Ethological Experiments: Self-Test Material
Author Paul Kenyon


The influence of egg size and colour on egg retrieval by herring gulls

This page allows you to test your understanding of a technique used by ethologists to study behavioural preferences

You can review material on ethological techniques here.

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:


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?"



Test your understanding of the titration experiment by clicking on the egg you think a gull would choose.
A tip appears along the bottom of the browser as you pass the cursor over the egg.

This is the smaller of two natural coloured eggs. This is the larger of two natural coloured eggs. Brown eggs are less preferred than natural coloured eggsNatural coloured eggs are more preferred than brown eggsGreen eggs are more preferred than natural coloured eggsNatural coloured eggs are less preferred than green eggsGreen eggs are less preferred than green speckled eggsGreen speckled eggs are more preferred than green eggsNatural coloured eggs are less preferred than green speckled eggsGreen speckled eggs are more preferred than natural coloured eggsGreen eggs are more preferred than brown eggsBrown eggs are less preferred than green eggsGreen speckled eggs are more preferred than brown eggsBrown eggs are less preferred than green speckled eggsGreen eggs are preferred over slightly larger natural coloured eggsLarger natural coloured eggs are less preferred than slightly smaller green eggsGreen speckled eggs are more attractive than much larger natural coloured eggs Natural coloured eggs are less attractive than much smaller green speckled eggs