Wild hummingbirds have plenty of thoughts of food despite their minuscule brains, say researchers who note that the tiny birds' "episodic" memory for nectar feedings is so exact that it's unique among wildlife.

In fact, rufous hummingbirds in the Canadian Rockies show episodic memory similar to the way humans use a stopwatch, researchers Andrew Hurly at the University of Lethbridge in Alberta and his colleagues in Britain have found.

The scientists' study, in Tuesday's issue of the journal Current Biology, says hummingbirds are adept at remembering the timing of their feedings.

Rufous hummingbird is picky about its nectar timing. (Courtesy: Susan Healy, University of Edinburgh)

"Not only is this the first time that this degree of timing ability has been shown in wild animals, but these hummingbirds also exhibit two of the fundamental aspects of episodic-like memory (where and when), the kind of memory for specific events often thought to be exclusive to humans," they wrote.

The team tracked how often wild hummingbirds visited eight artificial flowers filled with a sugar solution in June and July of 2000.

Half of the flowers were refilled every 10 minutes, while the others were loaded with the sugar every 20 minutes.

The birds returned to the flowers on schedule, going to the 10-minute refills sooner than those that were topped up every 20 minutes, the researchers found.

"Hummingbirds remembered the locations and timing of eight rewards, updating this information throughout the day," the biologists wrote.

The scientists suspect hummingbirds – which weigh about 20 grams each and have brains the size of a grain of rice – developed such memory to avoid wasting time and energy searching for food during their 3,200-kilometre annual migration from Canada to Mexico.

Territorial hummingbirds' brains may be wired to work on short time intervals so they avoid revisiting flowers emptied of nectar before they replenish.

The study was sponsored by the Natural Sciences and Engineering Research Council of Canada and the National Environment Research Council in Britain.