The development of pecking in ring doves is described and
analyzed as a model system for understanding the roles of learning
in behavioral development. Ring dove squab go from complete
dependence on their parents to independent feeding during the third
and fourth week post-hatch. They learn to identify food and to
consume it through their interaction with food and their parents.
This chapter describes experiments that analyze the specific
learning mechanisms involved in the development of pecking and what
it is that squabs learn from their experience. More generally,
the chapter illustrates the utility of applying learning principles
to the analysis of behavioral development.
I. Learning and Behavioral Development
Two relatively independent traditions in psychology
emerged for the study of how experience affects behavior (Balsam &
Silver, 1994). Developmentalists have long acknowledged a central role for
the effects of experience on behavioral development and, quite independently,
studies of learning and memory have occupied a central place in psychology.
Yet there has been relatively little specific application of knowledge and
principles in one domain to specific studies in the other domain. This lack
of cross fertilization has occurred for a number of reasons (see Balsam &
Silver, 1994) but, most importantly, the developmentalist has been most
deeply concerned with the emergence of new response forms over time (Kagan,
1971; Lewis & Starr, 1979; Werner, 1957), whereas the learning
psychologist has been most concerned with changes in controlling stimuli
rather than changes in response topography (Balsam, 1988). However, it is clear that both areas
have identified and analyzed important processes and that a complete account
of both behavioral development and learning will require a synthesis of these
areas (Balsam & Silver, 1994).
chapter we focus on how learning principles can be used to understand the
behavior by describing our work on the development of pecking
in the ring-necked
dove (Steptopelia risoria). We study this system as a model for
understanding how learning contributes to behavioral development (Balsam
& Silver, 1994). First, we describe the character of adult
pecking. Next, we describe how this response emerges during development
followed by a summary of our research on how birds learn what to peck at and
how they learn to make the skilled adult motor response. Lastly, we
return to the general implications of this work for an analysis of behavioral
The beak is the primary means
by which most birds influence the world. It is used for prehension in a
similar way to how primates use fingers and hands to reach, grasp, and
manipulate objects. The beak is used to pick up and ingest food, preen the
feathers, and build the nest. In many altricial species the beak is used to
feed the young, either by grasping the beaks of the young and delivering food
regurgitatively or by using the beak to seize food and carry it back to the
nest. Additionally, the beak is sometimes used to drink, inflict injury, and
to produce song. All of these actions involve different topographies of
movement that are tuned to the specific stimuli and functions. These behaviors
are highly skilled in adults.
For example, the video clip
illustrates how picking
up a piece of seed is a highly skilled action. The views from above and
below expose the precision of the movement. An accurate response
requires that information about the spatial location and properties of the
target seed be rapidly acquired. This information is then used to precisely
guide the motion of the head and the coordinated movement of the beak in grasping
a seed. This is a remarkably flexible system in the fine adjustments that are
made during each unique occurrence of the behavior. The trajectory of the
head must change with each change in body position and the opening of the
beak must be scaled to each change in seed size. In adults, the
behavior is stereotyped and efficient, but it does not start out that
Prehension in avians, like that
in primates (Jeannerod, 1981, 1984), requires the coordination of two
response components (Deich, Allan, & Zeigler, 1988; Klein, Deich, &
Zeigler, 1985; Zeigler, Levitt, & Levine, 1980;): the effector must
be transported to the target object and then closed around it. In primates
the transport component is typically called reaching or simply transport; for
avians we prefer the term thrust. Closing around and securing of the target
is called grasping. A successful peck requires that thrusting and grasping be
The duration of thrusting and
the accompanying opening phase of grasping is so short (about 50-120 ms, Deich, Klein, & Zeigler, 1985;
Bermejo, Houben, Allan, Deich, &
Zeigler, 1989) in the Columbidae that either high speed motion
photography (Klein, Deich, & Zeigler, 1985) or direct transduction by
attached sensors is necessary to measure these movements precisely . A system for the direct
measurement of the gape component was developed
in the pigeon (Deich, Houben, Allan, & Zeigler, 1985). A small rare-earth
magnet is glued to the bird's lower beak and a small magnetic sensor to the
upper beak. The intensity of the magnetic field detected by the sensor
decreases as the beak opens. The signal from the sensor is input into a
computer and the changes in interbeak distance are recorded in real
We employed this system to
record the gape of
adult ring doves (from Deich & Balsam, 1994).
