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Invasion success is dependent on the ability of a species to discover and exploit novel food resources. Within this context, individuals must be willing to taste novel foods. They must also be capable of evaluating the nutritional content of new foods, and selecting their relative intake in order to fulfil their nutritional needs. Whereas the former capacity is well studied, little is known about the latter capacity. First, using the common myna as a model avian invader species, we quantified the willingness of mynas to taste novel foods relative to familiar ones. Mynas readily tasted high protein (HP) novel foods and consumed them in higher quantities compared to a familiar food. Data showed that at three different levels – mixes, ingredients and macronutrients – intake could not be explained by a random model. In experiment 2, we confirmed that mynas were making their selection based on protein (P) content rather than a selection for novelty per se. When given the choice of three equally unfamiliar foods, mynas again ate disproportionately from the high protein relative to high lipid and high carbohydrate foods. Analysis revealed that mynas consumed amounts of protein that were closer to the ones in their natural diet. Finally, in experiment 3, we measured inter-individual variation in innovation and exploration propensities, and examined associations with inter-individual variation in consumption of specific macronutrients. This analysis revealed that individuals that selected HP pellets were more exploratory and individuals that selected HC pellets were quicker to solve the innovative foraging task. These findings indicate that not only the willingness to taste novel foods, but also the capacity to evaluate their nutritional content, might be central to the myna's substantial ecological success.
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Tasting novel foods and selecting nutrient content in a highly
successful ecological invader, the common myna
Chloe Peneaux, Gabriel E. Machovsky-Capuska, David Raubenheimer, Francoise Lermite,
Charlotte Rousseau, Tanya Ruhan, John C. Rodger and Andrea S. Griffin
C. Peneaux (http://orcid.org/0000-0003-2746-5820), F. Lermite, C. Rousseau and A. S. Griffin (andrea.griffin@newcastle.edu.au), School of
Psychology, Univ. of Newcastle, Callaghan, NSW, Australia. – G. E. Machovsky-Capuska and D. Raubenheimer, Charles Perkins Centre and
Faculty of Veterinary Science, e Univ. of Sydney, Sydney, Australia. – T. Ruhan and J. C. Rodger, School of Environmental and Life Sciences,
Univ. of Newcastle, Callaghan, NSW, Australia.
Invasion success is dependent on the ability of a species to discover and exploit novel food resources. Within this context,
individuals must be willing to taste novel foods. ey must also be capable of evaluating the nutritional content of new
foods, and selecting their relative intake in order to fulfil their nutritional needs. Whereas the former capacity is well studied,
little is known about the latter capacity. First, using the common myna as a model avian invader species, we quantified
the willingness of mynas to taste novel foods relative to familiar ones. Mynas readily tasted high protein (HP) novel foods
and consumed them in higher quantities compared to a familiar food. Data showed that at three different levels – mixes,
ingredients and macronutrients – intake could not be explained by a random model. In experiment 2, we confirmed that
mynas were making their selection based on protein (P) content rather than a selection for novelty per se. When given the
choice of three equally unfamiliar foods, mynas again ate disproportionately from the high protein relative to high lipid and
high carbohydrate foods. Analysis revealed that mynas consumed amounts of protein that were closer to the ones in their
natural diet. Finally, in experiment 3, we measured inter-individual variation in innovation and exploration propensities,
and examined associations with inter-individual variation in consumption of specific macronutrients. is analysis revealed
that individuals that selected HP pellets were more exploratory and individuals that selected HC pellets were quicker to
solve the innovative foraging task. ese findings indicate that not only the willingness to taste novel foods, but also the
capacity to evaluate their nutritional content, might be central to the myna’s substantial ecological success.
To become established in novel environments, invasive
birds have to face new ecological challenges such as finding
new shelter and recognizing new predators and competitors
(Coleman and Mellgren 1994, Griffin et al. 2016). Within
this context, there is evidence that the ability to exploit
novel food resources is particularly important (Sol et al.
2011, 2012a). Avian species with high numbers of reports
of novel feeding behaviours, including consuming novel
foods and developing novel foraging techniques, are more
likely to become established when introduced to environ-
ments outside their native range than species with fewer such
reports (Sol et al. 2002, 2005, Wright et al. 2010). us,
to date, research has established that success in new envi-
ronments depends heavily upon an invader’s ability to con-
sume novel food, develop new foraging behaviours and/or
use pre-existing ones in novel contexts (Mayr 1965, Sol and
Lefebvre 2000, Sol et al. 2005, 2008, Griffin et al. 2016).
Foods are complex blends of many nutrients each of which
has their own effect on the forager (Raubenheimer et al.
