We're often advised to feed our birds plenty of fruits and vegetables, and it's recommended that we humans eat a lot of it too.  But fruits and vegetables are not created equal, and vegetables are nutritionally superior.  Veggies tend to be higher in vitamins and minerals, while fruit is higher in sugar. Veggies also have a stronger impact on longevity, in humans at least (Washington Post). So it's best to focus on getting our birds to eat vegetables, and view fruit more as a treat. 

What kind of fruit?

When we talk about fruit, we usually mean the sweet juicy fruits that we find in the supermarket, which have been changed by centuries of selective breeding into something that is usually very different from the original wild fruit.  There are several different categories of wild fruit, which can be radically different from supermarket fruit, and quite different from each other in their nutritional composition and other characteristics.  So when we hear that a certain parrot species eats fruit in the wild, we need to ask: what kind of fruit are they eating? And what part of the fruit are they eating? The answers might be something very different than what we're thinking.

Nutrient-dilute or nutrient-dense? From Comparative Avian Nutrition (called CAN from now on), page 89:

"fruits vary widely in size, nutrient content, numbers and sizes of seeds, toughness of fruit coat, and presence of toxic compounds... The energy and protein content of fruit is extremely variable and generalizations are difficult. In general, the nutritional value of fleshy fruits diverges into two categories: nutrient-dilute and nutrient-dense."

"Nutrient-dilute fruits usually have a relatively indigestible exocarp (skin) and a large proportion of seeds, although domesticated forms have been bred to have few or no seeds. The pulp contains large amounts of water and, on a dry-matter basis, is high in carbohydrates and low in fiber and amino acids. In general, the most striking nutritional characteristic of these fruits is a very high ratio of energy to amino acids and other nutrients"

In plain English, these nutrient-dilute fruits have a disproportionate number of sugar calories and not that much of anything else.  This is the type of fruit that we find at the grocery store.

What about nutrient-dense fruits?  Their nutritional profile is very different from that of sweet nutrient-dilute fruits. These fruits are usually not sweet. Back to Comparative Avian Nutrition:

"Nutrient-dense fruits (e.g. many laurels, including avocados, palms, dates, bay, sassafras, camphor, cinnamon, mistletoe) contain relatively large amounts of lipid [fat] (10-70% of the dry matter) and sometimes protein (5-20% of the dry matter), but less water and sugars. These fruits are sometimes referred to as 'dry fruits,' because of their low moisture content and soft, oily texture. Storage of protein and lipid in the fruit requires a large nutritional investment for the plant. Consequently, these fruits are less common but they are nutritionally superior to nutrient-dilute fruits, containing about three times the energy density on a fresh-matter basis." (CAN page 89) 

Palm fruit is a nutrient-dense fruit that is a popular parrot food wherever palms and wild parrots are found together. This is the fruit that red palm oil is made from. Unlike sweet fruits, palm fruit is high in fat and low in carbohydrates. The charts later in this article show how dramatically different it is from the domesticated sweet fruits eaten by humans. 

"The type of sugar in the pulp is variable among fruits. Most fruits dispersed by mammals contain high levels of sucrose, and a subset are cultivated for human consumption (e.g. apples, strawberries, blueberries). Most bird-dispersed fruits are rich in glucose and fructose and are more rarely cultivated commercially." (CAN page 90)

It's interesting that mammals and birds have different tastes when it comes to sweet fruit.  Of course they're talking about birds that are interested in eating the flesh of the fruit, not seed predators like parrots who are likely to collect the fruit before it's had time to produce much sugar. 

"The pulp of the fruit is typically designed as the minimal investment by the plant that will ensure consumption by the dispersers... An unanswered question in ecology concerns the reason for the poor nutritional balance and presence of toxins found in many nondomestic fruits. A suggestion, yet to be confirmed, is that the nutritional imbalance forces the birds to leave the plant frequently in search of other types of fruits or insects in order to balance their diet. The plant benefits because the bird's increased movement distributes seeds more widely. According to this hypothesis, trees with nutritionally balanced, toxin-free fruits would be inhabited by birds that would not leave the area to disperse the seeds... Not surprisingly, soft-bodied arthropods are often consumed as a supplement to a frugivorous diet... The high protein content of these animal foods nutritionally balances the low protein levels of most fruits." (CAN page 95)

Amazing. The relatively low level of nutrients in fruits might actually be a clever plot by the plant to make a bird (or other animal) go somewhere else for a while to find something else to eat, and drop some undigested seeds while it's there. The sugar calories alone are probably enough incentive to get an animal to take the bait, and anything else in the fruit is probably there because the plant couldn't produce the fruit without it.

