Physiological Preparation for Autumnal Migration in White-Crowned Sparrows

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Abstract
Termination of breeding, the development of photorefractoriness, subsequent postnuptial molt and fat deposition precede autumnal migration in white-crowned sparrows (Zonotrichia leucophrys). The timing of these post-reproductive functions differs from that of analogous vernal functions because they are neither synchronous within the population nor uniformly coincident with environmental stimuli. It is hypothesized that the expressions of autumnal functions are internally coupled and that they result from a separate process initiated by the increasing vernal daylength. The data concerning the effect of various photoregimes on testicular regression, postnuptial molt and fat deposition of Z. l. gambelli are consistent with the following generalizations (1) on daylengths that exceed 16 h these processes begin after 40-60 days of photostimulation, inversely related to daylength; transfer of photostimulated (20L [light] 4D [dark]) males to an intermediate daylength of 12 h near the end of the testicular growth phase blocks the expression of autumnal functions, although their expression is not blocked if the birds are photorefractory when transferred; males transferred from 12L 12D to 20L 4D in various stages of testicular development begin postnuptial molt after a fixed number of days regardless of the stage of testicular development at the time of transfer; rates of postnuptial molt and fat deposition are inversely related to the daylength to which the birds are exposed when these functions occur, which may explain the acceleration of preparation for autumnal migration in late-starting individuals. The systems that control autumnal processes appear not only to be independent of those that regulate analogous vernal processes, but also appear to rely more on internal coupling to integrate these functions than do the latter.