2012). Lipid (L) and carbohydrate (C), in particular, play a
central role in metabolism and energy storage whereas pro-
tein (P) is fundamental to animals’ growth and reproduction
(McWilliams 2011). Nutritional goals can vary among indi-
viduals, for example as a function of age and/or sex (Simpson
et al. 2010), but also among geographically isolated popula-
tions and among species (Tait et al. 2014). e complexities
of nutrient content and their impact on individual health
generate a challenge for invaders. Novel foods are likely to
differ in their nutritional composition and physiological
properties to those foods that invaders have encountered
previously (Machovsky-Capuska et al. 2016a). erefore,
not only must invaders be willing to consume novel foods,
they must also have the ability to identify the nutritional
content of these novel foods and combine their proportional
intake to fulfil their nutritional goals (Machovsky-Capuska
et al. 2016a).
Exploration and innovativeness in the foraging context
have been consistently linked to invasion and colonization
of novel habitats in birds (Holway and Suarez 1999, Sol and
Lefebvre 2000, Sol et al. 2002, Russell et al. 2010, Wright
et al. 2010). Nutritional intake might impact behaviour in
several ways. First, it is known that nutritional deficiencies
can produce personality biases in later life (Fraňková 1973,
Barnes et al. 1976, Almeida et al. 1991, 1993, 1994, 1996).
© 2017 e Authors. Journal of Avian Biology © 2017 Nordic Society Oikos
Subject Editor: Paul McDonald. Editor-in-Chief: Jan-Åke Nilsson. Accepted 19 July 2017
Journal of Avian Biology 48: 1432–1440, 2017
doi: 10.1111/jav.01456
1433
For example, low P intake during ontogeny is linked to
increased exploration in adult rats Rattus norvegicus. is
link between low P intake and activity might be maintained
through life as suggested by research in invertebrates. Crickets
Anabrus simplex deficient in P locomote more than crickets
satiated in protein (Simpson et al. 2006). Second, it is pos-
sible that the tendency to forage on certain nutrients specifi-
cally might enhance certain types of foraging behaviours. For
example, urban exploiters, which rely heavily upon human
discards rich in L and C, might have an enhanced motor
diversity due to their frequent handling of packaged human
discards in school playgrounds, supermarket carparks and
around fast food outlets. Higher motor diversity is known to
facilitate innovative foraging (Griffin et al. 2013a, Diquelou
et al. 2016). ese nutrient–behavioural relationships have
yet to be explored in invasive birds.
Classified as one of the world’s 100 worst invasive species
(Lowe et al. 2000), the common ‘Indian’ myna Acridotheres
tristis (recently proposed to be reclassified as Sturnus tristis
by Christidis and Boles (2008); hereafter mynas) is an excel-
lent model to investigate the physiological and behavioural
attributes of ecological invaders (Griffin et al. 2010, 2013a,
b, 2014, Sol et al. 2011, 2012b). Introduced to Australia
late 19th century (Martin 1996, Pell and Tidemann 1997,
Tidemann 2005), mynas are now present along much of the
east coast (Grarock et al. 2013). e species is well known
for its ability to learn about novel environmental stimuli.
Previous research has demonstrated learning of bait avoid-
ance (Feare 2010), the location of human hunters (Dhami
and Nagle 2009), novel predators (Griffin 2008), and dan-
gerous places (Griffin et al. 2010, Griffin and Haythorpe
2011). e species is also known to reduce flight distances in
areas highly frequented by humans (McGiffin et al. 2013).
Based on the traditional approach of quantifying dietary
generalism in terms of foods consumed, mynas are docu-
mented to be generalist omnivores. Indeed, the species has
been reported to consume a broad range of foods includ-
ing invertebrates, plants, fruits, and human discards (Moeed
1975, 1976, Sengupta 1976, Pell and Tidemann 1997,
Machovsky-Capuska et al. 2016b). Diet generalism is a
well-established predictor of invasion success (Cassey 2002,
Blackburn et al. 2009) and is likely to explain some part of
the myna’s extraordinary worldwide ecological success. On
the other hand, mynas are known to taste novel foods (Sol
et al. 2012b), but it is not known whether they would choose
novel foods when given the choice. It is also not known
whether mynas have the capacity to identify the nutri-
tional composition of novel foods and adjust their relative
consumption to achieve their nutritional goals. Finally, it
is not known whether the tendency to selectively consume
certain nutrients is associated with individual variation in
exploration and/or innovative foraging behaviour.
In the present study we combined behavioural observa-
tions with captive feeding trials and nutritional geometry to
examine food and nutritional selections in common myna
birds. In particular, we aimed to: 1) determine whether
mynas fed with novel foods select their foods, ingredients
and nutrients in non-random proportions, 2) establish the
macronutrient composition of myna diets using high-protein
(HP), high-lipid (HL) and high-carbohydrate (HC) pellets,
and 3) determine whether consumptions of particular
macronutrient were associated with innovation and explora-
tion propensities.