But sometimes the parrots manage to foil the plant's strongest defenses.  Check out this amusing paper by Filardi & Tewksbury about palm cockatoos eating (and destroying) otherwise-unavailable seeds by picking them out of cassowary poop.  The cassowary ate the fruit first, processed the part that the cockatoos couldn't manage, and excreted the seed. This is a happy outcome for the plant - intact seed is now sitting on the ground in a nice pile of fertilizer  But then the cockatoos came along and finished it off. 

Fruit seeds or fruit pulp?  Wild birds eat primarily to meet their energy requirement (CAN page 129). So when they're seen foraging on nutrient-dilute fruit, the question arises of what they're really after.  The seeds are the most nutrient-dense part of any plant, in terms of calories at least. So when a fruit has edible seeds, it often makes more sense to eat the seed than the fruit. This does happen a lot, and fruit seeds seem to be more popular with wild parrots than fruit pulp. Even the fig parrot - a bird named after a fruit - eats fig seeds as their primary diet, not the fruit pulp (Australasian Zookeeping).

"When foraging parrots other than lories and Pesquet’s Parrots target fruit, they may consume pulp but at least as often they discard it in favor of the seeds inside. Moreover, parrots eat seeds when the fruits are unripe and therefore before seeds are ready for dispersal.

"Parrots of the 17 species observed in this study fed from a total of 102 species of plants... Parrots fed from both reproductive and non-reproductive parts of these plants, but by far most plant species (92%) were exploited for their reproductive parts... For 56 percent of the plant species exploited, seeds were consumed directly... Parrots foraged on unripe fruits and seeds of 21 percent of plant species exploited.

"Parrots ate the whole fruits only of 15 percent of plant species exploited. Of the plant parts used by parrots, protein and lipid levels were relatively high, as were fiber levels. Seeds contained significantly more protein and lipid and less fiber than other plant parts... Seeds averaged 19 percent protein, compared to 9 percent for whole fruits, and 7 percent for fruit pulp. Similarly, seeds averaged 11 percent lipid, compared to 5 percent for whole fruit and 2 percent for fruit pulp. In contrast, seeds averaged 16 percent fiber, compared to 46 percent for whole fruit, 22 percent for fruit pulp, and 40 percent for other plant parts."

Ripe or unripe? There were several mentions of parrots feeding on unripe fruit in the previous paragraphs. The consumption of unripe fruit is probably geared mostly toward eating the seed, not the pulp.  Parrots are seed predators who destroy seeds instead of spreading them to new places where they can grow. So no plant in its right mind is going to put out delicious fruits in order to attract parrots.

Getting sugar calories is the primary reason that wild animals eat ripe sweet fruit, since it doesn't contain a lot of other nutrients.  But unripe fruit is not sweet, and is frequently bitter and nasty. The sugar develops in the last stage of ripening, because that's the point where the plant 'wants' the fruit to be eaten.  Foraging on unripe fruit is a smart competitive move for birds who mainly want to get at the seeds and are not very interested in the rest of the fruit. If they wait until the fruit is ripe, something else may eat it before they do. Taking the seed before the fruit is ripe enough for any other animal to want it also saves the parrots from the risk of fights with other species, for example a large monkey who doesn't want to share the fruit.

Eating unripe seeds might be a smart nutritional move too.  There's almost no information available on the nutritional differences between unripe seed and mature seed, but the evidence that we've got indicates that unripe seed is superior in at least some respects.  Unripe grass seed had higher-quality protein than ripe seed (Allen & Hume). Unripe fruit seed also has better protein than ripe, and the buds of flowers and leaves are also a desirable protein source for many animals (White).

