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by Clara da Silva

The remarkable migration has impressed humans inspiring people to monitor population size and health, potential causes for changes in population, and conditions most favorable to monarchs. Observers have reported population decline associated with droughts, habitat loss in both breeding and overwintering grounds, pesticide and land-use changes. The migration itself takes a toll; estimates of death rates suggest 90% of northern born migrating butterflies do not reach the overwintering grounds. 



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