Methods
Subjects and husbandry
A total of 21 birds were caught in and around Newcastle
(NSW, Australia) using a trap specifically designed for this
species (Tidemann and ANU Fenner School 2009). All
twenty-one participated in experiment 1, and a subset of
ten also participated in experiment 2. Upon capture, birds
were transported to the Central Animal House (CAH) at the
Univ. of Newcastle, where they were measured (tarsus, beak
and wing), weighed and individually marked with coloured
plastic leg bands. Birds were then released into outdoor
group aviaries (2.0 1.0 2.0 m, H D W) and treated
for internal parasites during a period of ten days. Mynas were
then transferred to a large flight aviary (2.2 1.2 4.4 m)
where they were held until testing commenced. Food and
water was available ad libitum at all times. Birds were fed
a commercial brand of puppy dog pellets (SUPERCOAT®
Puppy with Real Meat). Dog pellets were used because they
are small enough to be swallowed by mynas easily. Dog pel-
lets were the most widely used food to bait myna traps in
Australia and were originally selected as suitable bait based
on myna food choice tests undertaken in the context of
developing a species-specific trap (Tidemann and ANU
Fenner School 2009). In addition, free-ranging mynas fre-
quently consume dog pellets from suburban back gardens
with dogs (Parsons et al. 2006). Based on these observations
and on veterinarian advice, they were an adequate food to
feed captive-held mynas and have been used since 2006 in
our myna research program.
During the food and macronutrient selection experiments,
birds were held in individual cages (0.6 0.6 0.6 m)
located outdoors but protected from weather conditions by
a roof. Individual cages were equipped with a perch and a
nest box and spatially arranged such that birds were in close
visual and acoustic contact. is arrangement facilitates
mynas’ adjustment to individual housing (Griffin and Boyce
2009, Griffin and Haythorpe 2011, Sol et al. 2012a, Griffin
et al. 2013a).
Procedure
Experiment 1: novel food selection
To measure the food choices of mynas in the presence
of new, unfamiliar foods, 21 mynas were transferred to
individual cages and left for two days to acclimatize to their
new holding conditions. During this time, each myna had
ad libitum access to water, as well as ad libitum access to the
now-familiar dog pellet food.
Over the course of the following three days, birds were
presented with a simultaneous choice of three foods. To
create these foods, we mixed ingredients that differed in
their macronutrient composition: dog pellets (DP, 14.0%
L, 29.0% P and 46.6% C), commercial insectivore powder
named Wombaroo® (W, 12.0% L, 52.0% P and 18.0% C)
and commercial parrot food called Superior Egg & Biscuit
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Vetafarm® (EB, 8.0% L, 16.0% P and 63.0% C). e foods
were created manually under laboratory conditions in order
to produce experimental pellets that were visually identical
and novel to the captive birds. e ingredients were selected
with the primary aim of gradually reducing the proportion
of familiar content (dog pellets), and increasing the propor-
tion of novel content. is allowed us to quantify the mynas’
willingness to consume increasingly novel foods. e foods
varied along a gradient of gustatory familiarity, but also dif-
fered in their nutritional composition. e first food (Mix
1), contained solely the birds’ familiar DP and the nutri-
tional composition of the mix (dry mass) was: 15.6% L,
32.4% P and 52.0% C; the second food (Mix 2) contained
different proportions of DP and W and the nutritional
composition of the mix (dry mass) was: 15.2% L, 44.1%
P and 40.6% C; and the third food (Mix 3) contained dif-
ferent proportions of DP, W and EB, and the nutritional
composition of the mix (dry mass) was: 13.5% L, 36.7%
P and 49.7% C.
In order to measure which of the three foods mynas
selected, 7.5 g of each food was placed in one of three small
individual dishes (3 cm diameter, 2 cm deep). e three
dishes were attached to a wooden board so that they could
not be tipped over (see photography in Supplementary
material Appendix 1 Fig. A1). e 3-dish board was then
offered to each bird for three successive 24-h time periods.
Each food was presented in each dish once across the three
testing days in order to avoid specific location preferences.
At the end of each 24-h period, any remaining food was col-
lected and weighed with a high-precision scale. Each cage
was equipped with a removable floor tray that collected
any dropped food. Any food in the tray was collected and
incorporated to the left-over food before weighing. A con-
trol board with equivalent amounts of food was placed in an
adjacent empty cage in order to measure any change in mass
that might have occurred as a consequence of desiccation or
humidity. As weight changes of all control foods placed in
an empty myna cage were negligible (weight changes across
the three days (in percentage of total food consumed): Mix
1: –1.1%; Mix 2: –0.91%; Mix 3: –0.92%), they were not
considered further.
