New research shows that in the span of just fifty years songbirds have become slightly smaller, probably as a consequence of global warming. This is no big surprise as it has long been understood that size is inversely correlated with temperature. The Darwin contemporary Christian Bergmann first observed this trend, in terms of a correlation with latitude, and the trend became known as Bergmann's Rule. But how did the change come about?
The new research, which accounts for more than 100 species and almost half a million birds, shows a rapid reduction in size as the earth warmed in recent decades. The change is slight, but the statistics in the massive study are undeniable.
Perhaps the change is a consequence of natural selection shifting the populations to slightly smaller sizes, as the smaller individuals have greater reproductive success. Or perhaps the change is a result of built-in biological responses to the temperature change.
However the changes arose, the process involved complex structures and mechanisms--structures and mechanisms that must have evolved if the theory of evolution is true. Under evolution, we must believe that not only have the world's species evolved, but their various response mechanisms have as well.
Evolutionists explain that evolution happened to construct such response mechanisms. When needed those mechanisms were helpful, and so were selected for.