Polyandry occurs when females form social bonds and gain simultaneous parental care from multiple male mates. It is thought to be rare in birds and to occur more often in territorial species when the Operational Sex Ratio (OSR; ratio of mature males to females) exceeds one. We asked if variation in the OSR affected the rate of polyandry in Song Sparrows (Melospiza melodia) over 30 years on Mandarte Island, British Columbia, Canada. We found no correlation between OSR and polyandry (R_s = 0.04, df = 28, P = 0.86), but positive correlations between OSR and percent females with more than one social mate (R_s = 0.44, df = 28, P = 0.01), and percent females sharing a territory with a replacement male and her dependent young (R_s = 0.46, df = 28, P = 0.009). We suggest that polyandry in Song Sparrows is limited by the intolerance of territorial males towards intruders, but that it occurs when females occupy the territories of two or more males and gain their simultaneous care for the dependent young of a single brood as a consequence.