Experiment 2: macronutrient selection
Experiment one manipulated food familiarity, but in doing
so, simultaneously manipulated nutrient content. Hence,
the purpose of experiment 2 was to manipulate macronu-
trient content while holding energy content and familiarity
constant (i.e. all foods were novel). Ten common mynas were
transferred from the group flight aviary to individual cages
identical to those used in experiment 1 and allowed two days
to acclimatize to their new surroundings. As in experiment
1, birds had access to food (DP) and water ad libitum during
acclimatization. Over the course of the following three days,
each bird was presented with a simultaneous choice of three
semi-synthetic pellet foods that were isoenergetic (2600 kcal
g–1), but differed in their macronutrient composition (P, L,
C) (for details see Machovsky-Capuska et al. (2016b)). Dur-
ing tests, 25 g of each food was placed in one of the three
small individual plastic bottles attached to one side of the
cage so that they could not be tipped over (see photography
in Supplementary material Appendix 1 Fig. A1). e three
bottles were then offered to each bird for three successive
24-h time periods, as in experiment 1. At the end of each 24
h period, any remaining semi-synthetic food was collected
and weighed with a high-precision scale.
Experiment 3: behavioural analysis
e aim of experiment 3 was to explore potential
relationships between individual variation in macronutri-
ent intake and individual variation in exploration propensity
and innovativeness.
Innovation test one potential approach to quantifying
innovative foraging is to measure an individual’s propensity
to solve novel foraging problems (Webster and Lefebvre
2001, Griffin and Guez 2014). To this end, each myna was
presented with either a puzzle box consisting of a closed Petri
dish (presented right way up with a flexible handle or upside
with a hook attached to the top part) or a small Plexiglas box
with a lid and a handle (for examples, see schematic in Sup-
plementary material Appendix 1 Fig. A2). Mynas could solve
the extractive foraging problem by pulling the small handle
or the hook off the top of the puzzle box, or, alternatively, by
either levering or pushing the lid upwards. e reward (a dog
pellet or a mealworm) was visible but the apparatus needed
to be opened to access the food.
ese tests were conducted early in the morning following
an overnight food deprivation period. To avoid a neophobia
response to the tasks, the apparatus had been placed in the
same location as the mynas’ daily feeding dishes with an
available food reward the evening before the trial. To begin
the trial, the tasks were presented with one visible reward
inside but unavailable without solving the task. Trials were
video recorded for 30 min. following the introduction of
the apparatus. If the bird failed to make contact with the
task within 30-min the first time round, they were given a
second opportunity at least 0.5–1.5 h later. To investigate
differences between birds in problem-solving propensities,
an innovation score was calculated using latency to solve (s)
minus latency to first contact the apparatus (s). A score of
1801s was attributed to birds that failed to solve the task.
Exploration test – exploration propensity was assessed in
an unfamiliar room under artificial light (Dingemanse et al.
2002) (see room’s plan in Supplementary material Appendix
1 Fig. A3). e room included five artificial trees each bearing
five perches. e ground was divided in four areas (87 116
cm). Birds were moved from the aviary to an individual cage
placed on wheels. e individual cages were covered with a
large piece of opaque material and rolled into the unfamil-
iar testing room. e opaque material was gently lifted and
the bird left alone to acclimate 10 min to this environment
before the experimentor remotely opened the doors of their
cage (1 27 55 cm). e bird was given a 15-min latency
to exit its home-cage, if it had not departed from the cage by
the end of that period the experimenter approached the cage
to induce exit. Once the bird exited the cage, it was given 10
min to move freely around the room. Birds had the ability
to return to the home cage during the test, however the nest
box remained closed at all times during testing. White noise
was played back through a loudspeaker to mask the sounds
of birds in a nearby room.
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JMP® ver. 10 (SAS Inst., Cary, NC, 1989-2007.10) (for
PCA). Two-tailed tests were used throughout and alpha
levels were set at 0.05. All visual representations, including
RMT models, were created with SPSS Statistics 21 (IBM
2012).
Data deposition
Data available from the Dryad Digital Repository: < http://
dx.doi.org/10.5061/dryad.3q7k6 > (Peneaux et al. 2017).
Results
Experiment 1
e amount of food consumed by mynas differed signifi-
cantly across the three food mixes (Mix 1: median 12.03 g,
range 17.53 g (4.56–22.09 g); Mix 2: median 17.59 g,
range 15.08 g (6.36–21.44 g); Mix 3: median
16.20 g, range 16.05 g (5.81–21.86 g); approximative gen-
eral independence test: n 21, c² 8.75, p 0.01, Fig. 1a).
Mix 1 was consumed in significantly smaller quantities than
Mix 3 and Mix 2 (approximate 2-sample permutation test
with Bonferroni–Holm correction: Mix 1/2: p 0.04;
Mix 2/3: p 0.04), indicating that the food novel in its
appearance, but most familiar in terms of its content (DP),
was the least consumed food. Mixes 2 and 3, which were
equally novel in appearance, but contained increasing propor-
tions of novel contents, were not consumed in significantly
different proportions (approximate 2-sample permutation
test with Bonferroni–Holm correction: p 0.98).
e amount consumed of each of the three ingredients
and nutrients also differed significantly after the three days
of experimentation (approximative general independence
test: ingredients: n 21, c² 38.67, p 0.0001, Fig. 1b;
nutrients: n 21, c² 39.75, p 0.0001, Fig. 1c), showing
that mynas fed non-randomly from their foods dishes at the
three differente scales analysed.
e macronutrient composition of the diet (dry mass)
estimated during the experiment 1 was 37.3 % P ( 0.2 SE):
14.8 % L ( 0.1 SE): 47.9% C ( 0.2 SE).
