior to the approach interruption training.

disk. If the pigeons succeeded to interrupt their peck they avoided the time-out penalty consequent on green disk contacts. After the pigeons re- achieved the 80% correct pecks criterion during six consecutive sessions of 100 trials each, the gate then was lowered to 60 mm and the red-go, green-stop condition re-instituted until the pigeons re-achieved the 80% correct criterion over three consecutive sessions of 100 trials each. Afterwards the gate was lowered to 50 mm and the red-green condition run again until criterion and so on successively for 40, 35, 30, 25, and 20 mm gate heights above the pecking disk. This implied that in the corresponding sessions and trials the green stop light was turned on at increasingly later instants of the peck downward motion. The proce- dure served to assess the stage of the approach phase up to which the pigeons could still abort the pecking motion.

The second group of seven pigeons were treated in exactly the same way except that they experi- enced two interspersed sessions of 100 trials with the gate positioned at 45 mm height under red light only condition while the gate was still otherwise set at a 70 mm height and before they were submitted to the green-interruption training. Single sessions of this same type were subsequently interspersed after training with each of the eight lower gate heights had been completed. These sessions served to assess the peck approach durations under a standardized condition throughout the experiment.

7.2. Results

None of the animals ever interrupted their pecks when the disk remained lit red throughout the trial (0% peck interruptions). All animals, however, interrupted at least a proportion of their pecks on trials where the disk illumination switched to green. The proportion of correct stops depended at which stage the interruption signal was presented, that is the height at which the gate was positioned (Fig. 9). When the stop signal occurred at a gate-disk distance of 70 mm the percent of correct interruptions varied between 74 and 99% with an average of 89%. At a height of 60 mm the correct rate decreased to between 74 and 96%, average 85% and so on until at the lowest 20 mm gate position only between I and 22%, average 8% of the stop signals led to peck abortions (Table I).

Table I

Percent mean interruption scores (with Standard Deviation (S.D.) and range) of 15 pigeons at different gate heights (in mm)

Gate height 70 60 50 40 35 30 25 20

Mean 90 85 70 43 36 23 15 8

S.D. 8 6 13 15 19 16 II 6

Worst 74 74 44 23 17 7 4 I

Best 99 96 90 76 69 65 47 22

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As the average number of correct approach interruptions at the final gate height ·U 20 mm dropped to less than 10% it seemed possible that the green diode light could have lost some of its conditioned interruption signalling function.

Therefore, ten pigeons were re-exposed to a 45 mm gate height, green-stop condition. They in- deed only yielded an average of 38% interruptions but after two or at most three retraining sessions they achieved an average 64% interruptions, a level that was higher than the original combined average of 56% at the 50 and 40 mm gate heights.

Although the pigeons never interrupted their pecks during the trials when the pecking disk remained red throughout, they nevertheless modified their pecking action under this condi- tion. Fig. 9 also shows the average approach durations (interval between gate and disk signals) exhibited by the second group of seven pigeons during the interspersed red-only sessions with the gate positioned 45 mm above the disk. It can be seen that during the baseline no green-stop signal phase of the experiment the pigeons exhibited mean durations of 73 ms (ranging between 59 and 91 ms) but that after the stop training came into operation they lengthened these approach dura- tions to an average 194 ms (ranging between 153 and 252 ms), without showing any regard to the gate height that had been in operation during the immediately preceding sessions. In other words, as the interruption training with the stop signal came into force the pigeons had switched from an average velocity of about 55 cm/s to a slower one of about 21 cm/s, these estimates being again based on the factual path length of about 40 mm instead of the nominal 45 mm (see earlier under Experiment IV). This speed reduction measured during the end phase of pecks presumably gave the pigeons more time to abort the approach movement should the stop signal light up. Despite a considerable inter-individ ual variability, the video recordings confirmed a marked reduction in the overall approach speed against the baseline condition. This was partly achieved by a more frequent insertion of a second stop late in the downward motion but not through any marked prolongation of the movement stops. The remain- der of the overall slow-down, however, was due to

lower head movement velocities. The pecks con- nected with the approach interruption training generally took the same form as the slower single pecks with two midway stops described in Experi- ment I. Four pigeons in fact did not fully inter- rupt the approach motions but managed to avoid triggering the pecking-disk signal by just touching it softly (two pigeons) or by redirecting the mo- tion past the disk to hit the base-plate (two pi- geons). The remaining II pigeons did actually arrest the approach movement in mid-course (ex- cept perhaps at the lowest gate position) by adopting rather idiosyncratic movement courses.

