RESUMO
Seasonal variations of plasma androgen levels in free-living adult and juvenile male Great Tits (Parus major major L.) were measured by radioimmunoassay over a period of 5 years. In 4 of the 5 years a peak of androgen concentrations was observed during the breeding season (in 1 year during the prebreeding season, followed by a nadir during winter. Besides these circannual fluctuations, considerable variations in plasma androgen levels between comparable behavioral periods of successive years were noticed (187 to 18.5 ng/dl during the breeding season and 86 to 16 ng/dl during winter). No relationship was found between these variations and possible causative factors such as age and origin of the birds and the time of trapping and blood sampling during the day. The large between year variations in androgen levels may be caused by changes in the birds' environment including density of the population and the availability of food. Additional data are required to test this hypothesis.