Experiment 2
e amount consumed of HL, HP and HP foods differed
significantly (HL: median 4.77 g, range 17.21 g
(0.55–17.75 g); HP: median 30.19 g, range 23.44 g
(20.08–43.52 g); HC: median 7.00 g, range 24.40 g
(2.19–26.58 g); approximative general independence test:
n 10, c² 15; p 0.0001). HP pellets were consumed
in significantly greater quantities than HL and HC pellets
(approximate 2-sample permutation test with Bonferroni–
Holm correction: HP/HL: p 0.01; HP/HC: p 0.01).
e macronutrient composition of the diet (dry mass)
estimated during the experiment 2 was 59.8% P ( 0.8 SE):
22.0 % L ( 0.7 SE): 18.4% C ( 1.0 SE).
e RMT model showed the differences of the P:L:C ratios
of the diets estimated for experiments 1 (P:L:C 2.5:1.0:3.2)
and 2 (P:L:C 2.7:1.0:0.8) with previously reported
In order to determine exploration tendency, several
variables were scored from the videos, including: number
of trees, perches and ground areas visited. To assess the
bird’s activity, we scored the number of movements (flights
between trees, hops between perches, walks on the ground
and returns to cage). e latency to exit the cage was also
scored.
Statistical analyses
Variables scored for the exploration (i.e. number of move-
ments, number of zones visited, number of floor-zones
visited, number of trees visited, number of branches vis-
ited, and latency to exit home cage) were compiled in a
principal component analysis (PCA), using the correla-
tion matrix and a Varimax rotation, in order to reduce the
number of independent variables and obtain an individual
exploration score for each bird. Values on the first prin-
cipal component were used as exploration score for each
individual bird.
Results were analysed using non-parametric statistics in
the form of permutation test (sometimes called a random-
ization test, a resampling method similar to bootstrapping
(Good 2000)). In experiment 1, we tested whether mynas
ate randomly from the novel foods provided by comparing
the consumption of foods, ingredients and nutrients with a
null expectation in which mynas ate equal amounts using
approximative general independence tests (repeated mea-
sures). In experiment 2, the same approach was used to com-
pare the amounts of each HP, HL and HC foods consumed
by the birds. Descriptive statistics for food consumption
were also presented as median and range. Significant effects
were followed up with post-hoc pairwise comparisons using
approximate 2-sample permutation tests stratified by indi-
viduals and p-values were corrected using the Bonferroni–
Holm correction (noted p’).
For each semi-synthetic food consumed in experiment
2, correlations between the total amount of macronutrient
(P, C, and L) consumed and behavioural data were examined
using another form of permutation test, a general Indepen-
dence test.
Following Raubenheimer (2011), we used nutritional
geometry (right-angled mixture triangles, RMT) to portray
the experimental food choices of mynas faced with unfamiliar
foods. For experiment 1, we determined food consumption
and diet of mynas at three different scales (foods, ingredients
and nutrients) and we compared observed intakes against a
null hypothesis that mynas consumed equal amounts. For
experiment 2, we obtained the macronutrient composition
of diets (expressed as P:L:C dry mass ratios) from the con-
sumption of semi-synthetic foods. We then compared our
results with the 1) null hypothesis of consumption of equal
amounts (dry mass), 2) macronutrient composition (dry
mass) of the diet of free-ranging mynas as determined by
stomach content analyses (Sengupta 1976, see Machovsky-
Capuska et al. 2016b for more details), and 3) macronutri-
ent composition (dry mass) of the diet obtained using an
experimental ‘cafeteria’ design (Machovsky-Capuska et al.
2016b).
All statistical analyses were carried out using R ver. 3.1.1
(R Development Core Team) (for Permutation tests) and
1436
Experiment 3
Correlation analysis revealed the existence of two relation-
ships between behavioural traits and the consumption of
specific macronutrients (Fig. 3, see full correlation table and
scatter plot panel of the correlation analysis in Supplemen-
tary material Appendix 2 Table A1 and Fig. A2). Individuals
that consumed larger total amounts of carbohydrate pellets
in a three-way choice between C, L and P were faster to
solve a novel extractive foraging task (Fig. 3a; approximate
general independence test: n 10, Z –1.89, p 0.04).
We found no evidence of a relationship between innovation
latencies and intake of the other two macronutrients (P and
L) (approximate general independence test: protein: n 10,
Z –0.63, p 0.57; lipid: n 10, Z 1.37, p 0.17).
Behavioural variables used to describe levels of exploration
were aggregated into a PCA. e KMO test suggested the
matrix was appropriate for use in a PCA (KMO 0.74). e
first axis explained 58.2% of the variance in the data and was
considered to be a good summary of the data (Budaev 2010).
