Experiments: Self-Test Material
Egg retrieval by herring gulls
Herring gulls lay eggs in a shallow nest on the ground. The eggs are green or brown in colour and are covered in dark blotches. If an egg rolls out of the nest it is retrieved by the parent.
In an elegant series of experiments, Baerends & Kruijt investigated what properties of the egg signalled to the gull that it should be retrieved back into the nest. They removed two eggs from the nest and placed two dummies on the nest rim. The dummies were of various sizes, shapes and colours.
The investigators were interested in exploring the birds' retrieval preferences:
- Are bigger eggs more attractive than smaller eggs?
- What is the effect of egg colour on this behaviour?
Position preference and egg retrieval by gulls
It proved impossible to carry out a simple preference test because when two identical sized dummies were placed on the nest rim, the gull usually revealed a marked position preference, retrieving the dummy on the left first (or vice versa).
To overcome this problem, Baerends & Kruijt employed a very elegant titration technique in which position was titrated againt size. In anthropomorphic terms they said to the gull
"OK, we know you prefer the egg on your left / right hand side, but if we put this bigger egg on the right / left hand side will you retrieve it first?"