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Age dependant lateralized prey delivery and chick rearing
of Caspian Terns (Sterna caspia)
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JACQUELYN K. GRACE and DAVID P. CRAIG
Willamette University
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ABSTRACT
Laterality or the preferential use of one or the other side of the body is often associated
with differential brain activity and increased specialization of brain function. Right or left
biases are wide spread among vertebrates and have been documented among some birds.
We monitored Caspian terns (Sterna caspia) feeding young at a colony and recorded more
than 2,000 chick feeding events for analysis of lateralization of prey within the bills of
adult birds, size and shape of prey items delivered to chicks in relation to chick age, and the
development of prey handling skills among chicks. No initial lateralization of prey head
position was found as adults flew onto the colony, but head position of successful feeds
was found to be significantly lateralized with a bias to the right side. Adults switched prey
orientation from left to right without dropping the prey item a significantly greater number
of times than from right to left, suggesting an adult bias. Adults appeared to adjust the
length, but not shape of prey delivered according to chick age. Ability of chicks to
manipulate prey items increased with age. The first week of a Caspian Tern’s life is a
critical time in which rapid changes in ability to manipulate and consume prey items occur,
and that adults modify their delivery behavior to respond to these changes. We believe this
is the first documentation of lateralization in the Laridae.
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INTRODUCTION
The Caspian Tern is ideal for studies of reproductive strategies and parental care
because prey are highly visible and play an important role in courtship and reproductive
success (Cuthbert and Wires 1999). In this study, we focused on three areas of chick
introduction to prey: a) lateralization of prey head position delivered by adults, b) the
association between chick age and diet, and c) the development of prey manipulation skills
among chicks.
Caspian Tern young (Sterna caspia) are semiprecocial and seminidifugous. Upon
hatching, chicks have open eyes, are covered with down, and capable of limited mobility.
The first week of life is characterized by rapid size and mobility increases. Chicks
generally spend the majority of time near the nest scrape, but after one week are highly
ambulatory (Cuthbert and Wires 1999). Adults begin delivering whole prey items to young
upon hatching (Quinn 1990). Typically, an adult lands near the nest scrape with a bill load
and proceeds to walk toward its chick(s), which runs up, grabs the prey item from the
adult’s bill, and swallows it whole. If a chick is not aggressive in taking the item, the adult
will lean down and make the item easier to obtain (Bent 1921).
Laterality is the preferential use of one or the other side of the body, and is often
associated with differential brain activity and increased specialization of brain function
(Gunturkun et al. 2000; Ventolini et al. 2005). Population level lateralization (directional)
is widespread among vertebrates and has been documented in a number of avian orders,
including discriminative and navigational eye use in Columbiformes (Gunturkun et al.
2000; Prior et al. 2004), ground scratching and visual discrimination in Galliformes
(Dharmaretnam et al. 2002; Rogers 1997; Valenti et al. 2003), spatial memory in Paridae
(Clayton 1993, Clayton & Krebs 1993), cloacal contact, discriminative, anti-predator, and
navigational eye use in Passeriformes (Nyland et al. 2003, Templeton & Gonzalez 2004;
Wiltschko et al. 2004), tool manufacture in Corvidae (Rutledge and Hunt 2004: Weir et al.
2004), body rotation and foot use in Falconiformes, and Strigiformes (Csermely 2004),
predatory and sexual behaviour in Charadriiformes (Ventolini et al. 2005). Yet, no
lateralization has of yet been determined in fish eating birds, and specifically, the laridae.
This study looked at the orientation of prey within the bills of adult Caspian Terns.
Lateralization of tool use in New Caledonian Crow is perhaps the closest analogy to this
behavior as both involve carrying an item in the bill which is then maneuvered into a hole
(the mouth of a chick in terns). Two independent studies have found that New Caledonian
Crows exhibit individual laterality with the working end of a stick oriented to the right or
left of their beak (Rutledge and Hunt 2004; Weir et al. 2004).
