Question

Black-headed Gulls

Niko Tinbergen (1907-1988) was a Swedish Ethologist (animal behaviorist) famous for studying

animals in their native habitats. One of his classic experiments involved a bird called the black-headed gull (Larus ridibundus). Black-headed gulls build nests of twigs on the ground and lay light brown eggs with dark brown spots. However, the inside of the egg is white in color. Tinbergen noticed that adult gulls pick up the eggshells shortly after a chick has hatched and fly them to a location far from the nest. Since this behavior required expending energy and time that could have been spent feeding and protecting the chicks, Tinbergen wanted to know why the birds did this.

Problem: Why do black-headed gulls remove eggshells from the nest?

Hypothesis: The white interior of the shell is not camouflaged and attracts predators to the nest.

Therefore, the gulls remove the shells to decrease predation.

Test: Tinbergen collected gull eggs and painted 69 of them white and left 68 of them with their natural color. (Statistically, these numbers are close enough to be considered equal.) He then scattered the eggs next to a gull breeding area and observed from a nearby blind. Predation rates were recorded for white versus natural colored eggs.

Data:

Original Number of Eggs           Eggs Taken by Predators                    Eggs Not Taken

White Eggs                        69                                                        43                                                        26

Natural Eggs                      68                                                        13                                                        55

a. What are the independent and dependent variables in this experiment?

b. Do the results of this experiment support the hypothesis? Why, Why not?

c. Are you 100% sure (without a doubt) that your hypothesis is correct? (Is it proven?)

d. If you were working with Tinbergen, what would you suggest be done next? Why?

Answer 1

Answer a)

Independent variable - Colour of eggs
Dependent variable - How many eggs were taken by predators.

Answer b)

Yes, it appears that the results do support the hypothesis because many more white eggs were taken
by the predators than the natural ones.

Answer c)

No, there is still some doubt because not 100% of the white eggs were taken, and more testing must
be done.

Answer d)

I would do further tests - repeat the experiment with more eggs, and in more places.