Relationships between individual testosterone concentration in male barn swallows and extrinsic factors (n = 51).

Day of capture was expressed as the number of days before or after the social mate's clutch initiation, breeding density as the number of active nests within 6 meters of the focal bird's nest, and opportunities for extra-pair activity as the number of fertile females present in the barn per male on the day of capture. Ambient tempature ranged from 33.8 to 73.4°F (average 51.8°F), breeding density ranged from 1–10 nests (average 6), and opportunities for extra-pair activity ranged from 2 to 12 fertile females (average 8). Summaries derived from the mixed modelling procedure in MLwiN.