Reproductive seasonality in an opportunistic breeder, the red crossbill, Loxia curvirostra

Crossbills (genus Loxia) are thought to breed opportunistically whenever food is abundant. If strictly true, this would distinguish them from most temperate zone birds that use photoperiod and supplementary cues (e.g., temperature, food supply) to time changes in reproductive physiology and behavior. I describe patterns of changing reproductive physiology in free-living Red Crossbills (Loxia curvirostra) as functions of season and conifer phenology. Nonbreeding spring migrants had elevated plasma luteinizing hormone (LH) and began gonadal development prior to finding a summer breeding site. The birds began breeding in July while feeding on maturing seeds of western hemlock (Tsuga heterophylla). Circulating LH declined in August, and gonads regressed completely as molt accelerated in September, although seed availability was at an annual high. Slight gonadal growth occurred after molt in late autumn and early winter, but reproductive maturation and nesting did not occur during winter in this study, possibly because western hemlock cones had shed most of their seeds. Winter/spring breeding (reproductive opportunism) at other locations was inferred from the presence of independent juveniles in early summer. Thus, activation of the reproductive system in summer and termination of breeding during molt in autumn appear to be regular seasonal phenomena in Red Crossbills, as in other temperate zone birds. Their opportunistic capabilities are superimposed on this underlying pattern.