South African Journal of Botany

Sugar preference in common mynas (Acridotheres tristis)

S.T. Gumede

School of Life Sciences, University of KwaZulu-Natal, Private Bag X 01, Scottsville, 3209,

Pietermaritzburg, South Africa.

ABSTRACT

Nectarivorous and frugivorous birds have been found to select their diet according to sugar type.

Consequently many studies of sugar preference have been conducted on various avian species.

Common mynas (Acridotheres tristis) are a common invasive worldwide and do damage to fruit

crops. However, their sugar preferences have never been studied. Previous studies revealed that

some avian species with the enzyme sucrase can digest sucrose whereas those without this

enzyme cannot and therefore they prefer fructose or hexose over sucrose. In particular the

Sturnidae (starlings) do not have the sucrase enzyme. Consequently we investigated sugar

preference, effect of concentration and assimilation efficiency in common mynas (n=7). These

birds were given pairwise choice of sugars (fructose, sucrose and glucose) of 5g/ml (5%) to test

sugar preference. Common mynas preferred glucose over sucrose and fructose. To test what

concentrations they preferred of glucose, they were offered three different concentrations (5%,

10% and 25%). They showed distinct preference for the 10% concentration of glucose and

concentration did not influence energy intake. The birds maintained body mass. Common mynas

failed to digest and absorb sucrose but fructose and glucose were digested and assimilated

efficiently. Results of this study showed that common mynas neither prefer glucose that is high

in concentration nor too dilute. Nest eradication, spraying the roosts might help in managing

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population increase of common mynas and people must not feed common mynas. However,

further studies needs to be done.

1. Introduction

Food selection in nectarivorous and frugivorous birds is influenced by several factors

including: gut physiology and sugar preference (Avery et al., 1999; Gatica et al., 2006;

Witteveen et al., 2014). According to Ayala-Berdon et al. (2012) food selection in nectar feeding

animals is influenced by two main mechanisms, the ability to digest sucrose and the ability to

taste the sweetness of the sugar. Sugar preference also depends on the enzyme sucrase that is

responsible for digestion of sucrose (Downs, 1997; Witteveen et al., 2014). Generally, species

lacking enzyme sucrase unable to digest sucrose (Lane, 1997). These species usually prefer

fructose and hexose (Lane, 1997). It has been reported that sucrose ingestion may cause osmotic

diarrhea (Avery et al., 1999; Graves, 2015; Lane, 1997). However, some species are able to

digest sucrose but they prefer hexose over sucrose when given a choice (Avery et al., 1999;

Lane, 1997). Some of the fruit feeding birds are able to hydrolyze sucrose but not as efficiently

as they do with glucose or fructose (Lane, 1997). The enzyme sucrase have been found in

passerine and non-passerine bird families (Lotz and Schondube, 2006).

Sugar concentrations also play an important role in influencing the diet of nectar feeding

birds (Brown et al., 2010). There is a variation of nectar sugar concentrations among plants and

this variation influences the preference in nectarivorous birds (López-Calleja et al., 1997; Mata

and Bosque, 2004; Roberts, 1996). Sugar concentration may be fundamental in managing the

daily intake energy in nectar feeding birds (López-Calleja et al., 1997). Viscosity and nectar

concentration are directly proportional, one increases with the other (López-Calleja et al., 1997).

Some nectar feeding birds prefer higher concentrations in order to capitalize on their level of

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energy ingestion but they reduce the intake as the concentration increases (Jackson et al., 1998).

Sugar and concentration preferences appear to be the outcome of the close relationship between

the pollinators and the plants they pollinate (Brown et al., 2010; Leseigneur and Nicolson, 2009;

López-Calleja et al., 1997). Plants that are pollinated by these birds have the similar sugar type

and concentration that they prefer (Leseigneur and Nicolson, 2009). Previous studies revealed

that starlings (Sturnidae), American robin (Turdus migratorius), and brown-eared bulbuls

(Hypsipetes amaurotis) are unable to digest sucrose therefore they prefer fructose or hexose over

sucrose (Avery et al., 1999; Lane, 1997). Sugarbirds (Promerops) and sunbirds (Nectariniidae)

absorbed sucrose efficiently as glucose (Jackson et al., 1998). Bananaquits (Coereba flaveola)

showed no preference when offered sucrose, glucose or fructose in pairwise choice tests and

were able to digest or absorb sucrose and hexoses efficiently (Mata and Bosque, 2004).

