Hurly study explores hummingbird habits

By TAMERA JONES, UK (Natural Environment Research Council, with U of L Communications staff and researcher input)

Hummingbirds are a welcome sign of spring, and a colourful reminder that the flowers in your garden are not just nice to look at, but are also an important food source.

But hummingbirds don't rely on taste alone when deciding how much nectar to drink from a new flower. Instead, they wait to see how they feel after their first meal, a new study reveals.

This is unexpected, because scientists know that hummingbirds can detect minuscule changes in nectar concentrations from one flower to the next. This "choice behaviour" research is one result of nearly 20 years of hummingbird study at the University of Lethbridge Westcastle Field Station by U of L biological sciences researcher Dr. Andrew Hurly, Dr. Sue Healy and numerous colleagues and students from the University of Lethbridge, the University of Edinburgh and the University of St. Andrews.

A hummingbird feeds from a constructed feeder. Photo by Dr. Andrew Hurly.

Hurly and Healy have been studying cognition, behaviour and ecology of rufous hummingbirds since 1992.

Approximately 40 undergraduate and graduate students from the University of Lethbridge, the University of Edinburgh and the University of St. Andrews have been involved in the hummingbird research project.

Like their extraordinary talent for flying thousands of kilometres from Mexico to Canada, taking the right amount of nectar onboard is a bit of an art – especially for such a tiny bird that weighs just three grams.

If they drink too much weak nectar, the extra load makes flying inefficient; if they don't take on enough of the same nectar, they'll have to eat again much sooner. On the other hand, if they drink too much of a rich nectar, they could easily get fat.

"The birds will almost certainly be able to detect the change in taste, but it seems they choose to ignore it until they get post-ingestive information before altering how much they drink the next time," explains Healy from the University of St. Andrews, a co-author of the study.

The researchers wanted to see how a group of wild rufous hummingbirds that breed in the Canadian Rocky Mountains responded to changes in food quality.
They dotted feeders containing different concentrations of nectar solution along the Westcastle river valley to be ready for the birds' return to the area from their overwintering grounds in Mexico.

Nectar concentrations in the flowers hummingbirds feed on vary from as low as seven per cent sugar to as high as 60 per cent. The researchers started by filling the feeders with a 14 per cent sugar solution.

Once the birds got used to this concentration, Healy and her colleagues changed to a 25 per cent solution. They found that the birds didn't change how much nectar they drank, sometimes persisting until the fourth meal.

"They take the same volume irrespective of concentration," explains Healy.

When they trained the birds to expect more concentrated sucrose in the feeders and then switched to the weaker solution, they got exactly the same result. Hummingbirds usually feed every 10 or 15 minutes, and only drink for a few seconds at a time. But because they digest nectar so quickly, they don't have to wait long to get the information they need to decide how much to drink for their next meal.

Other researchers have found that other creatures, like molluscs, wolves and cattle also rely on post-ingestive information about food before deciding how much to eat the next time. So even though hummingbirds certainly prefer sweeter nectar, it may not be surprising that taste alone is not enough to make a decision.

This story first appeared in the Legend. For a look at the Legend in a flipbook format, follow this link.