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Nutritional analysis

The first chart below illustrates the dramatic difference between nutrient-dense fruit (palm fruit) and the nutrient-dilute fruits that we humans like to eat (everything else listed in the chart). Tropical fruits are generally considered to be more nutritious than temperate-zone fruits, so they make up most of the comparison. To be fair, the nutrient content of the sweet fruits is skewed by their high water content. So I included a comparison between fresh and dried versions where information was available. There's still a huge difference between palm fruit and sweet fruit even after we've leveled the playing field this way.

The discussion below focuses on the palm fruit from Elaeis guineensis, the African palm species that red palm oil is made from.  It appears that palm fruit in general is a fatty fruit, not sweet, but there are considerable variations in the nutritional content of palm fruit from different species. 

Keep in mind that the wild diet has the wrong nutrient balance for a pet bird (see separate article), and you shouldn't feed your bird a high level of fat or sugar no matter what the wild birds are eating.  But when you hear that a particular wild bird eats fruit, the comparison below clearly shows that you shouldn't assume that this fruit bears much resemblance to cultivated fruit. For a wild parrot, high-nutrient high-fat fruits are a better nutritional bargain than low-nutrient high-sugar fruits. When they do go for a low-nutrient fruit, in many cases it's really the seed that they're after, and the fruit pulp is an incidental item that may or may not be eaten.  

I estimated some of the amounts in the chart, because official data was unavailable but there was enough information to make an educated guess.  These estimated amounts are in light purple to make them identifiable.

The next chart shows how high-fat palm fruit compares to some high-fat seeds and nuts. As expected, palm fruit beats them significantly on Vitamin A, since palm fruit is a rich source and seeds and nuts are deficient.  But the seeds and nuts beat palm fruit significantly on protein, which is harder to obtain in the diet than Vitamin A is.

Now let's see how high-carbohydrate sweet fruits compare to other high-carbohydrate foods, namely vegetables and grains.  For easy comparison, the chart below groups the fresh fruits together with the high-moisture vegetables and the dried fruits with the low-moisture grains. In general, the vegetables blow the fresh fruits out of the water for most nutrients, despite having a higher water content than the fruit.  What's most surprising is how poor a Vitamin A source mango is compared to kale or carrots. The grains are superior to the dried fruits for many nutrients but not all, and grains are particularly deficient in Vitamin A.

But just look at the difference in the proportion of sugar to total carbs!  The fruits have a significantly higher percentage of total carbohydrates to begin with (especially when compared to the vegetables).  On top of that, the ratio of sugar to total carbs for the fruits is 50-80%.  Carrots are the only item in the vegetable and grain category with a ratio that comes close to the 50% level. Broccoli has a ratio of about 25%, and the others have a negligible amount of sugar. Fruit's high sugar content and relatively low content of other nutrients is the reason that it's less desirable than vegetables.

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Fruit consumption by wild African greys

Let's look at the documented fruit-eating habits of African greys as an example of how different fruit eating in the wild can be from the supermarket-fruit experience. Wild African grey parrots are widely reported as including fruit in their diet, and it seems to be universally agreed that they are particularly fond of the fruit of the African palm Elaeis guineensis. It is not clear how much of their diet consists of palm fruit, but a study by Tamungang et al found a high density of parrots on a palm oil plantation during the dry season, while other nearby locations were preferred during other seasons.  The study area was in and near Korup National park in Cameroon.

A different paper by Tamungang et al provides information on what else they were eating (English translation of abstract).  An obnoxiously hard-to-read text-only version is available here. Tell your computer to find "Diversity of food of the Grey Parrot" on the page. 

The paper reported that the birds ate a surprising amount of flowers, and 20% of the diet came from three flower sources. It isn't clear how much of this was flower buds and how much was fully developed flowers. Flower buds are a rich source of protein, as discussed later in the article. But eating mature flowers probably means that the birds were consuming sugar.  The nectar is the most nutritious part of a flower; it's there as a lure for pollinators and is the only part of the flower that the plant "wants" to be eaten. So it would be a bad strategy for the plant to have anything else nearby that was good to eat. The pollen is mostly indigestible and the rest of the flower is low in nutrients.  The paper indicates that flower resources were available for about 11 months out of the year, so most of the time the birds wouldn't need sweet fruit if they wanted to eat sugar. In most cases, the fruits that the parrots were feeding on were not sweet anyway, so sugar was apparently not the reason for eating them.