Values on this first axis were used as the exploration score of
each individual, where high scores indicated high tendency to
explore (Table 1). Correlational analyses between each indi-
vidual’s PC1 score and its relative intake of each macronutri-
ent during experiment 2 revealed an association between an
individual’s relative protein intake and exploration tendency.
Mynas that consumed more protein had higher explora-
tion PC scores (Fig. 3b; approximate general independence
test: n 10, Z 1.91, p 0.04). No relationships were
found between an individual’s exploration score and either
its carbohydrate (approximate general Independence test:
n 10, Z –0.59, p 0.58); or its lipid intake (approximate
general independence test: n 10, Z –1.31, p 0.20).
Discussion
In this study, we aimed to analyse mynas’ food and nutri-
tional selection through the presentation of novel foods. We
experimental feeding trials for mynas (P:L:C 6.3:1.0:0.1)
and also with the reconstructed natural diet (P:L:C
3.3:1.0:1.4, see Machovsky-Capuska et al. 2016b for more
details) (Fig. 2).
Figure 1. Right-angled mixture triangle showing (as a % of dry weight) the foraging choices of captive mynas during the three days at
different scales. (a) Myna diet (black hollow circles) clusters around the range of Mix 3 intakes, (b) the model shows the region of
ingredients’ space (dotted line) that was accessible to the birds (black hollow circles) given the three mixes (black hollow triangles) they were
provided during the experiment and (c) nutritional niche accessible to mynas delineated by dotted lines as defined by the three different
mixtures (black filled squares) offered to the birds. e three plots compare the consumption of foods with a null hypothesis that mynas
consume equal amounts (black filled circles).
Figure 2. Right-angled mixture triangle showing (as a % of dry
mass) the macronutrient preferences in common mynas. Empty
circles represent macronutrient intakes of individual birds and the
squares represent macronutrient compositions of the three semisyn-
thetic food pellets offered during the captive feeding trials (hollow
blue square HP diet; solid green square HC diet; solid grey
square HL diet). e green star represents the macronutrient
composition of the diet estimated in experiment 1; the gold circle
represents the macronutrient composition of the diet estimated in
experiment 2; the solid red circle represents the natural diet esti-
mated from Sengupta (1976, for more details see Machovsky-
Capuska et al. 2016b) and the hollow red circle represents the diet
estimated from free-ranging mynas obtained from Machovsky-
Capuska et al. (2016b). e black triangle represents the null
hypothesis for a balance dietary nutrient intake consuming equal
amounts of the three macronutrients.
1437
intended to identify food selection patterns and also whether
the patterns of consumption of some macronutrients were
correlated with innovation and exploration propensities. In
experiment 1, birds showed significantly less interest for a
familiar dog food relative to two other foods, despite the dog
food mix being the most familiar in terms of content. Our
analyses also showed that mynas fed in non-random pro-
portions at three different scales analysed: foods, ingredients
and nutrients. In experiment 2, mynas selectively consumed
HP when simultaneously offered a choice between HP, HC,
Figure 3. Correlations between: (a) mean ( SE) innovation score and total amount of HC pellets after a 3-d period (g); (b) mean ( SE)
exploration score and total amount of HP pellets consumed after a 3-d period (g). Innovation score solving latency – first contact
latency.
Table 1. Orthogonally (Varimax) rotated component loadings on
first axis for the exploration test. Bold indicates variables contribut-
ing to a component’s meaning.
Behavioural variables PC1
Number of trees visited 0.87
Number of branches visited 0.97
Total number of zones visited (ground air) 0.93
Number of movements 0.87
Latency to first exit the cage –0.09
Number of floor-zones visited 0.42
1438
e present study revealed that mynas consuming higher
amounts of HP foods were more exploratory when con-
fronted with a novel environment. Moreover, individual’s
tendency to explore a novel space is a repeatable trait in
mynas (Lermite et al. 2017). To reduce the impacts of behav-
iour unrelated to exploration during this open-field test, spe-
cific methodologies were used (i.e. individuals were not food
deprived and were free to exit home cage) to limit the expres-
sion of stress-related responses and facilitate information
acquisition (Dingemanse et al. 2002, Mettke-Hofmann et al.