This figure shows the
movement of the head synchronized with that of the beak. The gape sensor
record, shown in the lower panels, is also synchronized with the drawings.
The top panel of the gape sensor record shows displacement and the bottom
panel velocity. Negative velocities indicate reclosure of the beak. As an
ingestive sequence begins, the head is briefly fixated above the seed
(Goodale, 1983) and then drives downward toward the seed. During this
thrusting movement the beak opens. Gape reaches its maximum
(1-2mm wider than the seed) while the head is being thrust downward. As the
beak reaches the seed it is reclosed around it creating a plateau in the
record. After reclosure with the seed secured at the beak tip, there is a set
of higher velocity beak opening movements that typically have only a single
velocity peak and result from moving the seed to the back of the beak for
swallowing. The movement of the seed in the beak is called mandibulation, and
is often assisted by the tongue which sticks to the seed and pulls it
backwards ( Van Gennip, 1988; Zweers, 1982b). At the end of the ingestive
sequence the beak opens one final time. This opening appears on the gape
record as a peak with rounded shoulders. According to Van Gennip (1988), this
epoch of the signal is caused by movement of the tongue to allow
Development of Pecking
The Columbidae family (pigeons
and doves) provides good model of how adult feeding responses emerge during
development (Balsam, Deich & Hirose, 1992). The general developmental
pattern is similar in all members of this family and has been described in
detail for the ring dove (Lehrman, 1955; Silver, 1978; Wortis, 1969).
After a male and female ring
dove have mated, the female typically lays a clutch of two eggs
that are incubated by both parents for 14-15 days prior to hatching.
Initially, the young are unable to feed themselves. The parents feed the
squab "crop-milk" (Wortis, 1969), a cheese-like substance produced
in the crop. Both parents participate in the regurgitative feeding
of the young (see a video of this behavior). These feedings are initiated by the parents after hatching, but by
the time the young leave the nest on about post-natal day 10 (PD 10), the
squab beg vigorously to obtain parental feeding. As can be seen in the next
video clip, the begging consists of the
squab thrusting its beak at the parent's beak while making very rapid
fluttering motions with its wings. The begging is sometimes accompanied by a
chirping-like call. During regurgitative feeding, the
parent grasps the bill
of the squab between its mandibles and makes vigorous pumping movements with
its upper body, particularly with its neck and head. From around the third
day post-hatching until the squab begin getting food on their own, the
parents feed the squab crop-milk mixed with increasing amounts of seed (Graf,
Balsam & Silver, 1985). The amount of crop-milk
fed to the squab increases through about PD 5 and declines thereafter.
The amount of seed fed to the squab steadily increases until they are eating
seed on their own during the fourth week after hatching.
The squab's and parents'
behavior surrounding feeding interactions changes during the
dependent to independent feeding (Hirose & Balsam, 1995). All of the
young that we have observed will beg
for food at least through the time of weaning (PD 21).
The next figure shows
feeding (see triangles) of young begins to decline near the end of the
third week and usually ceases by the
fourth week post-hatch. Pecking begins
around PD 14 and increases in frequency. Squab begin to successfully ingest
seed by about PD 16 and continue to improve in efficiency (see
circles). By PD 28 they are nearly as efficient as adults at ingesting
seeds (Balsam et al., 1992). We (Deich, Tankoos & Balsam, 1995) used the hall-effect
system to follow the changes in gaping from the time squab start pecking until they independently feed.
