What's Harming the Robins?

Suppose a scientist observes an abnormality in the growth of robin embryos in a certain area. She knows the area has been sprayed with a pesticide and suspects the chemical may be causing the abnormalities she has observed.

To test this hypothesis, the scientist carries out a controlled experiment. She maintains two groups of robin embryos of the same age in the laboratory. Each group is exposed to exactly the same conditions of light, temperature, sound, and so on, except the embryos in the experimental group are exposed to a known amount of the pesticide in question.

The embryos in both groups are then examined over an identical period of time for the abnormality. If she finds a significantly larger number of the abnormalities in the experimental group than in the control group, the results support the idea that the pesticide is the culprit.

To be sure no errors occur during the procedure, the original researcher should repeat the experiment several times. Ideally, one or more other scientists should repeat the experiment independently.

Question: What flaws, if any, can you find in this experiment that might lead you to question the scientist's conclusions? (Hint: What other factors in nature-not in the laboratory-and in the embryos themselves could possibly explain the results?)