2002, but see discussions in Carter et al. 2013 and Lermite
et al. 2017). Together, these findings suggest that mynas with
higher exploratory tendencies are likely to be more sensitive
to dietary fluctuations in protein. A relationship between
low-protein diet and exploration has been demonstrated
in rats (Almeida et al. 1991, 1993, 1994, 1996). A lack of
dietary-protein has been suggested to cause an increase of
‘impulsiveness’, driving malnourished rats to explore more
open-arm novelty (Almeida et al. 1991, 1993, 1994, 1996).
is behavioural pattern reflects a decrease in anxiety in
the protein-deficient individuals. Protein-malnutrition
is suspected by the authors to cause deleterious effects on
brain structures underlying inhibitory behaviours in situ-
ations promoting anxiety (Almeida et al. 1994). Hence,
lower anxiety levels induced by protein malnutrition could
act as a mediator for higher levels of exploration in these
protein-seeking birds. Moreover, in the context of an urban
environment, proteins are expected to be rare and extremely
valuable (Eagle and Pelton 1983, Pierotti and Annett 1987,
Murphy 1993, Machovsky-Capuska et al. 2016b). Foraging
for HP foods in urban environments (e.g. insects) would
therefore require longer periods of time spent exploring the
environment, which could in turn produce a feedback loop
reinforcing the expression of exploratory behaviour in these
P deficient individuals.
e present study also revealed that mynas consuming
higher amounts of HC foods were faster to solve a novel
extractive foraging task. Innovative foraging behaviour has
been linked to the ability to use a greater variety of motor
actions in a foraging context (Griffin et al. 2014). Urban
mynas are often seen foraging in school playgrounds,
food outlets and supermarket carparks (Sol et al. 2012b).
Anthropogenic food sources are often wrapped in differ-
ent packaging that are likely to substantially challenge and
enhance mynas motor diversity skills. Human discards
contain high-levels of C relative to P (Pierotti and Annett
1987), and birds adapted to access and consume these food
items are likely to develop a taste for HC foods. is might
explain the present association between innovative foraging
and consumption of C found here.
e mechanisms underlying food and nutritional choices
and post-ingestive processing in invaders are likely to be
a key to understand how these species succeed in novel
environments (Machovsky-Capuska et al. 2016b). Here, we
demonstrated that common mynas select their foods based
on their nutritional composition. e tendency to sample
unknown foods to fulfil nutritional requirements is likely to
contribute to the invasion success of mynas, but this remains
to be tested. Our findings linking macronutrient intake
and exploration and foraging innovation highlight the
HL pellets. e RMT analyses also revealed that mynas
consumed different amounts of protein to their natural diet
described by Machovsky-Capuska et al. (2016b). Finally,
experiment 3 revealed the existence of two nutrient–behav-
ioural relationships. Mynas that consumed greater amounts
of HP pellets had a stronger tendency to explore whereas
those mynas that ate more HC pellets were faster to solve a
novel foraging problem.
e history of captive myna feeding in the present study,
together with their patterns of food choices during test-
ing, strongly suggest that mynas selectively consume novel
foods relatively to the nutritional content. In experiment 1,
we showed that when mynas were given a choice between
foods containing a gradually reduced proportion of famil-
iar content (DP), they chose to consume higher amount of
the foods that contained increased proportion of unfamiliar
content (EB and W). Birds actually chose novel combination
of HP foods.
Experiment 2 confirmed that it was not novelty per se
that was driving this selection, but rather protein content,
by showing that mynas selected HP relative to HC or HL
when all three foods were equally novel. ese observations
are consistent with the findings of Machovsky-Capuska et al.
(2016b). In that study, free-ranging urban mynas selectively
chose to consume almost exclusively protein-enriched foods,
leading the authors to speculate that free-ranging mynas
might be protein deficient. Here, we extend these findings
by confirming that mynas are willing to consume novel
foods in such proportions that would enable them to reach
their nutritional goals.
Animals are likely to detect certain nutrients using
olfactory and gustatory cues (Simpson and Raubenheimer
2012). For example, gallinaceous chicks display an unlearned
predisposition to peck at foods (Suboski and Bartashunas
1984), which presumably enables them to perceptually
locate salt and glucose. In contrast, the identity of plants
containing HP, which lacks a perceptual signature, is socially
transmitted from mother to chick in gallinaceous chicks
(Allen and Clarke 2005). As our mynas were individually
held and all foods were visually identical and offered ran-
domly, mynas were not able to mimic each other’s food
choices. Furthermore, our novel foods had no resemblance to
insects (that are also HP foods), meaning that mynas could
not have been able to rely on previous experience to select
foods more likely to contain protein. Hence, their detec-
tion and selection of HP foods were likely to be triggered
by different physiological mechanisms including systemic
nutrient sensing mechanisms, neural circuits that control
feeding behaviour, hormonal feedback from body reserves
(Morton and Schwartz 2011) and also post-ingestive regula-
tory responses that assist in the adjustment of imbalanced
nutrients (Simpson and Raubenheimer 2012). A challenge
ahead should aim to determine the mechanisms by which
mynas sense nutrients and in particular detect HP foods.
Also, further experiments will be undertaken to rule out the
alternative possibility that mynas only demonstrated taste
preferences for particular foods instead of actively regulat-
ing their macronutrient intake, although a large body of
literature spanning many animal taxa suggest this is unlikely
(Simpson and Raubenheimer 2012).
1439
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and behaviourally flexible phenotypes in a very successful
invader.
