Question

what is the most likely reason the population of these butterflies has been declining? the number of milkweed plants along the butterflies migratory route has decreased. the butterflies are evolving to tolerate cold weather in the northern united states and southern canada. the number of bird species evolving to prey on butterflies has increased. the increase in yearly temperatures has warmed the northern habitats.

Explanation:

Milkweed is the primary food source for butterfly larvae. A decrease in milkweed plants along the migratory route would limit the food available for the butterflies, likely causing a decline in their population. Warmer temperatures might not necessarily be a negative for butterflies in terms of survival directly as they can adapt to some temperature changes. An increase in bird - predators preying on butterflies is a concern but not as fundamental as a loss of food source. Butterflies evolving to tolerate cold weather is more of an adaptation factor and not a likely cause of decline.

Answer:

The number of milkweed plants along the butterflies’ migratory route has decreased.