The development of prey handling skills is of vital importance to the evolutionary
and life history of the Caspian Tern, as skill level determines the reproductive success of
individuals (chicks will only survive if provided with adequate prey). A few studies have
attempted to quantify changes in prey delivery by adults to meet the increased energetic
demands of growing chicks. The length of prey delivered to both Roseate Tern chicks and
Black Guillemots increases linearly with age (Ramos et al. 1997; Cairn 1987), and the
availability of appropriate species and size of prey has been found to be critical to the
reproductive success of the Common and Arctic Tern (Uttley et al. 1989). Elucidating the
prey selection criteria of Caspian Terns in particular is crucial in light of recent concern for
the depredation of endangered salmon species (U.S. Fish and Wildlife Service 2005; Roby
et al. 2003; Ryan et al. 2003). However, the relationship between chick age and length or
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species of prey delivered or ingested by chicks has not been quantified in the Caspian Tern.
METHODS
We monitored Caspian Terns (Sterna caspia) feeding young primarily on East
Sand Island in the Columbia River estuary, Oregon, the largest such colony in the world
with an estimated 8,200 breeding pairs. Observations were conducted from three blinds
that surround the colony. Our secondary site was Brooks Island in San Francisco Bay, a
breeding colony of an estimated 520 pairs. Observations on Brooks were conducted from a
blind located behind the colony from which about two-thirds of the colony was visible.
Data was collected for a total of nine weeks following the hatching of chicks at East Sand
Island at the end of May until fledging at the end of July 2005. Eight weeks were spent at
East Sand Island and one week at Brooks Island during the middle of the study period.
Researchers opportunistically selected adult Caspian Terns flying onto the colony
with bill loads and recorded prey species, length, and initial orientation in the bill (right,
left, lengthwise), age and status (alpha, beta, pirate, adult) of birds attempting to acquire the
food item, number and direction (head, tail, middle) of each feeding attempt, and final
orientation of successful feeding events, or outcome of the event if unsuccessful
(kleptoparasitism, adult flew away or wandered on the colony). A feeding attempt was
defined as the act of touching the prey item with the bill, or visibly lunging at the prey item.
The primary bird was that which was successful or, if the feed was unsuccessful, that which
attempted the most. Only primary birds were considered in this study. Video footage of
feeding events was also taken and reviewed to aid in accurate observations of rapid chick
feeds.
Orientation of prey was defined as the head position of the prey item relative to the
bill of the adult Caspian Tern carrying it (Figure 1). Initial orientation was determined as
soon as feasible after the adult landed on the colony. Final orientation was defined as the
last position of the prey item before ingestion.
Prey size was estimated in relation to the adult Caspian Tern bill length (exposed
culmen = 7 cm, mandible gape = 9 cm). Prey species were visually identified based on a
number of features and familiarity with the local species. Bill-loads were identified with
the aid of spotting scopes and binoculars, but in most cases were only identifiable to the
family, or general shape (fusiform, ovate) of the item. If identification was impossible, the
item was classified as unknown. Chick age was estimated to the nearest week based upon
plumage and size differences (Figure 2).
The families Engraulidae (anchovies) and Embiotocidae (perch) were used in
fusiform and ovate comparisons. Although other fusiform species were observed as well,
Engraulids were the most distinctly fusiform of common prey items delivered.
Embiotocids were the primary ovate prey items recorded on colony. Chi-square was used
to quantify the variation between initial and final prey orientation within the bills of adult
Caspian Terns, and between the means and regression lines developed for prey length and
attempts/drops as a function of chick age.
RESULTS
Caspian Tern chicks which dropped their food item at least once were found to have a
higher rate of feeding failure than chicks which did not drop the prey delivered to them (p ≤
0.01). Out of 1344 successful chick feeds, 99.6% were swallowed head first. In
lateralization analysis, only left and right orientations were considered as they composed
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99.6% of prey deliveries.
Lateralization
No significant initial bias for right or left orientation of the head of the prey item
was found in adult Caspian Terns as they landed on the colony. Final prey orientation
within the bill of the adult prior to successful chick feeds was significantly lateralized with
a preference for the right side (p ≤ 0.01). Prey initially oriented to the left did not result in a
greater number of feeding failures, single drops, total drops, or attempts by chicks than
prey initially oriented to the right. However, prey initially oriented to the left were
switched to the right before a successful feeding significantly more often than those
switched from right to left (p ≤ 0.01). This switch was found to occur without dropping the
prey item, as adults switched from a left to right orientation significantly more often than
from a right to left orientation when no prey drops were recorded (p ≤ 0.01).
When the final prey orientation of successful feeds was broken down by chick age
class only one-week-old chicks exhibited any significant bias with a preference shown for
the right side (p ≤ 0.05). However, a greater number of prey items were oriented to the right
than to the left in all age classes of chicks. When two-week-old chicks are grouped with all
successive age classes, a significant bias toward the right is found (p ≤ 0.05).
