Leaving home for the first time can often mean a diet of hot chips, noodles and cheap bread.

Turns out it is not so different for the Australian white ibis, sometimes known as Sydney's "bin chicken".

Far removed from its natural environment in the marshes of western NSW, in the city the white ibis is giving up its protein-rich diet and is loading up on carbs.

That's the finding of University of Sydney doctoral candidate Sean Coogan, a Canadian who usually studies the feeding habits of grizzly bears.

"The urban white ibis seems to be taking advantage of the abundance of high-carb human foods available in the city," he said.

Since the 1970s, the white ibis has gradually moved from its traditional habitat west of the Great Divide to urban centres on the coast.

It is thought this is due to increased agricultural water extraction, drought and habitat loss combined with what seems an opportunistic ability to utilise a variety of food resources.

Dr Richard Major from the Australian Museum estimates that "a quarter of the state's white ibis population is now living in Sydney, compared with virtually none prior to the 1970s".

The authors of the study, published in Behavioral Ecology, are not sure if the ibis preference for carbs is inherent or an adaptation to city life.

Mr Coogan said: "In their natural environment the ibis eat an animal-based food: insects, earthworms, crustaceans such as yabbies. This is basically a carnivorous diet.

"So their preferences in the city are clearly outside the range of their natural foods."

Mr Coogan's paper points out that "humans are believed to have adaptively evolved behavioural and physiological regulatory systems that find foods high in carbohydrates and fats appetising, because such foods may have been rare in the ancestral environment".

He suggests a similar process might be driving ibises' eating habits.

"Another possible reason could be down to a rapid adaptation to urban environments where carbohydrates are more readily available."

Mr Coogan said he didn't quantify what ibises scavenge from the city's bins, "but from what I saw it was bread, rice, noodles - really high-carb foods that are relatively abundant".

So is a carb-rich diet making the ibis unhealthy? Mr Coogan said this should be an area for future research.

"We don't know what effect human foods have on ibis. But on the surface, they seem to be doing pretty well as evidenced by their population growth and breeding success."

He said other further study would be to replicate the experiment in the Macquarie marshes south of Brewarrina to see if that population also prefers carbohydrates

For his experiment, Mr Coogan offered ibis in Hyde Park and the Royal Botanic Garden a choice between energy-equivalent pellets that were either rich in proteins, fats or carbohydrates.

Following more than 60 feeding trials he found on average that the proportion of nutrients selected by ibis per feeding session was 52 per cent carbohydrate, 25 per cent protein and 23 per cent fat.

One interesting variability to the ibis's urban eating habits was during rainfall.

In his honours thesis in 2015, Matthew Chard at the University of Wollongong noticed that the birds would move to the Domain after rainfall where earthworms were readily available, increasing their protein intake.

During dry periods the ibis would move back to places like Belmore Park near Central Station where people feed the birds and a lot of discarded food is available.

Mr Coogan was surprised that when he tested food preference straight after rain, there would be an increase in protein consumption from his experimental food.

"Their preference for fat and protein pellets went up after rain, maybe they are tuning in to what's available in the environment and hammering that."

At the start of his experiment Mr Coogan said he started going through the bins in Hyde Park to see what the birds were eating. He said he got a lot of strange looks.

"So I went and bought a high-vis vest that a tradie would wear. Once I did that it was instant respect. I could go anywhere I wanted.

"If you ever want to go anywhere, wear a high-vis vest."

Mr Coogan's study into ibis forms one chapter in his doctoral thesis which examines macronutrient balancing among foraging animals, particularly the grizzly bear.

He said when he first arrived in Sydney he thought the birds looked great but soon found them "gross" when he saw their bin-feeding habits.

"However, after working with them I really started to enjoy them. They're quite funny," he said.

"They're delightful, with great personalities. They'll squabble among themselves or with other birds but they're actually really gentle.

"They're smart and bold while kind of timid at the same time."