The fact that the current number of birds occupies about 16% of the species' historic range suggests a significant decline in the species' population. This is because a much smaller proportion of the historic range being occupied indicates that the population has decreased, as fewer birds would require less space. Option B correctly interprets this information by stating that the species has significantly declined from its historic numbers. Option A is incorrect because a thriving and expanding population would likely occupy a larger, not smaller, portion of its historic range. Option C is incorrect because a stable population would not typically result in such a drastic reduction in occupied range. Option D is incorrect because adapting to a smaller range without population loss would not explain the significant reduction in the occupied range, suggesting that adaptation alone is not the cause of the current situation.