Another 26.7% of the diet came from four seed sources, and 53.3% came from seven fruit sources.  Only one of these seven fruits looks similar to our idea of a sweet, juicy tropical fruit; the others were fatty fruits and/or their taste appeal ranged from not being very good to not being considered edible at all by humans. 

Unfortunately, in most of the fruit-feeding cases the paper did not say whether the birds were eating the fruit pulp, the seed, or both. There also wasn't information on how much of each specific fruit the birds were eating or how the different fruits ranked in their food preferences. It was specified that the birds were eating the pulp in one case and the seed in another, and the rest are unclear. But enough information is available about the fruit that we can make a reasonable guess in some cases.

Here are the seven fruit sources identified by the study, listed in the same order as Table 1 of the paper. It is not clear whether this order means anything; the table is not alphabetical or organized according to any other identifiable classification scheme:

Alstonia boonei.  There is nothing on this tree that is considered edible by humans, although its wood is useful and some parts have medicinal uses (Useful Tropical Plants). A study on wild animals found that the guenon (a type of primate) was the only frugivore that would eat the fruit of this tree. Several other frugivores including two types of birds (hornbills and turacos) didn't eat it, and neither did colobus monkeys or chimps (Seed Dispersal and Frugivory). It's likely that the parrots were after the seeds, which apparently grow in pods that we humans would not normally consider to be a fruit.

Macaranga spinosa. This is another tree that is completely inedible for humans but has other uses (Useful Tropical Plants). It's also another tree where it's most likely that the parrots were after the inch-long seed not the fruit pulp, but this one could go either way. Primate Ecology reports that chimps eat the bark but not the fruit, although Wildpro says that bonobos eat the fruit. Favier et al says that the fruits and seeds are small enough to be eaten by birds, but they don't say what kind of birds.  

Dacryodes edulis aka butterfruit. This is the kind of high-fat fruit that we DON'T think of when people talk about tropical fruit, because it's more like an avocado than a pineapple.  It's highly edible for humans and is grown on plantations for its oil. It's reported that oil makes up 33-65% of the pulp.  It also contains 7% protein which is high for a fruit (20-30% by dry weight) and has a lysine content similar to that in animal protein. There's plenty for a wild bird to like in this fruit, and it's likely that the parrots were eating the fruit pulp (Useful Tropical Plants, Lost Crops of Africa, Wikipedia). The seed kernel is also rich in oil so they may have eaten that too.

Dacryodes microphylla. Although this tree is in the same genus as butterfruit, it appears that the fruit is very different, and is the closest thing on the list to the tropical fruit at the grocery store. Information is sketchy, but the fruit is described as juicy with a delicious sweet/sour flavor (Lost Crops of Africa).  The fruit pulp is the only part of this tree that is used by humans (Plant Use; English translation).

The paper's only suggestion about the wild parrots' food preference was the statement that "during seasons with poor plum yield, the parrots consume more Macaranga spinosa and Elaeis guineensis." It's not clear what they mean by "plums" since they don't use this word anywhere else, but it seems likely that they mean Dacryodes microphylla. B&T World Seeds says the fruit is similar to black plums.

It seems likely that the parrots were eating this fruit for the pulp (assuming that they waited until the fruit was ripe, which was not specified in the paper).  There's no word on the edibility of the seed, but it's possible that the parrots ate that too. This fruit is only available for four months out of the year, from June to September, so the parrots' window of opportunity to eat sweet juicy fruit is pretty limited.

Elaeis guineensis.  Here's the palm fruit!  It was discussed extensively earlier in the article, but I'll reiterate:  it is a high-fat fruit that has very little in common with juicy tropical fruits like mango and papaya. Humans make oil from both the fruit and the seed (Useful Tropical Plants). Greys eat the flesh but apparently don't eat the seed (Parrots Daily News). This was the only food source that was available to the parrots all year round; the others had more limited availability, with most available for only 3 or 4 months out of the year.  But the availability was staggered enough that the birds had three to eight food sources available in any given month.

