Ghalambor and Martin (2001) Found That a North American Species

Ghalambor and Martin (2001) found that a North American species of thrush placed higher value on their current nest full of offspring than did a similar South American species. This differential response is due to

A) a more abundant food supply in North America.
B) South American birds generally living longer than those in North America.
C) South American birds generally laying only one egg per nest.
D) higher parasitism rates in North America.