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Risk-Sensitive Foraging

Humans and animals are sensitive to risk.  That is, we pay attention to variation in our income - be that money or food.

Hummingbirds are an excellent model species with which to study risk-sensitivity because they will feed frequently at artificial flowers.  The board in the photo to the right contains 18 wells that can be loaded with sugar solution of different volumes.  The coloured rings indicate the reward type.  For example, yellow flowers might contain 30µl of nectar and red flowers might contain 10 or 50 µl of nectar with equal probabilities.  The mean return is the same, but the level of variation in intake is very different.

Our studies tell us that hummingbirds are very sensitive to risk.  However, we have documented that they seek variable rewards under conditions that are not predicted by previous theory.  My twin-threshold model of risk sensitivity is a hypothesis that may expain these choices.
hummingbird at foraging board
peak system

Spatial Memory

A territorial male rufous hummingbird visits hundreds of flowers each day, havesting only a few µl of nectar from each flower.  It takes many hours for a flower to secrete additional nectar.  Thus, to forage efficiently a hummingbird should avoid re-visiting flowers for many hours.

Our studies indicate that hummingbirds remember flowers not by their colours, but by their spatial locations.  Recent work indicates that birds can also remember when they have last visited a flower, and can associate time and place for at least 8 different flowers.

The photo to the left shows Simone Franz setting up her undergraduate research project.  Simone used two video cameras (Peak Performance System) and a computer to map the flights of hummingbirds as they approached a rewarded flower.  After collecting baseline data for 5-6 different flights, the flower was removed and the bird's next flight was recorded.  It appears that a hummingbird navigates to within approximately 1.4 m of a flower before it notices that the flower is missing.  Thus, the birds are likely employing local landmarks to navigate and they depend on the colourful flower as a beacon only in the last stages of approach.

Rationality and Choices

During our studies of risk-sensitive foraging, we noticed that hummingbirds sometimes reversed their foraging preferences. Essentially, they violated the principle of regularity - an important hallmark of rationality.  Models of consumer behaviour and models of foraging behaviour assume that humans and animals assign absolute values to commodities. Recent research with hummingbirds, other animals, and humans indicates that humans and animals sometimes use relative rules to assess value and thus their choices often depend on local context.  That is, assigned value is often dependent on the other options available in the choice set.

Because a territorial male hummingbird will visit our feeding arrays every 10 or 15 mintues throughout the day, they are excellent models for this research.
hummingbird foraging at flower