8. General discussion

The approach phase of pecks directed at single grains or at a heap of grains varied considerably between and even within each of the two condi- tions. Although we could grossly distinguish the kinematics of single pecks directed at single grains and serial pecks directed at grains in the heap, the details of the path and speed of the various points along the neck, head, and bill were virtually never the same from one peck to the next. Notice that this was so even though we made an effort to standardize the contexts in which the pecks were issued. Such invariable contexts are unusual in nature, where the location, shape, consistence, etc.

of food morsels that domestic pigeons consume are highly diverse (Haag, 1991). Indeed, in a study aimed at finding out how well adapted to deal with an experimentally construed diversity of feeding situations Siemann and Delius (1992) identified a considerable variety of feeding peck styles. Under the conditions that we examined in this study we found it practical to distinguish two types of pecking, single and serial that were con- nected with the pecking at single grains and the pecking of many grains. However, under more naturalistic conditions it is likely that a consider- able proportion of pecks cannot be fairly adjudi- cated to either type because of being intermediate, or even being aberrant in their temporo-spatial, kinematic course. The morphological basis for the remarkable diversity of approach courses is of course the multiple articulation, complex l11USCU-

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lature and differentiated innervation of the pi- geon's neck (Baumel et aI.,

1 979;

Horster et aI.,

1990). Thus, even before being subjected to any explicit conditioning pigeons possess a wide reper- toire of approach phase styles. This response vari- ability undoubtedly provides a more than ample behavioural substrate on which reinforcement can act selectively (Staddon, 1993).

The attempt at modifying the speed of pecking motion by imposing a fixed ratio schedule was not systematically effective. Even though the pigeons had the possibility of augmenting the frequency of rewards through an abbreviation of the interpeck- intervals they made only restricted use of this option. When between two and five consecutive inter-peck intervals per reward were demanded the mean duration of intervals were shortened compared against the one interval per reward baseline. But as more intervals per reward were required then the interpeck intervals increased again. The decreased interval duration at the in- termediate ratios was principally due to the fact that the pigeons resorted to a serial pecking mode involving a reduced path from the upper fixation position to the key contact position. The length- ening of the inter-peck durations with the higher ratios was mainly due to the occurrence of an increased number of occasional longer intervals.

Interestingly, earlier authors (Felton and Lyon, 1966; Powell, 1968) could not find any systematic changes of interpeck intervals duration dependent on fixed ratio increases. However, they specially separated the long post-reinforcement pauses that arose at higher response-to-reward ratios. We in turn largely succeeded in suppressing longer than 3 s intervals through a differential penalization procedure.

Regardless, a direct inter-peck timing criterion for differential reward and penalty deliveries (an

IRT < t schedule) adopted in Experiment III led

to clearer results. As the criterion interval dura- tion was successively reduced the interpeck inter- val decreased in a systematic and pronounced way as the criterion was successively lowered. But it was the case that the pigeons achieved this by increasingly adopting a short path pecking action similar to that described for serial grain pecks.

The birds had actually decreased rather than in-

creased the approach movement speed, thus pre- sumably avoiding any additional muscular effort.

When the procedure was modified in Experiment IV so that the beaks' transit times between a photoelectric gate and the pecking disk were mea- sured and true approach speed increases could be differentially reinforced then the pigeons did ac- celerate their peck movements considerably. The pigeons, with some individual variation, were found capable of increasing the average baseline head approach velocity of about 26 cm/s to an average velocity of about 36 cm/s. This no doubt involved an increased contraction effort by supinator muscles for the attainment of a higher peak velocity but also an increased effort by antagonist elevator muscles to prevent a too forceful, presumably aversive contact with the pecking disk (cf. Skinner, 1960). This indicates that normal disk-pecks are performed with an approach velocity that is markedly short of the maximum speed indicating that muscular effort or strain is likely to be a factor that moderates pecking movement velocities. Indeed, it is relevant to ask why pigeons peck for food as fast as they do. The functional answer probably lies in the fact that as highly social feeders they have to normally compete for food resources with flock compan- ions and this puts a special premium on speed (Murton et aI., 1972).

To complete the picture on the conditionability of the peck approach speed Experiment V was designed to achieve a slowing down. Since we expected this to be an easier task than condition- ing a speeding-up and we were intent on abbrevi- ating procedures, a rather high duration criterion was tried with some of the subjects. Though in retrospect these higher threshold durations were undoubtedly too steep to be fully effective, all the pigeons lengthened the approach phase durations as compared with the pre-conditioning baseline.

The mean approach speeds were reduced by from about 23 to about 18 cm/s. In Experiment VI we were successful in conditioning a more extreme form of speed reduction, including even a com- plete interruption of the approach movement. The pigeon's ability to learn to obey a visual stop signal during essentially all stages of the approach movement, although with decreasing efficiency,

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