Pycnanthus angolensis. The paper specifies that the parrots ate the seed of this fruit, and even included a picture showing the uneaten fruit pulp. The pulp looks more like a piece of wood than a nice juicy mango. This tree is a member of the nutmeg family, and the fruit has cartilaginous flesh that dries woody (Wikipedia). Chimpanzees will eat the aril of the fruit but it is too bitter for humans (Mahale Chimpanzees).  Humans sometimes use the seeds as a spice. The oil from this fruit is considered to be inedible although it does have non-food uses (Useful Tropical Plants).

Blighia welwitschii. The paper specifies that the parrots eat the aril attached to the seeds. This aril is the only part of this fruit that is edible because the seed and skin are toxic. The arils themselves are toxic when they're unripe but are safe enough when the fruit is so ripe that it splits open. The aril is only a small part of this fruit, which has large seeds and a woody skin. The aril is considered to be edible for humans, with a rather low edibility rating of 2 out of 5 (Useful Tropical Plants, Plant Use). A related species, B. sapida, is considered to be more desirable and is sometimes grown for commercial fruit production (Wikiwand).

I couldn't find a complete nutritional profile of B. welwitschii, but Lamperti et al indicate that it's 65% water, which is slightly less than the water content of an avocado. It has 20% crude protein, which is very high compared to the other fruits on the list. The carb and fiber content was too confusing for me to figure out but is sort of in the midrange compared to the other fruits on the list. The fat content wasn't listed so presumably it's a number too close to zero to be worth mentioning. It's not particularly juicy and apparently not particularly sweet. There's no word on the protein quality, but if it's any good that would be an obvious reason for the birds to eat it. Its cousin B. sapida is a straightforward high-fat fruit (Goldson et al).

It appears that for humans, B. welwitschii is used more for medicinal purposes than as a food source (African Indigenous Vegetables). But PROTA4U reports that in Gabon, it is a favorite food of the African grey parrot.

Trivia time - this tree's genus was named after Captain Bligh of the HMS Bounty. An infamous mutiny didn't stop him from getting botanical credit.

Albizia gummifera was listed as a seed source in Table 1 but described as a fruit elsewhere in the paper. It's a fruit in the botanical sense but comes in the shape of a flat seed pod that we wouldn't ordinarily describe as a fruit. The parrots ate only the seeds. The other seed sources (Dialum guineense, Ceiba pentandra, and Treculia africana) are also technically fruits, and Treculia africana is better known as the breadfruit tree. This illustrates the pitfalls awaiting us when we talk about fruit consumption by wild parrots.  A botanist's definition of a fruit is not the same as the common definition.

There's another paper by Chapman et al that describes a 10-week feeding frenzy by a large flock of CAGs in Uganda on the fruit of Pseudospondias microcarpa. The food resource is repeatedly described as fruit, but the birds only ate the seeds and threw the fruit pulp away. Which is ironic, because for human consumption it's described as a sweet fruit with enough juice to stain the fingers (Useful Tropical Plants). But that sweetness has an unpleasant edge to it, because the flavor is also described as resinous and astringent. Raw foodist Frederick Patenaude says the tannins leave one with a furry sensation in the mouth. Maybe humans can tolerate the tannins better than the parrots could. 

Because of these issues, humans usually eat this fruit in small quantities as a snack, and sometimes eat the seeds as well. It is occasionally cultivated for the edible fruit (Mansfeld's Encyclopedia). The common name is African Grape, although there are apparently several other fruits that are also called by that name.

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Fruit consumption by wild eclectus parrots

Eclectus parrots are one of the most extreme members of the parrot family. The color difference between males and females is so dramatic that it was initially believed that they were two different species. Their double-polygamy mating system, where each bird has an average of 7 mates, is certainly unique.  Their diet is different from the others too. Most parrots are classified as granivores (seed eaters) or omnivores (everything-eaters), but ekkies are frugivores (fruit eaters).  This is one bird where it looks like they really do eat a lot of sweet fruit that wouldn't be out of place at the supermarket.  But you shouldn't assume that anything in this section applies to any other species.  Ekkies tend to not do very well on a diet that's perfectly fine for most other parrots, and the reverse is probably also true.

