Tinbergen Observed That After Gull Nestlings Hatch, the Parents Remove

Tinbergen observed that after gull nestlings hatch, the parents remove the eggshells from the nest. To understand why (ultimate causation) , he painted chicken eggs to resemble gull eggs, which had camouflage coloration to allow them to be inconspicuous against the natural background. He distributed them throughout the area in which the gulls were nesting, placing broken eggshells with their prominent white interiors next to some of the eggs. As a control, he left other camouflaged eggs alone without eggshells. He then noted which eggs were found more easily by crows. Because the crows could use the white interior of a broken eggshell as a cue, they ate more of the camouflaged eggs that were near eggshells. Tinbergen concluded that eggshell removal behavior is adaptive: it reduces predation and thus increases the offspring's chances of survival.
-What legitimate factors might lead a scientist to do an imperfect version of an experiment, such as using chicken eggs as a stand-in for gull eggs? (Check all that apply.)

A) because the scientist doesn't care about proper procedure
B) the perfect experiment is too expensive
C) the perfect experiment would harm the study species
D) to quickly get preliminary data for a grant
E) ignorance
F) the perfect experiment would take too much time