Acknowledgements is project has been assisted by the New South
Wales Government through its Environmental Trust.
Permits – All husbandry and experimental procedures were approved
by the Animal Care and Ethics Committee of the Univ. of Newcastle
(Animal Research Authorities A-2011-127, A-2014-424, and
A-2014-425).
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... protein). 35,36 On a low-protein and low-carotenoid diet, controlled observations in captivity have revealed that eye patch coloration progressively decreases as time in captivity increases, until all coloration disappears and only a white skin surface is visible (C. Peneaux, personal communication; Figure 1). ...
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... Free-ranging urban Common (Indian) Mynas Acridotheres tristis select almost exclusively a high-protein food, and compete to gain access to it, when given the choice between high-protein, high-carbohydrate and high-lipid artificial pellets (Machovsky-Capuska et al. 2016). Moreover, protein-deprived mynas selectively consumed novel foods containing higher proportions of protein (Peneaux et al. 2017). Urban habitats may therefore be deficient in protein-rich foods, making this macronutrient a potentially limiting resource for ornamental production. ...
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Common Mynas Sturnus tristis, previously known as Acridotheres tristis, are considered among the world’s worst most invasive species. However, relatively little is known about the factors that affect their persistence and spread in new environments. They have been observed feeding on a wide range of foods, including anthropogenic foods in urban areas. Their diet preferences are relatively unknown. Therefore, we investigated the macronutrient preferences of Common Mynas in captivity. Common Mynas (n = 10) were given a pairwise choice of three different diets (high lipid, high protein, and high carbohydrate diets) in the laboratory to determine their preference. Common Mynas showed a preference for the high lipid food, followed by high protein, with the high carbohydrate food least preferred. Consequently, this suggests Common Mynas preferred food high in lipids compared with proteins and carbohydrates, but additional study is needed to confirm this. Implications are Common Mynas should not be a problem for South African agricultural areas, because this industry generally provides relatively few dietary items high in fat, so we expect the Common Myna will continue to be distributed mainly in urban areas of South Africa where anthropogenic foods relatively high in fat are more common.
... It may also help us to determine what nutrients may be limiting for a particular species in natural habitats (Coogan et al., 2014;Aryal et al., 2015;Coogan et al., 2018b). Such analyses have, for example, been made on several mammal and bird species (Panthi et al., 2015;Peneaux et al., 2017;Coogan et al., 2018b). ...
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... The time spent foraging by Red-winged Starlings at the UCT was seemingly not dependent on human presence and their associated foods on campus, as they did not respond with increasing foraging effort on days of anthropogenic food shortage. Future research should be aimed at the physiological effects of a predominantly anthropogenic diet as anthropogenic food might be of lower quality (e.g., Andersson et al. 2015;Toledo et al. 2016) or different nutrient composition (Machovsky-Capuska et al. 2015;Peneaux et al. 2017;Coogan et al. 2017) and a high consumption could be costly in terms of bird health (Shochat et al. 2010;Lowry et al. 2012). Exploring how other urban bird species cope with these short-term food fluctuations would improve our understanding of whether the strategy seen here is generalized to other species and may help to understand whether these behaviours are integral to allow species to exploit urban habitats. ...
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... The ability of invaders to adjust phenotypic characteristics via plasticity has been well studied (Wright et al. 2010;Hendry 2015;Peneaux et al. 2017) as have adaptive shifts by invasive species (Myles-Gonzalez et al. 2015;Gruber et al. 2017). However, these mechanisms are not mutually exclusive (Mery and Burns, 2010;Rollins et al. 2015), and synergistic changes have been suggested in other invasive species (Colautti and Lau 2015;Kilvitis et al. 2017;Reisinger et al. 2017). ...
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The dietary generalist-specialist distinction plays a pivotal role in theoretical and applied ecology, conservation, invasion biology, and evolution and yet the concept remains poorly characterised. Diets, which are commonly used to define niche breadth, are almost exclusively considered in terms of foods, with little regard for the mixtures of nutrients and other compounds they contain. We use nutritional geometry (NG) to integrate nutrition with food-level approaches to the dietary niche and illustrate the application of our framework in the important context of invasion biology. We use an example that involves a model with four hypothetical nonexclusive scenarios. We additionally show how this approach can provide fresh theoretical insight into the ways nutrition and food choices impact trait evolution and trophic interactions.
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The foraging behavior of animals in complex environments is affected by conflicting demands (cf. Sih 1980, Martindale 1982, 1983, Cerri & Fraser 1983) and may be dependent on more than one aspect of prey quality (Pulliam 1975, Westoby 1978, Nicotri 1980, Breitwisch et al. 1984). Various currencies have been suggested that could affect the dietary selection of a foraging animal. These include energy intake per unit of time (Schoener 1971), maximizing intake of an essential nutrient (Goss-Custard 1981), and mixing nutrient intake to ensure adequate nutrition (Westoby 1978, Nicotri 1980).
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