There were three types of
gapes that were dominant during different steps in the development of feeding. Over 90% of all
pecks could be sorted into the three types shown in the next
figure. All of the squab moved through the
same sequence of changes in topography as pecking became more efficient. Each transition in form was
associated with improved efficiency of pecking. These were just the sort of changes
that one might expect if the squab’s experience with seed was contributing to the changes in response
Thrusting and gaping
In our first approach to the
question of whether normal development of pecking depends on experience, we
(Graf et al., 1985) reared squab without any exposure to seed. We achieved
this by grinding seed into a fine powder and gradually making this the only
source of food available to parents prior to hatching. This powdered seed was
also the only source of food available to the families after the squab
hatched. We have found no differences between these powder-reared squabs and
seed-reared squabs in growth and general behavior. Beginning on PD 14 and on
all subsequent days, squabs were put into a chamber with seed on the floor
for twenty minutes. We found that powder-reared squabs pecked very little
during these test sessions as compared to seed-reared subjects indicating
that direct experience with seed is important for development of the adult
We then began to analyze
whether and what kind of experiences are necessary for normal development to
occur. First, we have found that
2-3 week old squabs have an unconditioned
tendency to thrust at seed (Graf et
al., 1985). This response will habituate unless the sight of seed is followed
by positive ingestional consequences (Balsam et al., 1992). These Pavlovian pairings
increase the rate of thrusting at seed (Deich & Balsam, 1993) and these
pairings occur in natural parent-squab interactions (Hirose & Balsam,
1995; see Akins & Hamilton (2001)
for the role of Pavlovian conditioning in avian sexual behavior).
The pairings of seed and food,
while increasing head thrusts toward seed, do not result in the release of
the skilled adult response. Initially, the opening and closing of the beak is
only loosely coupled to head movement (Deich & Balsam, 1993; Deich,
Tankoos & Balsam, 1995) and seems to be elicited by the head movement
(Wall, Saluja, Iskrant, Pierson, Deich & Balsam, 1996). These gapes are not scaled to seed
size and there is limited accuracy in the targeting of the peck. Suspecting that feedback from
successful and unsuccessful pecks was necessary for sustaining pecking and
for shaping the squab's behavior toward the adult form, we turned our
attention to analyzing the roles of feedback in the development and
maintenance of pecking.
Role of feedback
We guessed that feedback from
the beak during the movement and feedback about the consequent success and
failure of pecks was essential for the skill to develop. We thought that the
feedback might be important for sustaining and strengthening the peck
unless the mapping from the visual cue to the size of the gape was
"hard-wired", the afferent feedback from particular gapes must be
associated with the success and failure of pecks at specific targets. In two
different lines of experiments we have examined the role of feedback on the
development of pecking.
To examine the role that
feedback plays in sustaining the overall level of pecking, we exposed a group
of powder-reared squab to seed for 20 minutes each day during the second week
post hatch, but did not allow them to experience the seed in their beak. We
did this by gluing the seed to the floor of the test chamber. These squab were
immediately fed by the experimenter after the test session. The level of
pecking observed in this group on day 22 when they were tested with loose
seed on the floor was substantially less than the pecking observed in a
second group who was given unglued seed throughout the prior training and
testing days. Subjects exposed to the glued seed and deprived of feedback
provided by successful pecks responded considerably less than the subjects
that got to handle seed with the beak during testing. In fact, the glued-seed
subjects pecked at exactly the same level as subjects that only received
pairings of the sight of seed with food (Deich & Balsam , 1993;
1994). Pavlovian pairings
promote pecking but the opportunity to handle seed results in a higher peck
rate. Thus one role of the
feedback is to increase the overall rate of pecking. It is plausible that
when parents regurgitatively feed offspring the sensation of seed in the beak
becomes a conditoned reinforcer because of its association with positive
nutritional consequences. Once
the squab starts pecking successful responses provide this immediate reinforcement which would
lead to a higher peck rate.
We began to study the effects
of feedback in more detail in a collaboration with H.P. Zeigler's group at
Hunter College. Zeigler and his colleagues have studied the role of
trigeminal input on feeding in pigeons (Zeigler, Bermejo & Bout, 1994).
