REPRODUCTIVE SEASONALITY IN AN OPPORTUNISTIC BREEDER, THE RED CROSSBILL, LOXIA-CURVIROSTRA

Crossbills (genus Loxia) are thought to breed opportunistically whenev er food is abundant. If strictly true, this would distinguish them fro m most temperate zone birds that use photoperiod and supplementary cue s (e.g., temperature, food supply) to time changes in reproductive phy siology and behavior. I describe patterns of changing reproductive phy siology in free-living Red Crossbills (Loxia curvirostra) as functions of season and conifer phenology. Nonbreeding spring migrants had elev ated plasma luteinizing hormone (LH) and began gonadal development pri or to finding a summer breeding site. The birds began breeding in July while feeding on maturing seeds of western hemlock (Tsuga heterophyll a). Circulating LH declined in August, and gonads regressed completely as molt accelerated in September, although seed availability was at a n annual high. Slight gonadal growth occurred after molt in late autum n and early winter, but reproductive maturation and nesting did not oc cur during winter in this study, possibly because western hemlock cone s had shed most of their seeds. Winter/spring breeding (reproductive o pportunism) at other locations was inferred from the presence of indep endent juveniles in early summer. Thus, activation of the reproductive system in summer and termination of breeding during molt in autumn ap pear to be regular seasonal phenomena in Red Crossbills, as in other t emperate zone birds. Their opportunistic capabilities are superimposed on this underlying pattern.