Invasive species are organisms that have been accidentally or intentionally introduced

outside their normal distribution ranges (Richardson et al., 2000). Which then reproduce,

establish a breeding population, and spread rapidly in their new environment (Richardson et al.,

2000). These species are a major threat to biodiversity conservation and ecosystem worldwide

(Richardson et al., 2000). The damage to native species and ecosystems worldwide caused by

invasive non-native species is estimated to be as serious as the loss and degradation of habitats

(Richardson et al., 2000). Several invasive bird species have been introduced into new areas. For

instance common mynas (Acridotheres tristis) are native to southeast Asia and are constantly

encountered or widespread from Afghanistan, the Indian subcontinent, Burma to Indochina

(Baker and Moeed, 1987). Common mynas are recorded among the 100 worst invading species

worldwide (Holzapfel et al., 2006). They have successfully invaded many countries including

South Africa (Baker and Moeed, 1987; Peacock et al., 2007). Common mynas are found to be

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widely distributed in areas with an annual rainfall greater than 600 mm in southern Africa (Dean,

2000).

Common mynas have a broad diet, and are omnivorous (Peacock et al., 2007). This alien

invasive species is mostly associated with human habitation and urban areas (Peacock et al.,

2007){Lowe, 2011 #125}. Their population density increases with the level of habitat

modification (Lowe et al 2011). Common mynas are opportunistic and usually feed in pairs or

flocks which might be beneficial in reducing the predation risk (Peacock et al., 2007). Common

mynas’ flocks are known to damage fruit crops, counting grapes, pears, strawberries, figs,

gooseberries, apricots and apples (Barker and Moed 1987). These birds also have the ability to

endure different environmental conditions and all these characteristics help them to be

successful invaders (Baker and Moeed, 1987; Peacock et al., 2007). Common mynas are

generally believed to be aggressive birds, they may chase away other birds from their territory,

and resources, their broad diet is advantageous (Haythorpe et al., 2012). However, there is

insufficient evidence on the aggressiveness of common mynas (Lowe et al., 2011).

According to our knowledge, no study of sugar preference has been conducted on

common mynas. In this study, we examined the sugar type and concentration preference of

common mynas when given a choice of sugars. We also examined their ability to digest sugars.

Red-winged Starlings preferred hexose over sucrose and were unable to digest sucrose due to the

absence of enzyme sucrase (Brown et al., 2012). Therefore, in this case we predicted that

common mynas would prefer hexose over fructose and sucrose regardless of the concentration.

We also predicted that they will prefer low concentrations of hexose when given different

concentration choices.

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2. Methods

We captured seven adult common mynas using the mists net in Pietermaritzburg, KwaZulu-Natal

(29° 35' 23.9994"S 30° 23' 59.9994" W) in April 2015. Birds were acclimated for two weeks in

outside aviaries at the University of KwaZulu-Natal animal house. They were fed grated fruit

and vegetable mix (apples, pears, carrots and beetroot) and slices of pawpaw and banana as the

maintenance diet, supplemented with Aviplus Softbill Mynah Pellets (Aviproducts, Durban,

RSA). Birds were then moved into a constant environmental room after the outdoor

acclimatization where there were kept separately in cages (77×52×81 cm). The birds were then

acclimated for another week on the maintenance diet. Before experimental trials started, birds

were familiarized with burettes by offering them water in burettes daily. All seven birds were

used in all trials.

2.1 Sugar type preference trials

We conducted pairwise choice tests using the following pairs of solution: 5% of sucrose and 5%

fructose, 5% sucrose and 5% glucose, 5% fructose and 5% glucose which were offered to the

common mynas in burettes and at midday positions were changed to avoid positional bias. Birds

were weighed before and after trials which were conducted from 06:00 to 18:00h. During the

experimental trials the consumption of sugar solution was measured each hour from 07:00 to

18:00 h. No water was given to birds during the trials and no supplemental food overnight. Trials

were not run on consecutive days, one or two days were skipped before the next trial and they

were randomized. All birds in each day were offered the same choice of solutions. The total

volume consumed for each sugar type was calculated using the formula consumption of sugar

type divided by the total volume of solution drank. To determine the sugar type preference in

birds we used the Statistical Package for the Social Sciences (SPSS).

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2.2 Concentration preference trials

In additional to preference trials, to determine if common mynas have a concentration

preference, we offered each bird a choice of three glucose solutions of different concentrations

(5, 10 and 25%) simultaneously. These solutions were randomly placed in each bird cage. The

trials were conducted from 06:00 to 18:00 h and birds were weighed before and after each trial.

At midday the solutions positions were changed. The consumption of each glucose solution was

measured each hour from 07:00 to 18:00 h and the total consumption calculated for each

concentration. The volume of each solution consumed was converted into energy consumed per

gram body mass per day. Energy intake and volumetric intake were compared from each of the

three concentrations offered to birds at the same time using ANOVA to test the sugar

concentration preference in common mynas.