Length and Shape of Prey
Mean length of prey delivered by Caspian Tern adults and successfully swallowed
by chicks increased linearly with chick age, reaching the maximum length at three weeks
(Figure 3). These trends are also observed within observations for a single day (June 14,
2005), when prey availability is expected to be similar for all foraging adults (Figure 4).
Excluding five-week-olds, the proportion of ovate embiotocids delivered by adults
increased slightly with chick age, while the proportion of fusiform engraulids delivered
decreased slightly. A greater percentage of embiotocids than engraulids were consistently
delivered to chicks in all age classes (Figure 5). The proportion of both prey types ingested
is consistent with the proportion delivered in all age classes except for one-week-old chicks
(Figure 6). One-week-old chicks consume 60% of the embiotocids delivered to them,
compared to 85% of engraulids. The percent of each prey item successfully consumed
increases with age of chick until it reaches 100% at five weeks of age (Figure 7). Although
not described in the figures, adults were found to have the highest proportion of
embiotocids ingested out of all age groups. In this case, the adult either self-fed on the
colony, or fed its mate/copulation partner.
Development of Prey Handling Skills
The rate of feeding failure correlated positively with the number of times prey
items were dropped during the feeding event (Figure 8). While zero drops resulted in 6%
feeding failure, a single drop increased feeding failure to 38%, and two drops resulted in
82% feeding failure. The number of attempts and drops made by primary chicks in
obtaining a prey item decreased linearly with age of chicks (p ≤ 0.01; Figure 9).
One-week-old chicks attempted 1.76 times and dropped prey 0.32 times on average, while
two-week-old chicks attempted 1.26 times and dropped prey 0.09 times on average per
feeding event. The averages of each successive age class after two weeks remained
relatively constant.
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DISCUSSION
Lateralization
Caspian Terns exhibit laterality in the head position of prey items delivered to
chicks, with a bias for the right. Chicks almost invariably ingest prey head first, thus,
chicks will feed from the side of the adult on which the head of the prey item is located.
Lateralization of prey delivery may be related to brain asymmetries in visuoperceptual or
motor skills, and could stem from either chick bias as perceived by adults, or adult bias.
If chick bias is the cause of lateralization, young Caspian Terns are more successful
in obtaining prey from the right of the adult. Such a preference may be due to differential
visual ability as found in quail, domestic chicken, and pigeons, which exhibit significantly
higher visual accuracy in the right eye (Valenti et al. 2003; Gunturkun et al. 2000). Right
eye/left hemisphere dominance is attributed to an exposure of the embryonic right eye to
light during the last few days of incubation when the right eye is pressed against the shell,
while the left eye is positioned against the body of the chick and so remains in the dark
(Rogers 1997; Skiba et al. 2002; Valenti et al. 2003). In pigeons, embryonic light
stimulation leads to an increase in visuoperceptual processes in the left hemisphere, and a
decrease in visuomotor speed in the right hemisphere, while dark incubated pigeon young
exhibit no asymmetry (Skiba et al. 2002). If the embryonic right eye of the Caspian Tern is
likewise stimulated by light, then the visual discriminatory skills of the right eye could
result in chick bias for feeding from the right. That adults will switch orientation of prey
from right to left before the chick drops the item (i.e. fails at a feeding attempt) suggests
that adults have learned, or are genetically predisposed to accommodate for this asymmetry
in visuoperceptual ability.
Alternatively, laterality may be due to adult bias stemming from asymmetries in
motor skills or visuoperceptual ability. In order to feed small or unresponsive chicks, adults
must bend down and present the prey item to the chick. Such presentation might require
refined visual or motor skills, which may be controlled primarily by the left hemisphere of
the brain. Adult birds of prey (Falconiformes and some Strigiformes) also appear to exhibit
a left hemisphere dominance in manipulating prey items, which manifests itself as a bias
for right foot use (Csermerly 2004). The New Caledonian Crow behavior of directing a
stick or wire into a hole is strikingly similar to that of directing a prey item into the bill of a
chick as seen in Caspian Terns. Stick handling appears to be individually lateralized in this
crow, with equal numbers of subjects showing a bias for the working end positioned to the
right and left of the beak (Rutledge and Hunt 2004; Weir et al. 2004.) A thorough
understanding of the mechanism behind this differential tool use could aid in delineating
the process by which lateralization is determined in Caspian Terns.
