Effects of temperature on photoperiodically induced reproductive development, circulating plasma luteinizing hormone and thyroid hormones, body mass, fat deposition and molt in mountain white-crowned sparrows, Zonotrichia leucophrys oriantha

The mountain white-crowned sparrow, Zonotrichia leucophrys oriantha, breeds in subalpine meadows throughout many mountainous regions of western North America. Mathematical analysis of 20 years of egg-laying dates at Tioga Pass, California (3030m elevation) indicated a highly predictable breeding season suggesting that precise environmental cues such as the annual change in day length were important for regulating reproductive function. Additionally, it appeared that there was sufficient yearly variation in the timing of breeding to suggest that other environmental cues may also be important for regulating adjustments in reproductive development and regression. Captive populations of Z. l. oriantha showed strong responses in gonadal development following transfer to longs days (15L 913) and low temperature (5degreesC) slowed down photoperiodically induced gonadal growth and subsequent regression, in both males and females. High temperature of 30degreesC tended to accelerate gonadal development and regression whereas gonadal development was intermediate in a group exposed to 20 degreesC. Prior exposure to these temperature regimes while on short days (9L 15D) had no effect on body mass, fat, or plasma levels of luteinizing hormone (LH) and thyroid hormones. Curiously there was no effect of temperature on photoperiodically induced rises in LH in either sex despite marked effects on gonadal growth. Brood patch development was also enhanced in females exposed to 30degreesC. Corticosterone levels measured in a subset of plasma samples from this experiment indicated no effect of temperature suggesting that the retarded gonadal development at 5 degreesC was not a result of thermal stress. Although there was a robust effect of photo stimulation on thyroid hormone levels in blood of both sexes, temperature treatment had no effect on tri-iodothyronine (T3) concentrations. However, plasma levels of thyroxine (T4) were lower initially at 5 degreesC versus 20 and 30 degreesC treatments. This may be related to the protracted gonadal cycle at 5 degreesC versus the truncated gonadal cycle at 30 degreesC. Molt score, an indication of post-reproductive state and onset of photorefractoriness, was delayed in birds exposed to 5 degreesC. Body mass, and to a lesser extent fat score, tended to be lowest in birds exposed to 5 degreesC compared with those at 20 and 30 degreesC. These results demonstrate that ambient temperature significantly affected photoperiodically induced gonadal development and regression in these birds. The endocrine mechanisms underlying these effects require further study. (C) 2003 Published by Elsevier Science (USA).