In the wild, female Red-Winged Blackbirds (Agelaius phoeniceus) lay egg clutches that vary in size from just two eggs per clutch to seven eggs per clutch. Clutch size is a genetic trait, so a single female will lay clutches of a set size throughout her reproductive life.
Prior to a seven-year-long drought, a population ecologist surveys a breeding population of birds in a two-mile radius and determines the average number of eggs per clutch laid by females in that population to be five.
The ecologist surveys the same breeding population of birds at the end of the drought and finds the average number of eggs per clutch laid by females in that population to be two.
Three years later, during which time the area receives normal amounts of precipitation, the average clutch size in the population of breeding females has returned to five.
Which of the following BEST explains these observations?