Interestingly, when broken down by chick age class, significant laterality
disappears, except among feeds to one-week-old chicks. Although there is a bias toward
the right side in all age classes, after the first week, asymmetries become less pronounced
and are only significant when successive age classes are grouped together. If laterality is
due to adult bias, one-week-olds may warrant a greater degree of lateralization due to their
small size (Figure 2). An adult must bend down and present the prey item to one-week-olds
in a much more obvious manner than when delivering prey to older and larger chicks.
Older chicks will often grab prey items as soon as adults land on colony, or even before
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adults touch the ground, which allows adults no time to switch prey orientation. Thus,
feeding younger chicks may require greater motor or visuoperceptual ability and the
aggressiveness of older chicks may overwhelm parental right bias.
We have developed three possible causes for the observed lateralization. It may be
an ancestral trait which is nonadaptive at present, a byproduct of ontogenetic factors or of a
related brain asymmetry, or an adaptation to the current environment. If the latter,
kleptoparasitism may create such an environment where lateralization in chick feeding is
advantageous as it would facilitate rapid prey consumption by chicks. Before the adult
lands, the chick would already be aware of which side to approach from in order to
consume the prey item. We do believe that this is the first documentation of lateralization
within a pscivirous bird and specifically within the Laridae.
Length and Shape of Prey
The length of prey delivered and ingested by chicks increases linearly with age.
This shift in size of delivered prey can be explained by changes in the abundance of prey
species and changes in the adult’s selection criteria (Ramos et al. 1997). The onset of egg
laying may be related to the presence of many young, small fish with which to feed young,
small chicks. Yet, adult delivery in terms of prey length remained proportionate to chick
age throughout a single day, when prey availability is expected to be the same for all adults
regardless of the age of individual broods. Thus, differential availability of prey does not
completely account for the observed trend. Instead, adults appear to selectively deliver
prey as a response to increased energy demands (Cairn 1987) and improved ability of older
chicks to manipulate and swallow larger fish.
Five-week-old chicks experienced a decline in length of fish delivered by adults.
Because of the large size and comparative dexterity of five-week-olds, we do not believe
that this decrease is a response to the inability of this age class to consume large fish.
Nearly all five-week-old data were collected over a period of four days, and it is most
likely that availability of prey was low at this time due to external environmental factors.
In terms of prey shape, one week old chicks were able to consume a much lower
percentage of ovate fish compared to elongate fish. The rates of consumption of ovate prey
increased drastically after the first week, reaching 100% by the fifth week (Figure 7).
Thus, ovate embiotocids appear to pose a greater difficulty in consumption than fusiform
engraulids for one-week-old chicks in particular. Adult delivery, though, does not
correspond to the poor ability of young chicks to consume embiotocids. Hence, adults
appear to selectively deliver prey based on length but not shape in correlation with chick
age.
Development of Prey Handling Skills
The ability of Caspian Tern chicks to manipulate prey items increased with age, as
measured by a decrease in the number of attempts and drops per feeding event. Although
the relationship between attempts/drops and age is linear, the greatest increase in ability
occurs between the first and the second weeks after hatching. There is strong pressure on a
Caspian Tern colony to consume prey as quickly as possible. Dropping a prey item while
on colony increases the chances of feeding failure by 32% if dropped once, and 76% if
dropped twice. Feeding failure is often due to kleptoparasitic gulls or pirating chicks and
adults that are always on the look out for the possibility of a stolen meal. Dropping a prey
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item provides an opportunity for pirates when the prey item is not under the control of any
bird. Additionally, predation by gulls and bald eagles is common on East Sand Island, and
small, soft chicks are easily grabbed by predators. Thus, great pressure is put on a chick to
swiftly obtain food from its parent and to grow larger and stronger rapidly.
Caspian Tern chicks may respond to the pressures of starvation and predation by
rapidly learning prey handling skills in order to maximize their chances of a successful
feed. Alternatively, the selection forces of starvation and predation may result in the death
of all chicks which possess lesser manipulation skills within the first two weeks after
hatching. It is impossible to distinguish between these two explanations at this time.
