Complex sentence: It is hard to estimate the total number of spotted shags in New Zealand; estimates are between 10,000 and 50,000 breeding pairs (20,000 to 100,000 birds).
Split into simpler sentences: It is hard to estimate the total number of spotted shags in New Zealand. That's why estimates are between 10,000 and 50,000 breeding pairs (so 20,000 to 100,000 birds).