Birds are known for their eyesight, not their sense of smell. But the odors they emit may play a key role in helping them land mates, according to a new study. Most bird odors originate from the oils in the preen gland. Birds use their beak to rub preen oil over their feathers while grooming. To study the effects of different odors on mate choice, researchers collected preen-oil samples from 34 dark-eyed juncos, a common North American sparrow. They then put together a profile of odor compounds more typically found in males and those more typically found in females. Researchers assigned a score to birds based on the proportion of compounds in the preen oil. The higher the score, the greater abundance of malelike odor compounds. The high-scoring males produced the greater number of offspring (between six to seven fledglings) compared to the low-scoring males who had no surviving offspring. The study, to be published in the October issue of Animal Behaviour, also found that bird odor was a more reliable predictor of reproductive success than a male’s size or his plumage. Researchers also looked at social reproductive success, or the male's ability to raise offspring of their own as well as those sired by other males. Good “father” birds tended to have a more malelike odor.