Last winter was mild enough to allow most bird species to forage naturally, which explains why the proportion of birds visiting feeders was much lower than usual. The mild winter also allowed many species to stay in their summer range all winter without migrating south, thereby limiting the usual attrition accompanying migration. Hence, last year' s mild winter is responsible for this year' s larger-than-usual bird population. Which one of the following, if true, would most strengthen the reasoning in the argument? ['Birds sometimes visit feeders even when they are able to find sufficient food for survival by foraging naturally.', 'Increases in bird populations sometimes occur following unusual weather patterns.', "Birds that remain in their summer range all winter often exhaust that range's food supply before spring.", 'Birds eating at feeders are more vulnerable to predators than are birds foraging naturally.']