By sectioning some or all of the trigeminal branches, they have found that
trigeminal input plays a role in motivation, spatial accuracy, and the
efficiency of grasping and mandibulation.
The afferent feedback from the
beak is carried by the three
branches of the
maxillary, mandibular and ophthalmic branches. We (Ye,
Wild, Balsam & Zeigler, 1998) examined the circuit that carries the
feedback in adults and squab that were 24-48 hours old. We found that the
organization of the trigeminal ganglion, its somatotopic projections upon the
principal sensory nucleus (PrV) and the projection to the telencephalon are
similar in the adult and hatchling. Consequently, developmental changes in
pecking are unlikely to be due to maturation of circuitry but rather are
driven by the information carried in the system.
We have begun to look at the
role of the trigeminal input in more detail by reducing the input from the
beak. We first did this in adults by sectioning the peripheral nerve. The
most fascinating of these results is
revealed in the type of errors made after deafferentation. The link shows
types of errors made by an adult prior to and after bilateral ophthalmic
sectioning. The errors increase after the surgery, but notice that the immediate
effect is to produce a small increase in grasping errors. In these cases, the
bird made contact with the seed but could not hold it in its beak.
Subsequently grasp errors decline but spatial targeting errors increase. The
bird misses the seed toward which its peck is directed. This demonstrates
that the trigeminal input plays the important role of giving feedback about
the success versus failure of the spatial aspects of the peck. When that
feedback is attenuated the targeting becomes more variable. Sham controls do
not show decreased efficiency of eating, indicating a specific effect of
We have also sectioned the
trigeminal a few days after hatching and allowed the squab to be reared by
their parents. They grow normally and interestingly, they all eventually
learn to eat seed. However, the rate at which they learn is very much
affected by the quality of the sensory
feedback from the beak. The
eating efficiency of
squab that have intact trigeminal nerves improves much faster than the
efficiency of squab who have one or two branches of the nerve severed on day
3 post-hatch. The development of skilled pecking is highly dependent on this
iput. The more attenuated the input the slower the learning. We do not know
if squab could eventually learn to handle food without sensory input from the
beak but this input clearly plays a key role in normal development. There are
two likely components to the deficit produced by deafferentation. The reduced
feedback from the beak movement may interfere with learning to scale the gape
to seed size. Additionally, reduced feedback from seed in the beak is likely
to make it more difficult to distinguish successful and unsuccessful pecks.
Both uses of the feedback seem essential for the skill acquisition.
In sum, feedback about success
and failure seems to contribute to every aspect of the skill. It contributes to the overall level
of pecking and affects the targeting, grasping, and handling of the
seed. When normal feedback is
present during development the highly flexible adult response emerges. The bird pecks very precisely at a
wide range of seeds from a range of positions. We wondered what learning underlies this flexibility.
How does the bird build such a
Scaling of Gape to Seed Size
We already knew that maturation
alone could not account for the development of the adult gaping response
(Deich & Balsam 1994). Squabs reared on the powder diet do not show the
adult gape form even when tested at an age when seed-reared squabs are quite
proficient. The skilled adjustment to seeds of different size exhibited by
adults (Deich & Balsam, 1994) depends on experience. Thus we turned our
attention to identifying the specific experiences necessary for this aspect
of the adult skill.
Our doves are usually raised on
a standard dove mix. This mixture contains seeds of many sizes and shapes
which range from about 1mm (millet) to about 12 mm (peas). It is possible
that doves must separately learn to handle all of the different seeds before
they show the precise and flexible adult response. Alternatively, the doves
may acquire a generalized motor program. One characteristic of a generalized
motor program is that quantitative adjustments of behavior occur in novel
situations. For example, once we are skilled walkers we adapt to new terrain
with little loss of skill. Similarly, we adjust the opening phase of
our grasp to the width of any new object we pick-up. Presumably, once the motor
program is functioning we are able to extract the necessary information to
use the program from visual cues provided by the novel object.
