It is well known that ravens are smart but a new study shows the birds may be more intelligent than we previously thought.

Experiments conducted by Swedish researchers found that ravens can plan ahead for different types of events and they are willing to forgo an immediate reward for a better one in the future, reports Xinhua news agency.

This type of planning has historically been thought to be unique to humans and great apes, according to the study published this week in the US journal Science.

Previously, ravens and other members of the corvid family were known to cache food to eat at a future date.

However, some scientists considered this behaviour different from the planning exhibited by apes and said it might just reflect a specific adaptation confined to the food hoarding domain.

Now, researchers at Sweden's Lund University let ravens solve tasks that they don't encounter in the wild: using tools and bartering with humans.

First, ravens were trained to drop a simple tool, a stone, into a puzzle box through an opaque tube, causing the box to release a reward from an opening in the bottom.

Then, the box and the stone were removed. One hour later, the ravens were given the stone, as well as several "distractors" such as a wooden wheel, a wooden ball, a metal pipe and a toy car. Fifteen minutes later, the birds got the box back.

It turned out nearly every raven chose the stone, and upon being presented with the box, they used the tool to open it, with a success rate of 86 percent.

The researchers repeated the same experiment with a 17-hour delay in returning the box to the ravens and the success rate was 88.8 percent.

A high success rate was also seen in similar experiments where ravens used a token to later barter for a reward.

The researchers found the birds selected the tool or token about 73 percent of the time.

Even when the ravens knew that trading would only happen on the next day, they chose and stored these tokens as soon as they were offered to them.

"To be able to solve tasks like these, one needs a collection of cognitive abilities working in concert, such as inhibitory skills and different forms of memory," study author Mathias Osvath, associate professor in cognitive zoology at Lund University, said in a statement.

"That ravens show similar functions, and combine them in ways similar to apes, despite a last common ancestor as far back as 320 million years ago, suggests that evolution likes to re-run good productions," Osvath concluded.