Ornithologists were amazed to discover that a bird as small as a swallow could make a round - ​ trip of 14,000 miles [22,500 km] before returning to the same nest the following spring.
उत्तरी गोलार्ध के करोड़ों पक्षी दक्षिणी गोलार्ध में शीतकाल गुज़ारते हैं ।