It seems obvious that feeding a granivore's diet to a frugivore could be problematic, since a frugivore is designed for a different type of food.  It doesn't necessarily explain every quirk in the relationship between ekkies and diet, but it goes a long way toward helping us understand the probable source of the problem. A species that evolved for a diet of nutrient-dilute food like sweet fruit has to be super efficient at extracting the few nutrients that are in the food, and they might also have developed a lower requirement for certain nutrients.  Give them a lot of nutrient-dense foods like seeds, nuts and grains, and there's likely to be trouble. You shouldn't feed a lorikeet like a granivore either because they aren't granivores - they're nectarivores.

So how frugivorous are ekkies?  There isn't any formal documentation that I can find, but it seems to be generally agreed that they really are frugivores and not something else.  Sugarloaf Animal Hospital says "They are not obligate frugivores, but as much as 80% of their wild diet is reported to be fruit in some ecological studies."  An obligate frugivore is an animal that requires a lot of fruit in the diet to survive, while an animal that eats a lot of fruit but doesn't actually require it would be a facultative frugivore.

I couldn't find the studies or any reliable information on whether or not their frugivory is obligatory or not, but Parrots of the World (Forshaw) mentions three sources that reported lots of fruit-eating by ekkies. The book gives the authors' names, which are 1. Toxopeus, 2. Cain & Galbraith 1956, and 3. Hoogerwerf.  The references are apparently too old and/or obscure to show up on the internet.

The best information that I've found is a Youtube slideshow presentation by avian veterinarian Rob Marshall titled "Exploring the Wild Diet to Solve Functional Digestive Disorders in Eclectus Parrots".  The format is inconvenient - there are 38 short clips that have to be navigated while trying to read the notes below the video before the clip ends.  So I'll report the most significant parts here.

Be aware that there are always limitations in studies on the diet of wild birds. Observational studies that don't include an analysis of crop contents can only tell us part of the story.  Dr Marshall is an avian veterinarian not a biologist. We don't know where he made his observations or how well they measure up to official scientific standards. I've been told informally that the location was probably in far north Queensland, which is not easily accessible and requires a fairly major expedition to get there. We don't know how long the observation period was - probably more than a single day simply due to the effort required to get to the location, but possibly not a very long period either since people who have another job usually can't spend too much time off in a remote place. How many birds were observed - a small number or a lot? Observing the feeding activities of wild birds is often difficult, especially when there's dense forest to contend with, and ekkies are rainforest birds.  What did they eat when they flew off and went somewhere else to forage? Nobody knows the answer to that, obviously.But in any case the birds' preference for fruit pulp over other foods seems pretty clear, at least for the time period and location where they were observed.

The notes to Clip 2   say "Eclectus are classified as generalist frugivores. This means they eat mostly fruit pulp in the wild. We collected and identified rainforest foods known to be eaten by Eclectus. We also discovered new foods. Our findings reconfirmed the frugivory status of Eclectus."

The notes to Clip 3 say "Our study divided known Eclectus foods into 4 categories. The first and most important category consisted of fleshy pulp and soft skinned fruits. Most fleshy fruits collected were sugary. Others were clearly lipid. The fruit pulp wild diet of Eclectus parrots is considered nutritionally compromised because of its low protein content. Apart from fruit pulp Eclectus parrots also feed on seeds, aril, leaf buds, blossoms and nectar to satisfy additional nutritional needs."

The text on the actual video clip says that fleshy fruit pulp was more than 75% of the diet, and shows pictures of the fruit. Most of it was sugary, but the diet did include some palm fruit which is fatty not sweet. Presumably the birds were mostly eating ripe fruit, since unripe fruit is not sugary.

The Clip 4 notes say "The seeds eaten by Eclectus are small. It was expected that the seeds within hard cased fruits would compensate for the low protein levels of fruit pulp. Nutritional analysis of hop bush seeds and red ash seeds was undertaken. Crude protein levels were surprisingly low in these seeds."