It was an open question as to
whether experience is necessary for the development of motor programs. Though
skill surely improves with experience, but it is not clear whether or not the
initial induction of motor programs requires experience. Because we can limit
the doves’ experience with seed during development we can study this question
with respect to pecking. We know from our powder-reared subjects that they
are not able to pick up seed when they first encounter it – even at an age
when normally reared subjects would be quite proficient (Deich & Balsam,
1994). Perhaps experience with a single seed would be sufficient for inducing
the motor program. Alternatively, experience with two or more points along a
dimension may be necessary for the development of a generalizable
To test this hypothesis, we
compared the pecking of normally reared doves to a group reared on a single
size pellet. In the group
reared on a single size pellet their diet consisted solely of pellets, 2 mm
in diameter. This was the only food available to the parents and squab until
the time of weaning and the only food available to the squab until they were
18-24 months old. A second group of subjects was reared on mixed grain until
they were between 18 and 20 months old. This group was switched to a diet of
only 2 mm pellets by gradually increasing the proportion of their food that
consisted of pellets over the course of a week. These subjects were then
maintained solely on these pellets for at least two months prior to
On the first test day, subjects
were given 1 mm, 2 mm, and 3.6 mm pellets to eat. On test trials ten seeds of
each size were presented. All pecks were videotaped and scored for
accuracy. The set-up was similar to the one illustrated in the
video. The seeds and beak were visible from below and from the side of
the test aquarium. We classified the types of errors made by the birds
into the six categories described in the figure showing the data from this
test. The result of this test showed that subjects reared on mixed seed
(not shown in the figure) had little trouble
handling the 1 and 3.6 mm
pellets even though they had only handled 2 mm seeds in the recent past. The
subjects who had only experienced a 2 mm diet (shown in the figure) exhibited
a different pattern of results during the generalization test. These subjects had little trouble with the 1mm seed. They made a few errors
in which they contacted the seed but failed to grasp it. This was about
the same number and type of error that they made when handling the very
familiar 2 mm pellet. In contrast, the subjects reared on the restricted diet
did very poorly on the 3.6 mm seed. They often contacted the seed and failed
to grasp it or got their beaks around it briefly until the pellet squirted
out as the beak closed. They were not scaling the gape to the size of
the seed or positioning the beak to securely grasp the seed. It seems
likely that the gape they had learned for 2mm seeds was wide enough and
targeted well enough to handle the smaller, 1 mm, seed but not adequate for
the larger seed. Over the next few days they readily learned to handle the
larger seeds. In fact, they made very few errors after 50 or so experiences
with the 3.6 mm seed. After a week of this testing, we introduced a 7.2 mm
pellet during testing. None of the birds had any trouble with the new seed.
Apparently, it takes experience with two or three points on a dimension to
induce the generalizable skill.
Synopsis: Learning and the
development of pecking
There is an unconditioned
tendency for squabs to thrust at grain-like objects (Balsam et al., 1992).
However, this response habituates quickly unless the squab experiences
Pavlovian pairings of the sight of grain with feeding (Balsam et al, 1992;
Deich & Balsam, 1994). The Pavlovian induced thrusts do not have the
coordinated gape found in the adult peck (Deich & Balsam, 1994; Deich,
Tankoos, & Balsam, 1995). There is only a loose coordination of gape and
thrust, induced by the thrusting motion, itself (Wall et al., 1996). Squabs must
have experience with handling and ingesting seed for the adult topography and
coordination to emerge (Deich & Balsam, 1994). Through differential
feedback from successful and unsuccessful pecks, the gape component and its
coordination with the thrust component are moved toward the more effective
adult form by a process of response shaping. Operant conditioning is proposed
as the underlying operative process because neither maturation nor extensive
Pavlovian training is sufficient to produce the stereotyped adult response
(Deich & Balsam, 1994). Furthermore, reduction in the oro-sensory
feedback as a result of beak deafferentation interferes with the development
of the coordination.
