Meeting Abstract

20.3  Jan. 5  Seasonal components of the reproductive schedules of North American crossbills (Loxia sp.). HAHN, TP*; CORNELIUS, JM; KELSEY, TR; Univ. of California Davis; Univ. of California Davis; Univ. of California Davis tphahn@ucdavis.edu

Crossbills have been thought capable of initiating reproduction at any time of year, or maintaining it indefinitely, if food supply permits. However, at least the smallest North American form displays a fundamentally seasonal reproductive schedule, with autumn gonadal regression even if food supply remains at the late summer annual peak. It has been unclear whether this seasonality is unique to this highly specialized form or is a general feature of crossbills. Here we present evidence from several other crossbill forms indicating that the autumn collapse of the gonads, coincident with advancement of the annual plumage molt, is indeed a general feature of North American crossbills. Gonads of free-living birds regress during September or October even if food supply remains high enough to support subsequent winter breeding. This suggests some form of autumn refractoriness to environmental cues. Experiments already rule out �absolute photorefractoriness� typical of other temperate zone songbirds, leaving �relative photorefractoriness� or some kind of �food refractoriness� as possible explanations for the decline in reproductive competence in autumn. Viewed in a comparative context, this is highly unusual and likely represents an adaptive specialization to facilitate a flexible breeding schedule. Field work is ongoing to ascertain whether these birds ever maintain reproductive competence through the autumn under exceptionally favorable conditions. Comparative experiments will help clarify the extent to which crossbills� extraordinary reproductive patterns are the result of adaptive specializations of their cue response systems, or of conditionally plastic outputs of phylogenetically conserved cue response traits.