Western monarch butterflies, which crowd trees along the California coast every winter and flush them with color, have declined so dramatically since the 1980s that the species will likely go extinct in the next few decades if nothing is done, scientists said Thursday in a population study of the treasured creatures. Fewer than 300,000 of the brilliant orange and black insects were counted last year at some 300 locations stretching from Marin County to the Baja California peninsula, where millions of wintering monarchs historically took up shop, according to the report published in the journal Biological Conservation.