All of these processes are
important in the normal developmental context. From the time that parents
start to feed squab seed the feel of seed in the mouth becomes associated
with positive nutritional consequences. During the second week after hatching
the parents stop consistently feeding the begging squab and, instead, go to
peck at seed themselves (Wortis, 1969). The squabs follow the parents to the
seed and make some unconditioned thrusts at food. Additionally, we have found
that squabs tend to peck when their parents do (Iskrant & Balsam, 1994;
see Zentall & Akins (2001) for more on social
learning) and have documented that this social coordination is present in the
undisturbed interactions of parents and squabs (Hirose & Balsam, 1995).
The unconditioned tendency and the social enhancement guarantee the squabs
visual exposure to seed. During this stage, parents generally feed squabs
within a minute or two of this exposure (Hirose & Balsam, 1995). This
allows for the sight of seed to be paired with food. The frequency of
thrusting toward seed is increased by the Pavlovian pairings (Balsam et al.,
1992). The squab’s thrusting movement elicits loosely coordinated gapes (Wall
et al., 1996). Some of these thrust-gape coordinations result in seed in the
beak. Feedback from the seed in the beak reinforces the successful actions.
The gape component and its coordination with the thrust component are moved
toward the more effective adult form by a process of response shaping by
Learning and Behavioral
response is produced by an intricate interplay
between the squab’s reflexes, the teaching of the parent, and the very
precise feedback that is provided by the environment for success and failure.
In these regards the development of pecking shares many features of
behavioral development that are common to altricial species.
The highly skilled and
adaptable adult pecking
Within the context of the
parent-offspring interaction there is typically a great deal of flexibility
in development as a result of experience. For example, in the case of feeding, local conditions of
food availability can determine the rate at which weaning occurs, the
specific foods that a juvenile learns to find and to consume, and the
specific responses that are used for foraging and eating (Balsam et al.,
1992). Thus experiences are a
critical influence on development. An important question is how to understand the specific ways in which
experience affects these behaviors.
A number of developmentalists
have articulated the view that laboratory learning principles do not capture
the ways that experience affects development. Gottlieb (1976, p. 232) wrote, “Traditional forms of
learning (habituation, conditioning and the like) have not proven very useful
in explaining the species-typical development of behavior…” In this context,
we view the studies described in this paper as a true test of validity for
laboratory learning principles and believe that they have done pretty well. At least in the case of
pecking, laboratory-derived learning principles have served us well by
providing an analytic tool for understanding how specific experiences
contribute to the development of specific aspects of behavior.
Perhaps, the major contribution
of learning principles is to conceptualize development in terms of separate
stimulus and response transitions. Learning theory can provide the analytic tools for understanding
transitions in stimulus control (Alberts, 1978; Alberts & Brunges, 1979; Gottlieb, 1983; Hall
& Swithers-Mulvey, 1992) by looking for things like changes in predictive
validity, blocking, overshadowing and potentiation during development. Likewise, the use of learning
principles to understand response transitions is also fruitful (Balsam, Deich, Ohyama & Stokes, 1998).
The emergence of new behavior
is also a significant aspect of development that is influenced by
experience. New response
forms can be induced by elicitation, Pavlovian conditioning, habituation, and
shaping (Balsam & Silver, 1994). Again the learning principles seem useful for understanding the ways
in which experience accomplishes these changes in development. Furthermore, one can conceive of the
transitions in functionally equivalent behaviors as the product of changing
relative reinforcement rates. For example, the transition from dependent to independent feeding is
likely modulated by the pay-off that each affords. The selection of specific feeding topographies is likely
determined by the successes and failures of those actions in the past. In sum, we believe that
learning principles provide a very useful framework for understanding the
roles of experience in the ontogeny of behavior.
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thank all of our many students that contributed to the work on the
project. We also add a special
thank you to Alice Deich who helped in
many ways. The work was supported by NSF grant IBN-9727428.