When glaciation was at its peak the woodswallow population actually boomed. In boom times there is a larger pool of gene carriers ideal for genetic selection when times become harder. More of the birds die off. Yet, the survivors have traits that suite the changed environment.
Australia has the harshest of climates with long periods of drought. Consequently, birds such as pelicans live a longtime on a meagre diet so they can wait for good times to breed. Zebra finches seem to do well even when times are very hard.