2.3 Apparent assimilation efficiency

We determined the apparent assimilation efficiency (AE) of glucose, sucrose and fructose at 25%

concentrations. A tray of liquid paraffin was placed under the wire mesh cage where birds were

captured to prevent the evaporation of liquid excrete. Birds were fed a single sugar solution for a

period of 4h and deprived food for the next 2 h to make sure that food was digested and

processed and then returned to maintenance diet. Amount of sugar solution consumed was

recorded every hour for the duration of the trial. Excreta were collected from the liquid paraffin

using a syringe and weighed. Samples were then centrifuged at 1,300 rpm and sugar content was

analyzed using the Shimadzu (LC-20AT) high-performance liquid chromatograph (HPLC).

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3. Results

3.1 Sugar type preference trials

There was a significant difference between fructose and sucrose choice test in terms of volume

intake (T-test: P = 0.003; t = 8.062; df = 12, Fig. 1a). Mean volume intake of fructose was higher

than mean volume intake of sucrose (Fig. 1a). There was also a significant difference between

the choice of glucose and sucrose volume intake (T-test: P = 0.008, t = 6.828; df = 12, Fig. 1b).

Mean volume intake of glucose was higher than mean volume intake of sucrose (Fig. 1b). There

was a significant difference between glucose and fructose in terms of volume intake (T-test: P =

0.033; t = 6.334; df = 12, Fig. 1c). The mean volume intake of glucose was higher compared

with mean volume intake of fructose (Fig. 1c).

3.2 Glucose concentration preference trials

Common mynas showed no significant difference in terms of volume intake between three

different concentrations (5%, 10%, 25%) of glucose when offered these concurrently (ANOVA:

df = 2; F = 2.468; P = 0.113, Fig. 2a). There was high mean volume intake of glucose at 10%

compared with 5 and 25% concentrations (Fig. 2a). The lowest mean volume intake was at 5%

concentration (Fig. 2a). The concentration of glucose did not have an effect on energy intake in

common mynas (ANOVA: df = 2; F = 2.565; P = 0.105, Fig. 2b). Common mynas gained more

energy when feeding on 10 % concentration of glucose (Fig. 2b).

3.3 Apparent assimilation efficiency

Common mynas showed efficient assimilation of glucose and fructose, they absorbed 90% of

both sugars with only 10% of glucose and fructose found in their excreta (Table.1). With sucrose

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common mynas did not show efficient assimilation, about 80% of sucrose remained in their

excreta and only 20% was absorbed (Table.1).

4. Discussion

As much as common mynas are known to have a broad diet and omnivorous (Peacock et

al., 2007), in this study they showed the distinct preference of fructose over sucrose when

presented with fructose and sucrose of the same concentration (5%). As mentioned, common

mynas as members of Sturnidae were expected not to prefer sucrose (Brown et al., 2012) and

red-wing starlings showed lack of sucrase-isomaltase activity (Bizaare et al., 2012). Sturnidae

members are unable to digest sucrose because they are known to lack sucrase enzyme which is

responsible for digestion of sucrose (Brown et al., 2012; Gatica et al., 2006). Generally, birds

show the preference of sugars that are able to digest and absorb efficiently (Mata and Bosque,

2004; Medina-Tapia et al., 2012). Therefore, common mynas hexose preference over sucrose

showed that they may not be able to digest and absorb sucrose efficiently. Similarily,

bananaquits did not show any preference when offered sucrose, glucose and fructose and they

are able to absorb all these sugars efficiently (Mata and Bosque, 2004). Thrushes (Turdidae)

decreased their volume consumption when they were offered sucrose solution and are also

lacking sucrase activity (Gatica et al., 2006). Common mynas also decreased their volume intake

on sucrose solution. They did not to show sucrose preference when offered glucose and sucrose

choice. This behaviour confirmed that common mynas gut physiology is unable to digest sucrose

efficiently (Ayala-Berdon et al., 2013). Birds lacking sucrase enzyme experiences diarrhoea

when ingested sucrose therefore they avoid this sugar (Schondube and Del Rio, 2003). In

addition, the less volume intake of sucrose by common mynas may be showing avoidance of

osmoregulation from sucrose (Lotz and Schondube, 2006). When common mynas were

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presented with fructose and glucose they showed the distinct preference of glucose over fructose.

These results showed that even if common mynas prefer fructose and glucose but if they are

given a choice between these two sugars they mostly prefer glucose.