The one-week-old
In all aspects of this study, one-week-old chicks have been significantly different
from other age classes. Prey deliveries to one-week-old chicks exhibit significantly greater
laterality than those to older chicks and one-week-olds consume the shortest of all prey
items and have the most difficulty consuming ovate fish. In addition, one-week-old chicks
demonstrate a drastically lower ability to manipulate prey items when compared to
successive age classes. Due to their small size and limited mobility, the very young
hatchling is subject to great predation and starvation pressure. The chick responds to this
pressure with rapid increases in ability to manipulate and consume prey items, and adults
modify their delivery behavior in reaction to these changes.
AKNOWLEDGEMENTS
We would like to thank Willamette University and the Student Collaborative Research
Program, the Mary Stuart Rogers Foundation, the M.J. Murdock Charitable Trust, Paul F.
Swenson, Joel Shinn, Sam Lantz, and the Columbia River Avian Predation Project.
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Rogers, L. 1997. Early experiential effects on laterality: Research on chicks has relevance
to other species. Laterality, 2, 199-219.
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Right
Left
Lengthwise
Figure 1: Possible orientation of prey items within the bills of adult Caspian Terns
1 week
2 weeks
3 weeks
4 weeks
5 weeks
Figure 2: Average size and plumage of chicks in each age class (weeks 1-5)
12
18
Length of prey delivered = 0.445 age + 11.414
p ≤ 0.01
r = 0.208
Estimated prey length (cm)
16
14
12
10
8
Length of prey ingested = 0.509 age + 11.159
p ≤ 0.01
r = 0.251
6
4
2
0
0
1
2
3
4
5
Chick age class (weeks)
Figure 3: Estimated lengths of prey as a function of chick age. X’s are mean lengths of prey
delivered to chicks, solid dots are mean lengths of prey ingested by chicks, vertical bars are
standard deviations. The broken line indicates the regression for mean length of prey
delivered, the solid line is the regression for mean length of prey ingested. From 1215
deliveries and 1078 successful chick feeds in 2005.
20
Estimated prey length (cm)
18
16
14
12
10
8
6
Length of prey delivered = 0.838 age + 10.831
p ≤ 0.01
r = 0.242
4
2
0
0
1
2
3
4
5
Chick age class (weeks)
Figure 4: Length of prey delivered by adults as a function of chick age. Solid dots are
means, vertical bars are standard deviations. From 167 deliveries on June 14, 2005.
13
35%
Percent delivered
30%
25%
20%
15%
Embiotocidae
10%
5%
Engraulis
0%
0
1
2
3
4
5
Chick age class (weeks)
Figure 5: Percent each prey item contributed to total delivered prey by adults to chicks as a
function of chick age. Out of 1414 prey deliveries in 2005.
Percent ingested out of total diet
30%
25%
20%
15%
Embiotocidae
10%
5%
Engraulis
0%
0
1
2
3
4
5
Chick age class (weeks)
Figure 6: Percent each prey item contributes to the total diet of chicks as a function of
chick age. Out of 1254 successful chick feeds in 2005.
Percent ingested out of total delivered
14
100%
90%
80%
70%
60%
50%
40%
Embiotocidae
30%
Engraulis
20%
10%
0%
0
1
2
3
4
5
Chick age class (weeks)
Figure 7: Percent of prey item successfully ingested by chicks as a function of age. Out of
1254 successful chick feeds in 2005.
100%
Percent Failure
80%
60%
40%
y = 0.2585x + 0.1221
R2 = 0.8696
20%
0%
0
0.5
1
1.5
2
2.5
3
3.5
Number of drops
Figure 8: Percent of feeding failure as a function of the number of drops by the primary
chick involved in the feeding event. Feeding failure was due to kleptoparasitism by gulls,
pirating by conspecifics, or the adult wandering or flying away. Out of 1494 prey
deliveries.
15
3.5
Average number of attempts / drops
3
Attempts = – 0.137 age + 1.684
p ≤ 0.01
r = 0.159
2.5
2
1.5
1
0.5
0
-0.5 0
-1
1
2
3
4
5
6
Drops = – 0.120 age + 0.396
p ≤ 0.01
r = 0.154
-1.5
Chick age class (weeks)
Figure 9: Number of attempts and drops of a prey item as a function of chick age. Solid
dots are mean numbers of attempts, triangles are mean numbers of drops, vertical bars are
standard deviations. The solid line indicates the regression for mean number of attempts,
the broken line is the regression for mean number of drops. From 1343 and 1348 prey
deliveries in 2005.