Clip 5 comments on eating fruits with an aril but doesn't say anything particularly informative.  An aril is a bit of fruit attached to a seed that is encased within a larger, inedible covering.  Pomegranate is an example of a fruit with arils. The fruits shown in the clip are northern tamarind, red beech, and fire vine.

Clip 6 says,

The actual video clip says "Wild Diet:  bulky, watery, sugary fruit pulp; seeds much smaller part of diet; protein limited".

Certain species of leaf buds can be very high in protein, and this doesn't seem to be particularly well known. So the birds' protein intake might not be as limited as thought, even though the leaf buds are just a small part of the diet.  I've got some additional info on bud protein in the section of this article called "Do wild parrots eat vegetables?"

The hens would need a lot of protein during the breeding season to support egg laying and chick rearing.  Does the diet change at this time? Do they eat some bugs to provide protein? We don't know. Bug-eating might be harder to observe than a bird that's gnawing on a piece of fruit. Eating insects to supply certain nutrients is a common strategy for nectarivorous birds, which also have a high-sugar diet.

The wild diet in general is not appropriate for pet birds because of different nutrient requirements, and this definitely does NOT mean that if you have an ekkie you should mostly feed it sweet fruit. In fact I've heard through the grapevine that doing this can result in sick ekkies. But you do need to be aware that their requirements are somewhat different from those of most parrots, read up on the advice of reputable people who specialize in eclectus, and proceed with caution. This discussion is about WHY ekkies are different from other parrots, and not about the specifics of what you should feed them.

Digression - do ekkies have longer intestines? It's generally agreed that eclectus parrots have somewhat different dietary needs than other parrots and often don't do well on diets that are fine for other species.  The reason for this isn't really known, but one explanation that's often given is that they process food differently than the others, and one indication of this is that they have longer intestines than other parrots.

This seems to be anecdotal information based on a small number of observations.  I've heard that one vet did a necropsy and found that the gut was longer, and that a different vet hasn't observed a significant difference, and that's pretty much it.  There's simply not enough information available to know whether it's true or not.

Ekkie owners are strongly encouraged to feed their birds a lot of fiber, so even if longer intestines are observed it might simply be a reflection of their diet.  From Comparative Avian Nutrition by Kirk Klasing, page 113: "The length of the intestines increase with increased food intake or increased dietary fiber. For example, a 34% increase in dry-matter intake by Japanese Quail induces a 43% increase in the length of the small intestine."

The book also indicates that the size of the ceca changes (in birds that have them), and the size and strength of organs like the gizzard will also change. It takes a few weeks for these major differences to develop, but there are more subtle changes that can occur over a few days or even in the course of a single meal.

Changes in an individual's intestine length due to diet changes are called gut length plasticity, and they have been reported in fish, reptiles, and mammals (Zandonia et al, Naya et al), as well as several species of birds from a variety of families.  Presumably it happens with amphibians too; I haven't looked for info on them.  When a characteristic is observed in every major branch of the vertebrate family tree, from the oldest to the newest, it's fairly safe to assume that it evolved very early and is very widespread, and it would be an exceptional species that did NOT have this trait.  So until we receive information to the contrary, it's conservative to assume that diet influences the gut length of individual ekkies just as it does with so many other species. 

"These are the key features of Eclectus digestion. A large crop, wide thoracic esophagus and highly elastic proventriculus of Eclectus are morphological adaptations aimed at increasing food-holding capacity needed to accommodate a wet bulky wild diet. More time is needed to physically and chemically digest this increased volume of food in the proventriculus. Less time is required in the gizzard because fruit pulp lacks coarse fiber and seeds have already been broken into small pieces by Eclectus before they are swallowed. After the food is processed into small particle size by the milling action of gizzard muscles it moves into and rapidly fills the small intestine providing an expansive surface area for final absorption to take place. A precise hormonally regulated crop emptying process maximizes the speed of meal digestion by releasing the second part of the meal - retained in the crop - into the proventriculus as digesta moves into the duodenum from the gizzard."