Our results showed a distinct difference when common mynas were presented with three

concentrations of glucose (5, 10 and 25%). Common mynas showed preference for 10%

concentration of glucose with decreased intake of the low (5%) and high (25%) glucose

concentrations they decreased the volume consumption. These results are supported by the study

done by Brown et al (2012) that starlings are incapable of dealing with solutions of high

concentrations. Ingestion of hexose in high concentrations causes dehydration (Odendaal et al.,

2010). This could be the reason common mynas reduced the volume intake at higher

concentration of glucose. However, common mynas also decreased the volume consumption at

low concentration (5%). The taste of sugar might have an influence on preference between the

concentrations (5, 10 and 25%). In short term trials sweetness affect the preference of taste

(Downs, 1997). Sugars of high concentrations are sweeter than sugars of low concentrations and

common mynas preferred none of these concentrations. Furthermore, lesser double-collared

sunbirds (Cinnyris chalybeus) showed preference of glucose at the concentration of 10% or 20%

compared with higher concentration of 30% (Lotz and Nicolson, 1996). Village weavers

(Ploceus cucullatus) also did not show preference at high concentrations (20% and 25%) of

glucose (Odendaal et al., 2010). Furthermore, nectar feeding birds decrease their consumption

with an increase of concentration (Mata and Bosque, 2004). Concentration of glucose did not

have the influence on energy intake and birds were able to manage their body mass when offered

different concentrations of glucose. Common mynas consumed more 10% glucose concentration

than the other concentrations. However, they also consumed other concentrations (5% and 25%).

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Perhaps they balance these concentrations to avoid dehydration from higher concentration and to

balance the energy intake. The brown honeyeater (Lichmera indistincta) increased the volume

intake with less concentrated nectar to compensate for energy while a South African sunbird

decreased the volume intake with high nectar concentrated (Downs, 1997).

Common mynas have the ability to absorb glucose and fructose sugars efficiently (Table.

1). Common mynas absorbed fructose and glucose up to 90% whereas with sucrose solution only

22% was assimilated (Table 1). These results are supported by the study done by Brown et al

(2012) showing that starlings are lacking the enzyme sucrase (Bizaare et al., 2012). Common

mynas were avoiding the sucrose solution and the total consumption was very low compared

with other sugars which could be the results of lacking sucrase (Brown et al., 2012; Downs,

1997; Witteveen et al., 2014). Common mynas preferred hexose sugar because they are able to

digest and absorb it efficiently (Mata and Bosque, 2004; Medina-Tapia et al., 2012). Clearly

common mynas are lacking the sucrase activity because they are unable to assimilate sucrose.

As predicted common mynas have sugar preference and concentration. These birds are

unable to digest and absorb sucrose efficiently which shows that they are lacking the sucrase

activity and prefer hexose over sucrose and fructose. As expected they decreased the volume

intake of sucrose. Furthermore, they decrease the consumption of glucose at a very low

concentration (5%) and at a higher concentration (25%) and concentrations of glucose did not

have the influence on energy intake of common mynas; birds were able to maintain their body

mass. This suggests that common mynas prefer glucose not too sweet or too dilute. Managing

this alien species will be a challenge due to its broad diet, distribution and its association with

urban areas. As the urbanisation increases more natural habitats are modified. Therefore,

common mynas population should be expected to increase even more. In monitoring common

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mynas population I would recommend the nest eradication. Common mynas build their nest on

infrastructures accessing them will not be a problem. Nest eradication will also decrease

reproduction success of common mynas. Spraying the roosts with avicides may also contribute in

managing common mynas population. Community should also stop feeding common mynas.

Currently common mynas have not spread towards western side of South Africa but are

extending (Fig. 3). Therefore, there is a possibility of damaging grape crops which is a major

agricultural and economic concern. Further studies needs to be done.

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Legends of figures

Figure 1: the total mean volume intake of common mynas when presented with a choice of two

sugar types of 5% concentration: a) fructose and sucrose b) glucose and sucrose and c) fructose

and glucose

Figure 2: the total mean volume intake and energy intake of common mynas presented with

three choices of different concentrations (5, 10 and 25%) of glucose:

Figure 3: Distribution of Common myna in southern Africa constructed from statistical

smoothing of the records from SA Bird Atlas Project.

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Table 1: Showing calculated percentage of assimilated sugar.

Concentration(g/ml)

Sugar type Product Initial offeredLeft (in excrete) Assimilated%

Sucrose glucose 25 2.71 90

Sucrose 25 19.47 22.1

Fructose 25 2.82 89

Fructose Fructose 25 0.2 99

Glucose Glucose 25 0.21 99

a) b)

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c)

Figure 1

a) b)

Figure 2

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Figure 3

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