PEREYRA, M.E.*; MORTON, M.L.; HAHN, T.P.; U.C. Davis; U.C. Davis; U.C. Davis: Behavioral Indicators of Photorefractoriness in Free-living Dusky Flycatchers

Traditionally, spontaneous gonadal regression under stimulatory day lengths has been used to indicate the onset of photorefractoriness, particularly in captive birds. Under natural conditions, the loss of photosensitivity and onset of photorefractoriness are more difficult to detect. In free-living birds, behavioral changes linked to the onset of refractoriness and the associated seasonal termination of reproduction are often obscured by other behavioral states, such as those related to the maintenance of parental care. Renesting behavior is the most reliable indicator of these changes under natural conditions, and one of the most ecologically and evolutionarily relevant variables from the standpoint of life history theory. For many small passerines, however, detection of renests can become progressively more difficult late in the season. Species such as the Dusky Flycatcher (Empidonax oberholseri), provide a rare opportunity to observe progression of refractoriness at several behavioral and physiological levels. In this species, nest failures are usually followed by renesting until mid-July. Shortly after the loss of eggs or chicks, female flycatchers typically disassemble the failed nest and recycle material into a replacement nest. We used the disappearance of this behavior during the course of the season to determine when reproductive termination occurred. Renesting behavior in two widely separated populations of Dusky Flycatcher (Tioga Pass, CA and Atlin, BC) indicated that despite different photoregimes, the timing of reproductive termination was consistent between the two populations. Differences in behavior among individuals from late June to late July indicate that the onset of refractoriness is progressive rather than abrupt, and may be manifested behaviorally at several different levels.